Parental investment in southern elephant seals, Mirounga leonina

Is drift 'directional'? Unequal breeding sex ratio revisited

Unequal breeding sex ratio can significantly reduce effective population size, allowing a rare neutral allele to jump to a high frequency through genetic drift. However, this one-way alteration to allele frequency appears inconsistent with the concept that drift is non-directional. Based on binomial sampling distribution, this study developed a method to directly and exhaustively measure drift by calculating the mean deviation of change in allele frequency, then applied it to cases of unequal breeding sex ratio. The result shows that, under those cases, (1) the mean deviation can always be divided into two halves that are equal in size but opposite in direction; (2) each half consists of one or several categories represented by various allele proportions in the rare sex; (3) this proportion is another factor that determines the outcome of drift, in addition to effective population size and allele frequency; (4) drift is non-directional on a global scale, but whether an allele will drift up or down can be predicted based on the above factors. This method enables us to dissect every component of the expected change in allele frequency caused by drift and to find out the combined effect of population size, allele frequency and allele proportion in the rarer sex under neutrality but unequal breeding sex ratio.

Evolutionary Pathways to Communal and Cooperative Breeding in Carnivores

In animal societies, individuals can cooperate in a variety of tasks, including rearing young. Such cooperation is observed in complex social systems, including communal and cooperative breeding. In mammals, both these social systems are characterized by delayed dispersal and alloparenting, whereas only cooperative breeding involves reproductive suppression. While the evolution of communal breeding has been linked to direct fitness benefits of alloparenting, the direct fitness cost of reproductive suppression has led to the hypothesis that the evolution of cooperative breeding is driven by indirect fitness benefits accrued through raising the offspring of related individuals. To decipher between the evolutionary scenarios leading to communal and cooperative breeding in carnivores, we investigated the coevolution among delayed dispersal, reproductive suppression, and alloparenting. We reconstructed ancestral states and transition rates between these traits. We found that cooperative breeding and communal breeding evolved along separate pathways, with delayed dispersal as the first step for both. The three traits coevolved, enhancing and stabilizing one another, which resulted in cooperative social systems as opposed to intermediate configurations being stable. These findings promote the key role of coevolution among traits to stabilize cooperative social systems and highlight the specificities of evolutionary patterns of sociality in carnivores.

Male and female pups of the highly sexually dimorphic northern elephant seal (Mirounga angustirostris) differ slightly in body size

In mammals, males generally are larger than females, though such sexual-size differences have been documented primarily in adults and are relatively poorly known in early life. We studied sexual-size differences in pups of the northern elephant seal (Mirounga angustirostris (Gill, 1866)), which in adulthood is one of the most sexually dimorphic mammals. We studied body size at birth and weaning, at Islas San Benito, Mexico, at the southernmost limit of the species’ breeding range. Males were 10% heavier and 2% longer than females at birth. Sexes did not differ significantly in either measure of body size at weaning, although males were slightly heavier (4%) and longer (1%) than females. Neither growth rate nor suckling duration differed between the sexes. In previous studies in California, USA, pups at weaning were heavier than in our study, and males were heavier than females. These differences may reflect ecological, temporal, or life-history differences across populations. The modest difference in sexual-size dimorphism early in life in this species compared with the great difference in adulthood likely reflects multiple selective forces, including constraints on neonatal size set by body size of females, and the weakness of sexual selection at that stage of life.

Reproductive skipping as an optimal life history strategy in the southern elephant seal, Mirounga leonina

