Are Humans Seasonally Photoperiodic?

Humans exhibit seasonal variation in a wide variety of behavioral and physiological processes, and numerous investigators have suggested that this might be because we are sensitive to seasonal variation in day length. The evidence supporting this hypothesis is inconsistent. A new hypothesis is offered here—namely, that some humans indeed are seasonally photoresponsive, but others are not, and that individual variation may be the cause of the inconsistencies that have plagued the study of responsiveness to photoperiod in the past. This hypothesis is examined in relation to seasonal changes in the reproductive activity of humans, and it is developed by reviewing and combining five bodies of knowledge: correlations of human birthrates with photoperiod; seasonal changes in the activity of the neuroendocrine pathway that could link photoperiod to gonadal steroid secretion in humans; what is known about photoperiod, latitude, and reproduction of nonhuman primates; documentation of individual variation in photoresponsiveness in rodents and humans; and what is known about the evolutionary ecology of humans.

Climate change and seasonal reproduction in mammals

Seasonal reproduction is common among mammals at all latitudes, even in the deep tropics. This paper (i) discusses the neuroendocrine pathways via which foraging conditions and predictive cues such as photoperiod enforce seasonality, (ii) considers the kinds of seasonal challenges mammals actually face in natural habitats, and (iii) uses the information thus generated to suggest how seasonal reproduction might be influenced by global climate change. Food availability and ambient temperature determine energy balance, and variation in energy balance is the ultimate cause of seasonal breeding in all mammals and the proximate cause in many. Photoperiodic cueing is common among long-lived mammals from the highest latitudes down to the mid-tropics. It is much less common in shorter lived mammals at all latitudes. An unknown predictive cue triggers reproduction in some desert and dry grassland species when it rains. The available information suggests that as our climate changes the small rodents of the world may adapt rather easily but the longer lived mammals whose reproduction is regulated by photoperiod may not do so well. A major gap in our knowledge concerns the tropics; that is where most species live and where we have the least understanding of how reproduction is regulated by environmental factors.

Puberty in female mice is not associated with increases in either body fat or leptin

It has been hypothesized that puberty is triggered when body fat and hence circulating levels of leptin exceed critical thresholds. Four kinds of experiments tested that hypothesis in female mice. When age was the independent variable, body fat and circulating levels of leptin decreased rather than increased before the onset of puberty. When stage of reproductive development was the independent variable, neither body fat nor circulating levels of leptin correlated with the onset of puberty. In sharp contrast, reproductive development was well correlated with body weight. A significant nocturnal peak in circulating levels of leptin was seen before and at all stages of reproductive development, but the highest levels were seen after rather than before the first estrous cycle was initiated. Neither acceleration nor deceleration of puberty by varying the female's social environment had any effect on either body fat or leptin. There is no support in any of these experiments for the hypothesis that an increase in body fat and thus an increase in circulating levels of leptin triggers puberty in female mice.

Energy balance and ovulation: small cages versus natural habitats

In the laboratory, ovulation is suppressed when a mammal is in negative energy balance whether that state is caused by inadequate food intake, excessive locomotor activity or heavy thermoregulatory costs. In this paper, knowledge generated in the laboratory about the link between ovulation and energy balance is examined in relation to the kinds of energetic challenges mammals actually face in natural habitats. When viewed in that context, several conclusions can be drawn. First, females ovulate whenever extant energetic conditions permit unless the process is blocked by non-metabolic stress, social cues or a predictive seasonal cue such as photoperiod. In the latter case, most mammals show at least a seasonal tendency in their reproduction and the majority do not use a predictive cue; they reproduce opportunistically in relation to seasonal variation in the energetic characteristics of their environment. Second, the widely held assumption that a female's fat reserves must exceed a critical level in order that she may ovulate finds no support in the literature dealing with natural populations. Third, the surprisingly rapid responsiveness of the gonadotrophin releasing hormone (GnRH) pulse generator to energetic manipulation probably reflects the study of animals that are in a pure survival mode. Fourth, the complexity of the energetic challenges mammals face in the wild suggests that there are probably multiple metabolic and neural pathways coupling ovulation to energy balance and that these pathways are probably characterized by considerable overlap and redundancy. Thus, fifth, to develop a more realistic overview of these pathways there is a need for experimental designs that present mammals with the kinds of complex challenges they actually face in the wild habitats in which they evolved.

Do small voles require less food than large voles?

Many small mammals lose body mass in the fall and winter. Laboratory studies suggest that the cause of this varies from individual to individual, at least in meadow voles, Microtus pennsylvanicus. The short day lengths of fall induce this adaptation in some individuals, presumably to promote winter survival. Other individuals, side-by-side neighbors, are insensitive to this cue; they enter winter with a larger body mass that they may not be able to maintain if conditions become too harsh. A long-held assumption is that the photoregulated loss of mass allows energy to be conserved by decreasing the amount of food required for survival and, thus, decreasing foraging time. The present experiment tested the hypothesis that smaller, photoresponsive meadow voles do indeed consume less food and spend less time feeding than larger individuals, which are not photoresponsive. This hypothesis was tested in conditions meant to simulate some of the energy challenges faced by voles at the onset of winter. Representatives of each phenotype were housed in cages in which they had to leave their nests in order to feed, and in which food intake and time spent feeding could be monitored. At 25°C the two groups did not differ in either food intake or time spent feeding; the smaller animals required more food per gram of body mass. Food quality was reduced and, later, ambient temperature was decreased to 3°C. Food intake and feeding time were again almost identical in the two groups. It is suggested that the hypothesis failed to find support in this experiment because it does not take into account the higher thermoregulatory costs associated with a decrease in size.

Exposure to anabolic-androgenic steroids shortens life span of male mice

Adult male laboratory mice were exposed for 6 months to a combination of four anabolic-androgenic steroids of the kinds and at the relative levels to which human athletes and body builders expose themselves. The four steroids included testosterone, two 17-alkylated steroids, and an ester, and they were given at doses that totaled either 5 or 20 times normal androgenic maintenance levels for mice. By the time the survivors were 20 months old (1 yr after the termination of steroid exposure), 52% of the mice given the high dose of steroids had died compared with 35% of the mice given the low dose and only 12% of the control mice given no exogenous hormones (P < 0.001). Autopsy of the steroid-treated mice typically revealed tumors in the liver or kidney, other kinds of damage to these two organs, broadly invase lymphosarcomas, or heart damage, and usually more than one of these conditions. It can be concluded that the life span of male mice is decreased dramatically by exposing them for 6 months to the kinds and relative levels of anabolic steroids used by many athletes and body builders.

Running-induced testicular recrudescence in the meadow vole: role of the circadian system

Access to running wheels stimulates testicular recrudescence in meadow voles whose reproductive axes have been suppressed by short day lengths. The present experiments addressed the mechanism by which running stimulates the reproductive system. The results from two experiments suggest that running acts specifically to override the short day length suppression of the gonads: access to running wheels had no stimulatory effect on the testes of meadow voles housed in long day lengths, and the degree to which running stimulated the testes of meadow voles housed under short day lengths was significantly correlated with the degree to which the voles were reproductively photoresponsive. A third experiment queried whether running shifts the circadian clock in such a way as to cause an overlap between the short day length photoperiod and the period of sensitivity to light. This proved not to be the case: access to running wheels stimulated testicular recrudescence in meadow voles housed in constant darkness. Two experiments demonstrated that access to a running wheel did not alter short day length profiles of pineal melatonin content or the nocturnal rise in pineal melatonin content in the absence of light. Finally, daily patterns of circulating corticosterone levels did differ between voles with and without access to running wheels, although the difference could not be attributed to differences in the circadian system. Overall, these experiments suggest that running stimulates gonadal recrudescence by acting on the pathway by which photoperiod suppresses reproduction downstream of melatonin production.

Melatonin rhythms and pineal structure in a tropical bat, Anoura geoffroyi, that does not use photoperiod to regulate seasonal reproduction

It has been hypothesized that pineal structure and function might differ between temperate zone and tropical species of mammals because of lower amplitudes of seasonal change in photoperiod and, in some areas, less seasonal climatic variation. Anoura geoffroyi produce a single offspring in November or December of each year on the Caribbean island of Trinidad, at 10 degrees N latitude in the deep tropics. Previous work has shown that this population lacks reproductive responses to photoperiod, and must be enforcing seasonal breeding using a non-photoperiodic cue. Anoura geoffroyi have a minute, thin, and rod-like pineal gland. Throughout much of its length, the pineal courses irregularly within the ventrolateral wall of the great cerebral vein. This intimate relationship may have functional implications. Despite having a very small pineal gland, this species produced a nocturnal rise in serum melatonin. Serum melatonin levels in most individuals were below or near undetectable levels during the light period and rose to a peak averaging 100 pg/ml in the last third of the dark period. Our results indicate that, although the pineal gland of A. geoffroyi is extremely small, serum melatonin levels are comparable to those of other mammals.

Effects of prolonged exposure to anabolic steroids on the behavior of male and female mice

Adult male and female mice were exposed to a combination of four anabolic-androgenic steroids at pharmacological doses for 6 months. Males were exposed to either 5 or 20 times androgenic maintenance levels, as determined by bioassay; females were exposed to either one or five times the maintenance levels for males. Even the low doses were sufficient to reduce gonadal weight in both sexes and eliminate the estrous cycle in females. Steroid treatment blocked ejaculation in most males without influencing other facets of their sexual behavior. In some females steroid treatment elicited male-like mounting and pelvic thrusting. Exposure to steroids increased aggressiveness in females. Using a different testing protocol, steroid exposure had no effect on aggressiveness in males. Steroid treatment decreased the use of a running wheel significantly in females and marginally in males. Overall, the results of this experiment suggest no enhancement of normal androgen-mediated behavior in males, but potent effects on the behavior of females.

Effect of anabolic steroids on behavior and physiological characteristics of female mice

Adult female mice were exposed to a combination of four anabolic-androgenic steroids for 9 weeks at doses that were either one or five times the androgenic maintenance level for male mice. Relative to control females, steroid treatment depressed gonadotropin secretion and increased both dry body weight and fat content but without an increase in food intake. Steroid treatment depressed spontaneous use of a running wheel and open-field activity, and it increased aggressiveness. It also eliminated a behavior related to encounters between the sexes--the rejection of genital inspection. There was no effect of steroid treatment on the time required to recover from 10 h of enforced running on a treadmill. Overall, regardless of the test or measure, there was little or no difference in the effect of the high and low dose of steroids. This indicates a threshold of response below the low dose used in these studies, which itself is probably well below that used by many female athletes and body builders.

Reactions of reproductively photoresponsive versus unresponsive meadow voles to simulated winter conditions

At least some populations of meadow voles (Microtus pennsylvanicus) comprise individuals that vary greatly in the degree to which their reproduction can be controlled by day length. Some individuals respond to the short days of winter with complete gonadal inhibition, others are insensitive to this cue and thus have the capacity to reproduce opportunistically during the winter, and still others are intermediate in their responsiveness. The relative costs and benefits associated with some of the nonreproductive dimensions of these different strategies are explored. The two extreme phenotypes, reproductively photoresponsive and unresponsive individuals, were exposed in the laboratory to winter versus summer conditions, as defined by photoperiod, temperature, and quality of diet. This was done in cages that required the voles to leave their nests and subject themselves to ambient conditions in order to feed. The winter condition exerted a potent influence on body mass, body fat, food intake, nest building, pelage depth, and the amount and temporal pattern of feeding, as well as reproductive potential. The results suggest that the major nonreproductive advantage enjoyed by the photoregulated phenotype is a decrease in body mass and hence a decrease in required foraging time that anticipates harsh winter conditions. The opportunists also may lose mass in response to harsh conditions, but this is a direct and immediate response for which they may be poorly prepared.

Seasonal variation in human reproduction: environmental factors

Almost all human populations exhibit seasonal variation in births, owing mostly to seasonal variation in the frequency of conception. This review focuses on the degree to which environmental factors like nutrition, temperature and photoperiod contribute to these seasonal patterns by acting directly on the reproductive axis. The reproductive strategy of humans is basically that of the apes: Humans have the capacity to reproduce continuously, albeit slowly, unless inhibited by environmental influences. Two, and perhaps three, environmental factors probably act routinely as seasonal inhibitors in some human populations. First, it seems likely that ovulation is regulated seasonally in populations experiencing seasonal variation in food availability. More specifically, it seems likely that inadequate food intake or the increased energy expenditure required to obtain food, or both, can delay menarche, suppress the frequency of ovulation in the nonlactating adult, and prolong lactational amenorrhea in these populations on a seasonal basis. This action is most easily seen in tropical subsistence societies where food availability often varies greatly owing to seasonal variation in rainfall; hence births in these populations often correlate with rainfall. Second, it seems likely that seasonally high temperatures suppress spermatogenesis enough to influence the incidence of fertilization in hotter latitudes, but possibly only in males wearing clothing that diminishes scrotal cooling. Since most of our knowledge about this phenomenon comes from temperate latitudes, the sensitivity of spermatogenesis in both human and nonhuman primates to heat in the tropics needs further study. It is quite possible that high temperatures suppress ovulation and early embryo survival seasonally in some of these same populations. Since we know less than desired about the effect of heat stress on ovulation and early pregnancy in nonhuman mammals, and nothing at all about it in humans or any of the other primates, this is an important area for future research. Third, correlational data suggest that there may be some degree of regulation of reproduction by photoperiod in humans at middle to higher latitudes. Populations at these latitudes often show a peak in presumed conceptions associated with the vernal equinox. On the other hand, evidence gathered by neuroendocrinologists tends to argue against reproductive photoresponsiveness in humans.

Tonic-clonic convulsions in meadow voles

Some meadow voles (Microtus pennsylvanicus) exhibit prolonged tonic-clonic convulsions, possibly epileptiform seizures, when handled or exposed to a strange environment. These convulsions are often preceded by a period of slow head shaking and/or stiff-legged hopping, but never by the explosively wild running bouts that characterize convulsions in some mammals. Convulsions occasionally occur in meadow voles in response to mild disturbance, as when an individual in its home cage is carried from one room to another. In contrast, they can not be elicited by some of the auditory or olfactory insults used to induce epileptiform seizures in other mammals. Breeding experiments have established the genetic basis of the convulsions seen in meadow voles, and of particular interest here is the fact that some of the convulsing voles were caught in the wild. This raises the interesting possibility that wild voles in natural habitats might be susceptible to convulsions when startled.

The effects of running activity on the reproductive axes of rodents

Access to a running wheel causes gonadal recrudescence in Syrian hamsters whose reproductive axes have been suppressed by housing them under short day lengths (Borer et al. 1983). The first experiment tested the generality of this phenomenon in a population of rodents that is genetically heterogeneous for reproductive photoresponsiveness. Male meadow voles (Microtus pennsylvanicus) of the two extreme phenotypes--reproductively photoresponsive and non-responsive--were either provided with a running wheel or housed without one. After 4 weeks with a wheel, the responsive voles had recovered full reproductive function, while the reproductive axes of responsive voles housed without wheels remained suppressed. Three experiments queried whether the use of a wheel would have reproductively stimulative effects in other rodents. First, intact male mice given access to wheels showed no increase in testis size when compared to mice housed without wheels. Likewise, locomotor activity had no effect on male rats whose testes were partially regressed in response to testosterone implants or on female mice whose estrous cycles were pheromonally suppressed by housing them in groups. Thus the neuroendocrine pathway used by locomotor activity to enhance the secretion of gonadotropin is specifically allied with the pathway used by photoperiod to control GnRH secretion.

Variation in reproductive photoresponsiveness in a wild population of meadow voles

Individual variation in reproductive photoresponsiveness has been documented in laboratory colonies of several species of rodents. When maintained on short day lengths, some individuals experience complete gonadal regression, others undergo little if any gonadal regression, and still others show intermediate levels of responsiveness. In the present research, a combination of laboratory and field studies explored the potential importance of this kind of variation for the control of seasonal breeding in a wild population of meadow voles (Microtus pennsylvanicus). A sample of adult males was trapped in central Pennsylvania during the summer when all were in breeding condition. When exposed to short day lengths in the laboratory, these males showed the entire range of variation in responsiveness noted above and, correlatively, variation in the loss in body weight induced by this treatment. A sample of males trapped in the wild just before the winter solstice showed the same distribution of variation as did the males housed on short day lengths in the laboratory, and thus, as might be expected, a few pregnant and lactating females were also trapped at this time. Longitudinal studies over a 42-wk period revealed that the variation reflects the degree to which the testes regress in response to short-day exposure rather than the rate at which they regress. Finally, studies with laboratory-born voles demonstrated that the variation is independent of age. In total, these studies demonstrate that the variation in reproductive photoresponsiveness previously seen only in laboratory colonies of rodents indeed has relevance for understanding the seasonal control of reproduction in the wild.

An endogenous circannual rhythm of reproduction in a tropical bat, Anoura geoffroyi, is not entrained by photoperiod

Most species of mammals live in the tropics, and many breed seasonally, but little is known about the regulation of their seasonal cycles. Males of a tropical bat, Anoura geoffroyi (Order Chiroptera, Family Phyllostomidae), from 10 degrees latitude in Trinidad, were studied to test the role of photoperiod in regulating seasonal reproduction in the deep tropics. Groups of males were subjected to five treatments: 1) constant photoperiod; 2) a 12-mo cycle of civil twilight photoperiods mimicking those occurring at 10 degrees latitude; 3) civil twilight photoperiods of 10 degrees latitude, but accelerated to a 9-mo cycle; 4) civil twilight photoperiods characteristic of 30 degrees latitude, but accelerated to a 9-mo cycle; and 5) constant photoperiod, but with the timing of dark onset varied to match the timing of darkness at 10 degrees latitude, and accelerated to a 9-mo cycle. In all treatments, the first cycle of testis growth and regression matched that expected in the wild population, as reported previously for some of these groups. Subsequently, the testis cycle of bats in constant conditions free-ran for 20 mo with a peak-to-peak period of 7.3 +/- 0.3 mo. Period lengths in the four nonconstant groups, 7.2-7.7 mo, were not significantly different from that under constant conditions. Bats failed to entrain to any photoperiod cycle, including those mimicking changes at 10 degrees or 30 degrees latitude. They also failed to entrain to the cycle in which day length was held constant while time of sunset was varied, as occurs at the equator.(ABSTRACT TRUNCATED AT 250 WORDS)

Sensitivity of Syrian hamsters (Mesocricetus auratus) to amplitudes and rates of photoperiodic change typical of the tropics

Empirical data suggest that reproductive photoresponsiveness occurs in some populations of mammals above 13 degrees of latitude, but may be absent in populations from 0 degrees to 10 degrees of latitude. The present experiments examined the degree to which the low amplitude of change in photoperiod in the tropics constrains mammals from using daylength as a seasonal cue. The Syrian hamster, a temperate-zone species, was studied because of its well-documented ability to respond to small changes in photoperiod, and because of the absence of an alternative robustly responding species from the tropics. We subjected adult male hamsters to photoperiods that mimicked the amplitude and rate of photoperiod change of 30 degrees, 20 degrees, 10 degrees, and 5 degrees of latitude, but centered around an estimate of their critical daylength. For comparison, a fifth group was subjected to an abrupt change in daylength of a magnitude equal to the total annual variation occurring at 30 degrees. The two groups experiencing the gradually changing daylengths of 30 degrees and 20 degrees showed less within-group synchrony during testicular regression; in other dimensions of the annual testis cycle, including the degree of synchrony exhibited during recrudescence, they reacted similarly to the hamsters given the abrupt change in daylength. Some of the hamsters exposed to the gradually changing daylengths of 10 degrees responded to this challenge, as did a few in the 5 degrees treatment--in both cases, with poor within-group synchrony and a submaximal decrease in testis size. In an abbreviated second experiment, hamsters given abrupt decreases in daylength of magnitudes equal to those of the 10 degrees and 5 degrees groups responded slightly more frequently, and with maximal decreases in testis size. This suggests that mammals may not be constrained absolutely by an inability to respond to changes in photoperiod at 5 degrees to 10 degrees latitude. Seasonally breeding populations of mammals in the deep tropics that do not use photoperiod to regulate reproduction may use nonphotoperiodic cues because they offer a higher signal-to-noise ratio than do tropical changes in photoperiod.

Correlation between reproductive photoresponsiveness and photoregulated locomotor activity in meadow voles

Some rodent populations contain individuals that undergo complete gonadal regression under short day lengths, other individuals whose gonads are little affected by such treatment, and still others that are intermediate in their response. Meadow voles exhibiting this variation were used to explore the relationship between reproductive photoresponsiveness and photoregulated locomotor activity. The activity patterns of the two extreme phenotypes--reproductively photoresponsive vs. nonresponsive--were compared, first under short day lengths and then under long day lengths. The primary component of the daily running wheel activity pattern for both phenotypes under both conditions was ultradian. Within that framework, reproductively photoresponsive voles were predominantly nocturnal in their locomotor activity under both day lengths. In contrast, the nonresponsive individuals showed no significant circadian variation in their activity under either day length. These results suggest that reproductively photoresponsive and nonresponsive individuals may have fundamentally different patterns of activity throughout the year in the wild. Furthermore, the data suggest that the inability of some voles to respond reproductively to variations in photoperiod may be caused by a decoupling of the circadian system from the entraining effects of day length.

Failure of cryptorchidism to suppress fertility in a tropical rodent

The cane mouse (Zygodontomys brevicauda) breeds year-round on the hot llanos of Venezuela, only 8 degrees above the equator. The reproductive responses of the males of this species to heat were compared with those of a temperature zone rodent, the white-footed mouse (Peromyscus leucopus). When tested at different ambient temperatures, the movement of the testis in relation to the scrotum was similar in the two species, but the cane mouse's testis proved to be much more resistant to maintenance at core body temperature. In two experiments, cryptorchid cane mice experienced only a 24% and a 5% decrease in testis weight, and almost all of these males showed normal spermatogenesis and sperm storage. In comparison, cryptorchid white-footed mice showed a 58% drop in testis weight, a total inhibition of spermatogenesis, and a complete or near absence of stored sperm in all males. A mating test demonstrated that cane mice indeed remain fertile for at least 2 to 3 mo after being rendered cryptorchid. Normal numbers of these males when paired with females fathered normal litters. It is noted that the relative insensitivity of the testis of the cane mouse to heat might actually be more representative of scrotal mammals in general than is suggested by our present perspective, which has been developed on the basis of study of humans and mammals from cool climates rather than the tropics, where most mammals live.

Life history and bioeconomy of the house mouse

1. More is known about the western European house mouse, Mus (musculus) domesticus than any other non-human mammal. If laboratory and field information is combined, an extremely valuable understanding of the species' bioeconomy could be obtained. 2. The seven stages of mouse life-history are surveyed (up to birth, nest life, sex life, social structure, population statics and stability, senescence, and death), and the interactions between the changing phenotype and the environment are described. 3. These interactions can be used to build up a model of the opportunities and compromises which result in the fitness of individual mice. It is not yet possible to quantify such a model, but this should in principle be achievable.

Seasonal reproduction of a tropical bat, Anoura geoffroyi, in relation to photoperiod

Anoura geoffroyi (Chiroptera, Phyllostomidae, Glossophaginae), Geoffroy's hairy-legged long-tongued bat, were collected from September 1984 to August 1985, and these bats were found to breed seasonally in the wild on Trinidad, West Indies, at 10 degrees N latitude. Histological examination of these samples indicated that females became pregnant in July or August, and young were born in late November or early December. The testes and epididymides were small from September to mid-April, increased threefold in weight between mid-April and late May, reached a peak weight in July, and decreased in weight in August. Spermatogenesis occurred throughout the testes of males captured from May to August. In 1990, the timing of parturition in females that gave birth in the laboratory to young conceived in the wild was similar to the timing in the field in 1984-1985. Groups of 10-13 males were subjected in the laboratory to (i) a gradually changing, civil twilight photoperiod that mimicked the natural cycle of annual change at 10 degrees N latitude, (ii) the same gradually changing cycle of photoperiod accelerated to a six-month period, or (iii) a constant photoperiod (light 12:54 h: dark 11:06 h). These treatments began in mid-December, four months before the initiation of testicular recrudescence in the wild. In all three groups, testicular volume remained low until April, and then increased two- to threefold between late April and late June, rising to a peak in July, as occurred in the wild.(ABSTRACT TRUNCATED AT 250 WORDS)

A pseudoseasonal reproductive strategy in a tropical rodent, Peromyscus nudipes

A population of cloud forest mice (Peromyscus nudipes) at latitude 10 degrees N near Monteverde, Costa Rica, was sampled four times by live-trapping twice during the 7-8 month wet season and twice during the 4-5 month dry season in 1989 and 1990. Body weights were lower during the early part of the dry season in males and throughout the dry season in females than at other times. Testes and seminal vesicles were somewhat lighter early in the dry season, but epididymal spermatozoa were abundant in most males throughout the year. Adult females ovulated, mated and became pregnant in the wet and dry seasons, but young were produced only during the wet season. Most embryos failed to implant during the dry season, and the few that did complete implantation were reabsorbed before midpregnancy. Apparently, every year, the females in this population spend several months actively engaged in a behavioural and metabolically costly process that is doomed to be unsuccessful. This reproductive strategy is termed pseudoseasonal, because reproductive success is highly seasonal, but attempts to reproduce are nonseasonal. Implantation failures similar to those seen in the wild were induced in the laboratory using mild restriction of food or water. Field evidence points to food restriction as the more important cause of pregnancy losses in the wild. Exposure to the gradually changing daylengths typical of Costa Rica had no effect on the production of young by adults, and maintenance on light cycles of 8 h light: 16 h dark, 11 h light: 13 h dark, 13 h light: 11 h dark and 16 h light: 8 h dark had no effect on the reproductive development of young animals of either sex.

Lack of reproductive photoresponsiveness and correlative failure to respond to melatonin in a tropical rodent, the cane mouse

Cane mice (Zygodontomys brevicauda) are year-round breeders in Venezuela. As shown previously, these animals are not reproductively responsive to variation in photoperiod. In the present experiments, male cane mice were maintained on long or short day lengths (16L:8D or 8L:16D, respectively) and challenged with each of three experimental treatments known to "unmask" reproductive photoresponsiveness in laboratory rats: olfactory bulbectomy, prolonged food restriction, and exposure as neonates to a single injection of testosterone. Variation in photoperiod had no inhibitory effect on the responses of cane mice to any of these three treatments, as assessed by the weight of their testes and seminal vesicles. A fourth experiment demonstrated that cane mice are insensitive to 10 wk of continuous exposure to pharmacological levels of melatonin, again as assessed by reproductive organ weight. Likewise, a fifth experiment documented a lack of response to 10 wk of late-afternoon injections of massive amounts of melatonin. The cane mouse apparently is unique among the animals challenged so far in these ways in that it seems to have no vestige of reproductive photoresponsiveness.

Characteristics of a genetic polymorphism for reproductive photoresponsiveness in the white-footed mouse (Peromyscus leucopus)

Wild populations of Peromyscus are often composed of individuals that vary greatly in their reproductive response to photoperiod. A population of white-footed mice (P. leucopus) from Michigan (43 degrees N) was subjected to mass selection in the laboratory both for and against reproductive photoresponsiveness for four generations. The first generation of selection yielded one line of mice in which about 80% of the individuals were classified as reproductively photoresponsive (i.e., with undeveloped reproductive tracts when reared in short days, 8L: 16D) and another in which only about 20% were reproductively photoresponsive. Some and perhaps most of this difference was accounted for by changes in degree of responsiveness to photoperiod rather than by alterations in the proportion of discrete responsive vs. unresponsive phenotypes. Alteration of critical day length was not a factor. Three more generations of selection failed to change the proportions noted above significantly. Although the genetic control of reproductive photoresponsiveness is undoubtedly complex, a single variable locus may be responsible for much of the heritable variation present in this population. These results also suggest that natural populations contain genetically determined phenotypes that are intermediate between absolutely photoresponsive and absolutely unresponsive. The factors that might promote maintenance of heterogeneity of reproductive photoresponsiveness in a wild population of rodents are considered.

The energetic regulation of ovulation: a realistic role for body fat

This review weighs the evidence for and against the hypothesis that ovulation is regulated by a critical amount of body fat. The evidence supporting this hypothesis is correlative, and most of it stems from observations made in humans. On balance, the evidence from human studies does not support the hypothesis, however, and the results of animal studies argue strongly against it. In the latter regard, a variety of experimental approaches have been tried in both adult and peripubertal females of several species, and the results almost uniformly show little relationship between fatness and ovulation. There is no doubt that ovulation can be regulated somehow in relation to whole-body energy balance and that fat stores are an important component of energy balance, but there is no reason to accord body fat a direct causal role in regulating ovulation.

Suppression of puberty in rats by exercise: effects on hormone levels and reversal with GnRH infusion

In an effort to better understand the effects of prolonged exercise on the hypothalamic-pituitary-ovarian axis, we made a variety of comparisons between young female rats in two treatment groups: 1) prolonged exercise, in which growth and reproductive development were arrested at a peripubertal stage by requiring rats to run for long periods of time in order to obtain food; and 2) voluntary exercise, in which same-aged control rats were fed ad libitum and given free access to a running wheel. The pulsatile secretion of luteinizing hormone (LH) and growth hormone were completely suppressed by the prolonged exercise treatment. Mean levels of follicle-stimulating hormone, prolactin, and thyroid-stimulating hormone were not affected. Prolonged exercise elevated corticosterone titers, and the secretory pattern of this steroid was changed out of phase with running activity. Tissue levels of gonadotropin-releasing hormone (GnRH) in the hypothalamus and LH in the pituitary were enhanced, not suppressed, by prolonged exercise. Most importantly, pulsatile infusions of GnRH reinstated normal pubertal development and ovulation in rats still growth restricted by the prolonged exercise treatment. The results of this study indicate that the suppressive effects of prolonged exercise somehow affect the production of the hypothalamic GnRH-pulse generator signal.

Lability of fat stores in peripubertal wild house mice

Traditionally, the adaptive value of mammalian white fat stores is considered in relation to long-term needs such as providing protection against the vagaries of winter or signalling the reproductive system when energy reserves are sufficient to risk pregnancy. As shown here, the fat stores of young house mice could not serve such needs. Despite prolonged acclimation and excess nesting material, food deprivation at 10 degrees C significantly lowered the fat stores of peripubertal female house mice in only 12 h, and would exhaust them in 30 h. Even close to thermoneutrality (24 degrees C) the calculated time to exhaustion was only 70 h. The fat stores of a young house mouse are obviously too meager to offer any meaningful protection over a winter of several months duration, or even over a 5-6-week cycle of pregnancy and lactation. Furthermore, in a wild habitat where food availability and ambient temperature can vary rapidly and greatly, such fat stores would be too labile to effectively coordinate puberty with somatic development.

Photoperiod, melatonin secretion, and sexual maturation in a tropical rodent

Descendants of a sample of cane mice (Zygodontomys brevicauda) trapped at 8 degrees latitude in Venezuela were tested for reproductive photoresponsiveness. This species breeds continuously, year around, despite living in a seasonally harsh habitat. At 50 days of age there were no differences in the weights of the testes or seminal vesicles or in sperm counts of males born and reared on 16L:8D, 13L:11D, 11L:13D, or 8L:16D photoperiods, although there were small differences in body weight. Females born and reared on 16L:8D vs. 8L:16D cycles became pregnant at the same rates and ages when paired with males at 21 or 31 days of age. The daily duration of melatonin secretion depended on the length of the dark phase of the cycle in both sexes. Circulating levels of melatonin were elevated for 8 h on a 16L:8D cycle and for between 9 and 16 h on an 8L:16D cycle. In this tropical species, the neuroendocrine pathway that links photoperiod to reproduction apparently is disconnected somewhere between melatonin and gonadotropin secretion, causing cane mice to be reproductively unresponsive to variation in photoperiod.

Short-term hormonal responses to food intake in peripubertal female rats

These studies focused on the phenomenon of "catch-up" pubertal development. Circulating levels of several hormones were characterized in 8-wk-old female rats whose growth and reproductive development had been blocked before puberty by restricting their food intake. Some of these females were fed ad libitum for 24 h to initiate rapid pubertal development. Blood levels of luteinizing hormone (LH) and growth hormone (GH) were suppressed by food restriction and then partially restored to adult diestrus levels by 24 h of ad libitum feeding. Prolactin titers were also suppressed by food restriction but not significantly elevated by 24 h of ad libitum feeding. Circulating levels of follicle-stimulating hormone (FSH) and thyroid-stimulating hormone (TSH) were unaffected by either treatment. It is concluded that GH could play an active supplementary role to LH in eliciting catch-up pubertal development but that FSH and TSH could play only passive roles at best. The role of prolactin remains uncertain. On a finer time scale, when food-restricted females were examined in relation to the time of day at which they were fed, most showed high-amplitude LH pulses 2-4 h after eating but rarely at any other time. Thus under some conditions LH secretion can be modulated by food intake on an almost hour-by-hour basis. Overall, blood levels of corticosterone were generally but not always inversely correlated with the frequency of LH pulsing in these experiments. Finally, the present results argue against the concept that puberty is dependent on a critical whole body characteristic.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of prolonged exercise on puberty and luteinizing hormone secretion in female rats

Immature female rats were required to run for prolonged periods of time to obtain food. The amount of food they earned was adequate for full pubertal development and moderate growth under nonworking conditions, but both processes were blocked by the exercise requirement. Prolonged exercise also blocked the pulsatile release of luteinizing hormone (LH); only two LH pulses were seen in seven exercising females during a total of 24 h of monitoring at 8 wk of age. By comparison, almost 1 pulse/h was seen in postpubertal, normally growing females of this same age during metestrus. When the exercising females' running requirement was relaxed at 8 wk of age they experienced rapid catch-up growth and reproductive development. Both basal secretion and LH pulse frequency increased markedly within 48 h, and most of these females ovulated during the third dark period after relaxation. Altogether, the experimental paradigm and techniques employed here yield highly predictable results, and they should prove useful for exploring other neuroendocrine pathways through which excessive exercise antagonizes reproduction.

Epinephrine release in response to sexual activity in male versus female rats

Sequential blood samples were collected from adult male and female rats before, during and after they interacted with either a sexually-active or a gonadectomized adult of the opposite sex. As assessed by plasma levels of epinephrine, both sexes showed a much greater response to a sexual encounter than they did to a nonsexual encounter. The release of epinephrine in response to sexual activity differed in males and females in at least two ways, however. First, a sexual encounter caused an absolutely greater but a proportionally smaller response in females. Second, plasma levels of epinephrine in males but not in females reflected the alternating periods of copulation and noncopulation that characterize sexual activity in this species. Blood levels of epinephrine were low and stable in males between copulatory bouts and relatively high during these bouts. In contrast, circulating levels of epinephrine in females were almost as high in the interval between copulatory bouts as they were during copulation.

Patterns of sexual receptivity in the female musk shrew (Suncus murinus)

Female musk shrews (Suncus murinus) were tested daily to examine patterns of sexual receptivity. When only mounting was used as a criterion (to avoid pregnancy), nonpregnant females remained sexually receptive to males every day for 14 consecutive days. When insemination was allowed, most females continued to copulate for the first 5 days of pregnancy. Receptivity declined markedly around Day 10 of gestation, but a few females were receptive even into late pregnancy. Lactating females copulated with males 5 and 10 days after parturition. In general, unlike most mammals studied in the laboratory, the nonpregnant female musk shrew has no behavioral estrous cycle. Musk shrews are ready to mate anytime except in mild to late pregnancy, and even then occasionally mating is found.

A novel postcopulatory block of reproduction in white-footed mice

Female white-footed mice (Peromyscus leucopus) fail to produce offspring when paired with a male from weaning until 150 days of age if an adult female or her odor is also present. The present study delineates more clearly which stage of the young female's reproductive cycle is inhibited by the chemosignal of the older female. Age at vaginal opening and first estrus are delayed by the presence of the older female, but only for about 10 days. The presence of a male counteracts this delay. Thus, the basis for the block is not a delay in puberty. The older female's presence does not influence the number of estrous cycles experienced during the 30 days following first estrus, nor does it influence the number of corpora lutea found at autopsy. The presence of sperm in the vaginae of young females indicates that they were copulating. Likewise, examination of embryos 2 and 3 days after copulation reveals normal developmental progress. However, implantation does not occur in young females that have been exposed to an adult female. Thus, the block occurs either during the final stages of embryo transport or in relation to the implantation process itself.

Effect of food manipulation on the GnRH-LH-estradiol axis of young female rats

Female rats were prevented from growing and developing reproductively by restricting their food intake from the time they reached 80-85 g (27-29 days of age) until they were 2 mo old. A return to ad libitum feeding then typically yielded the pubertal ovulation during the third or fourth dark period. Ad libitum feeding for 48 h increased the frequency of luteinizing hormone (LH) pulsing in ovariectomized females. This treatment also depressed the level of circulating estradiol in ovariectomized females implanted with Silastic capsules. It had no effect on the rate at which estradiol was cleared from the blood in a 1-h test, however, nor did it affect the pool of assayable gonadotropin-releasing hormone (GnRH) in the hypothalamus, the pool of assayable LH in the pituitary, the response of the pituitary to GnRH, or the rate at which LH was cleared from the blood. In toto, the present results suggest that food restriction inhibits pubertal development by acting rather specifically on GnRH secretion via an ovarian steroid-independent pathway. The presumed supplemental role for enhanced negative-feedback sensitivity could not be evaluated because of the aberrant results with encapsulated estradiol.

Experiential and endocrine dependence of gonadotropin responses in male mice to conspecific urine

Previous research has shown that a urinary pheromone of female mice acts via the vomeronasal organ of the accessory olfactory system to elicit rapid release of luteinizing hormone (LH) in conspecific males. Several experiments were conducted to examine the importance of sexual experience for gonadotropin responses in male mice to female urine, male urine, saline, or mixtures of these stimuli. Both sexually naive and sexually experienced male mice had significantly higher plasma LH levels after presentations of female urine than after presentations of male urine. However, sexual experience appeared to increase the reliability of the short-latency gonadotropin response to female urine relative to a sexually neutral component of urine such as sodium chloride, and male urine appeared to suppress spontaneous LH secretion episodes in both naive and sexually experienced males. Subsequent experiments with sexually experienced subjects demonstrated that male mouse urine is a powerful suppressant of LH release in other males. Specifically, female mouse urine mixed with male urine failed to elicit LH responses in male subjects, whereas female urine mixed with saline was highly effective. Urine obtained from castrated male donors was as potent as urine from intact males in suppressing the gonadotropin response to female urine. The suppressive activity in male mouse urine thus does not appear to be critically dependent on gonadal hormones. The existence of a potent stimulatory pheromone in female urine and a potent suppressive pheromone in male urine makes male mice an excellent model system for studying the neural regulation of LH secretion.

Chemical properties of a female mouse pheromone that stimulates gonadotropin secretion in males

Female mouse urine contains a pheromone that acts via the vomeronasal organ of conspecific males to stimulate a rapid increase in circulating levels of luteinizing hormone. A bioassay based on this male response was used to test biochemical preparations of female urine. Retention of significant biological activity by the urine after dialysis indicated that the activity is associated with urinary protein. Complete loss of activity from the urine after adsorption chromatography on a neutral polystyrene column suggested that the protein functions as a pheromone carrier. Assay of gel permeation chromatography fractions, before and after degradation of the urinary proteins with proteolytic enzymes, demonstrated that the protein is not necessary for the male response in the bioassay. Its resistance to vigorous proteolytic enzyme treatment further indicates that the pheromone is not a peptide. High biological activity, indistinguishable from that of the unfractionated urine, was isolated in a protein-depleted, presumably low molecular weight fraction containing compounds that are retarded by adsorption on Sephadex. The chemical properties of this female mouse pheromone are markedly different from those of a recently purified female hamster pheromone that also acts via the vomeronasal organ.

Mammalian reproductive strategies: genes, photoperiod and latitude

This paper considers how and why natural selection might promote or block the photoperiodic regulation of a mammal's reproduction. The factors most important in making this decision would seem to be the following: life expectancy, length of the female's cycle, feeding strategy, the presence or absence of survival mechanisms like hibernation, and the nature of the seasonal challenges offered by the mammal's habitat. A speculative scheme is offered for the potential utility of this type of regulation dependent upon life expectancy and latitude of residence.

Variation in litter size encountered in utero influences the bioenergetic characteristics of adult female mice

This experiment establishes the importance of in utero litter size in shaping the bioenergetic characteristics of the adult female mouse. The potential number of fetuses carried by a pregnant female was reduced to half by blocking one oviduct prior to mating. The resulting offspring were reared in normal sized litters after birth. As adults these female offspring were heavier and they consumed less food than control females who had shared their mother's uterus with a normal complement of fetuses. As assessed with running wheels, females from reduced litters were less than half as active during adulthood as control females. Variation in prenatal litter size apparently has far reaching effects on the acquisition and use of energy by mice.

Reproductive response of a tropical mammal, the musk shrew (Suncus murinus), to photoperiod

Musk shrews (Suncus murinus) were maintained for 8 weeks in long (16 h light:8 h darkness) or short (8 h light:16 h darkness) daylengths. Males housed in short daylength had significantly lighter androgen-dependent sex accessory organs than did males kept in long daylengths. This same trend was noted in male sexual behaviour. However, the weights of the testes and epididymides and sperm numbers did not differ. Females housed in short daylengths had significantly lighter cervices and were less likely to demonstrate sex behaviour than animals kept in under long daylengths. Ovarian and uterine weights did not differ. These results suggest that the ability to respond to photoperiod can exist in tropical mammals, even if it is not used as a cue to time seasonal breeding.

Role of the ovary and adrenal gland in the sexual behavior of the musk shrew, Suncus murinus

The contributions of the ovary and the adrenal gland to sexual behavior were examined in the female musk shrew (Suncus murinus). Ovariectomy eliminated sexual behavior, and treatment with estradiol or testosterone restored it in almost all shrews. Progesterone was ineffective in this regard. Adrenalectomy, likewise, blocked sexual behavior in most females with intact ovaries. Taken together, these results suggest that sexual behavior in this primitive eutherian mammal is normally modulated by secretions of both the ovary and the adrenal gland.

Puberty in female rats: relative effect of exercise and food restriction

Reproductive development in relation to growth and fat deposition was compared in three groups of female rats: a group that was allowed to grow only slowly by requiring them to work hard on a running wheel for their food; a group in which the same slow rate of growth was imposed by restricting their food intake, but without an exercise requirement; and a normally growing, nonexercising, ad libitum-fed, control group. Animals forced to run for their food experienced vaginal opening at a significantly lower body weight than either of the other two groups. The same trend was apparent for the first ovulation, but not significant. Thus the present results suggest that, under some conditions, intense exercise may actually accelerate rather than decelerate reproductive development, at least relative to body weight. With the possible exception of body weight, none of the whole-body parameters measured in this experiment (body weight, growth rate, or amount of fat) were found to be critically related to the first ovulation when all three groups of females were considered as a unit. Thus the present results also argue against some of the current hypotheses, all developed using dietary manipulation, that the onset of fertility is somehow dependent on one of these factors.

Scent marking in the musk shrew (Suncus murinus)

The hormonal regulation of scent marking was examined in male and female musk shrews. In males castration depressed flank, perineal, and throat marking frequencies, and a physiological dose of testosterone restored perineal and throat marking to intact levels. Females exhibited high levels of marking whether tested in a non-pregnant state, late in gestation, or when post-partum. Moreover, ovariectomy had no influence on marking behavior. Adrenalectomy, in addition to ovariectomy, caused a significant decline in two of the three marking behaviors examined, flank and perineal marking. These results demonstrate that these sexually monomorphic scent marking behaviors are under gonadal control in male musk shrews, but not in females. Furthermore, two of the three different types of scent marking examined in female musk shrews appear to require adrenal hormones. Throat marking in the female musk shrew apparently is not regulated by steroidal hormones.

Susceptibility of the fat reserves of mice to natural challenges

Many small rodents living in the wild neither store food nor forage during the daytime. Thus they can feed only at night. Imposing this restriction upon young female laboratory mice maintained at 22 degrees C yields a dramatic daily cycle in their fat stores. Energy is rapidly stored as fat while feeding, and then rapidly utilized during the non-feeding period. Almost one-third of the extractable whole body fat is lost during a 14 hour non-feeding period. Less fat is stored while feeding at 11 degrees C. Thus missing a single feeding period at this cooler temperature results in a total depletion of fat stores. In an ultimate sense then, the daily challenge of surviving with such a paucity of fat reserves probably presents as great a problem to the small mammal as does the thermoregulatory cost of small body size itself. Strategies for solving this problem apparently vary immensely from population to population and from locale to locale.

Effects of food restriction and restoration on gonadotropin and growth hormone secretion in immature male rats

This experiment concerned the changing patterns in secretion of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and growth hormone (GH) under conditions of food restriction and subsequent catch-up growth. Weanling male rats were given either restricted (4 g food/day) or unrestricted access to food until 60 days of age. At this age, food-restricted rats weighed only 25% as much as rats fed ad libitum. Food restriction resulted in a dramatic decrease in the frequency of LH and GH pulses, and in the amplitude of GH pulses. It also slightly but significantly decreased mean blood levels of FSH (which was not secreted in a pulsatile manner in 60-day-old controls fed ad libitum). When restricted rats were given unrestricted access to food, frequency of LH and GH pulses and mean levels of FSH increased significantly and simultaneously within 2 days in half of the animals. Only an additional 8-10% of their body weight decrement was recovered at this time. After 10 days of food restoration, when restricted rats still weighed 50% less than controls, their secretory patterns of all three hormones were not significantly different from those of controls. Thus, recovery of gonadotropin and GH secretion was relatively rapid. Except for the quantitatively lesser impact of food restriction on FSH secretion, there was no evidence of any priorities in the secretion of the three hormones. Under conditions of rapid catch-up growth, the secretory patterns of LH, FSH, and GH appeared to develop simultaneously.

Genetic selection for reproductive photoresponsiveness in deer mice

Seasonal breeding is common in mammals, particularly in habitats outside the tropics. Climate and availability of food are the ultimate factors that usually dictate the optimal time of year for a mammal to breed; however, day length (photoperiod) often serves as the proximal cue to signal the onset or cessation of seasonal reproduction. Some individuals in some populations of deer mice are reproductively responsive to photoperiod, while other individuals in the same population are not. As shown here, selection can dramatically alter the frequency of photoresponsiveness in a laboratory population in only two generations. To our knowledge this is the first demonstration of selection for reproductive photoresponsiveness in any mammal. By implication, some wild populations of deer mice must use multiple, genetic-based reproductive strategies, and the degree to which each such strategy is exhibited must be subject to rapid change in response to both seasonally and momentarily changing climatic and dietary conditions.

Food-restricted, prepubertal, female rats: rapid recovery of luteinizing hormone pulsing with excess food, and full recovery of pubertal development with gonadotropin-releasing hormone

Prepubertal female rats were maintained continuously at 45% of their expected 50-day body weight by restricting their food intake. Uteri and ovaries declined in weight under these conditions. No evidence of pulsatile LH release was seen when these animals were examined at 50 days of age. Allowing unlimited access to food at this time caused rapid pubertal development. LH pulsing began in some females within 12 h; strong LH pulsing was seen in most females within 24 h, and all ovulated after only 2 1/2 or 3 1/2 days of ad libitum feeding. These were fertile ovulations, accompanied by mating and resulting in pregnancy. Administering GnRH in a pulsatile manner to 50-day-old, food-restricted animals also yielded full pubertal development. Uteri and ovaries gradually increased in weight, and ovulation occurred in 3 1/2 to 5 1/2 days. These findings support a contention that the major reproductive deficit resulting from food restriction relates to the control of GnRH secretion. In toto they also suggest a close metabolic coupling between some dimension of nutrient and/or energy processing and the GnRH pulse generator in the normally growing female.