Intermittent breeding by which organisms skip some current reproductive opportunities in order to enhance future reproductive success is a common life history trade-off among long-lived, iteroparous species. The southern elephant seal Mirounga leonina engages in intermediate breeding when body condition is low. While it is anticipated that this strategy may increase the lifetime reproductive output of this species, the conditions under which reproductive skipping are predicted to occur are not clear. Here I develop a dynamic state variable model based on published data that examines when southern elephant seals are predicted to optimally skip reproduction in order to maximize lifetime reproductive output as a function of current body mass, maternal age, and survivorship. I demonstrate that the optimal reproductive strategy for this species can include reproductive skipping, and that the conditions where this is optimal depend on patterns of mass-dependent adult female survival. I further show that intermittent breeding can increase lifetime reproductive output, and that the magnitude of this benefit increases with the ability of individual animals to replenish depleted body mass through foraging. Finally, I show that when the environment is variable and foraging is reduced in bad years, the benefit of adopting an optimal strategy that includes reproductive skipping increases asymptotically with the frequency of bad years. These results highlight the importance of characterizing the pattern of adult survival in this species, as well as the need to identify other factors that may influence the prevalence and benefits of reproductive skipping.

Acoustic measurements of post-dive cardiac responses in southern elephant seals (Mirounga leonina) during surfacing at sea

Measuring physiological data in free-ranging marine mammals remains challenging, owing to their far-ranging foraging habitat. Yet, it is important to understand how these divers recover from effort expended underwater, as marine mammals can perform deep and recurrent dives. Among them, southern elephant seals (Mirounga leonina) are one of the most extreme divers, diving continuously at great depth and for long duration while travelling over large distances within the Southern Ocean. To determine how they manage post-dive recovery, we deployed hydrophones on four post-breeding female southern elephant seals. Cardiac data were extracted from sound recordings when the animal was at the surface, breathing. Mean heart rate at the surface was 102.4±4.9 beats min^-1 and seals spent on average 121±20 s breathing. During these surface intervals, the instantaneous heart rate increased with time. Elephant seals are assumed to drastically slow their heart rate (bradycardia) while they are deep underwater, and increase it (tachycardia) during the ascent towards the surface. Our finding suggests that tachycardia continues while the animal stays breathing at the surface. Also, the measured mean heart rate at the surface was unrelated to the duration and swimming effort of the dive prior to the surface interval. Recovery (at the surface) after physical effort (underwater) appears to be related to the overall number of heart beats performed at the surface, and therefore total surface duration. Southern elephant seals recover from dives by adjusting the time spent at the surface rather than their heart rate.

Sizing ocean giants: patterns of intraspecific size variation in marine megafauna

What are the greatest sizes that the largest marine megafauna obtain? This is a simple question with a difficult and complex answer. Many of the largest-sized species occur in the world’s oceans. For many of these, rarity, remoteness, and quite simply the logistics of measuring these giants has made obtaining accurate size measurements difficult. Inaccurate reports of maximum sizes run rampant through the scientific literature and popular media. Moreover, how intraspecific variation in the body sizes of these animals relates to sex, population structure, the environment, and interactions with humans remains underappreciated. Here, we review and analyze body size for 25 ocean giants ranging across the animal kingdom. For each taxon we document body size for the largest known marine species of several clades. We also analyze intraspecific variation and identify the largest known individuals for each species. Where data allows, we analyze spatial and temporal intraspecific size variation. We also provide allometric scaling equations between different size measurements as resources to other researchers. In some cases, the lack of data prevents us from fully examining these topics and instead we specifically highlight these deficiencies and the barriers that exist for data collection. Overall, we found considerable variability in intraspecific size distributions from strongly left- to strongly right-skewed. We provide several allometric equations that allow for estimation of total lengths and weights from more easily obtained measurements. In several cases, we also quantify considerable geographic variation and decreases in size likely attributed to humans.

Variation in body condition during the post-moult foraging trip of southern elephant seals and its consequences on diving behaviour

Mature female southern elephant seals come ashore only in October to breed and in January to moult, spending the rest of the year foraging at sea. Mature females may lose as much as 50% of their body mass, mostly in lipid stores, during the breeding season due to fasting and lactation. When departing to sea, post-breeding females are negatively buoyant and the relative change in body condition (i.e. density) during the foraging trip has previously been assessed by monitoring descent rate during drift dives. However, relatively few drift dives are performed resulting in low resolution of the temporal reconstruction of body condition change. In this study, six post-breeding females were instrumented with time-depth recorders and accelerometers to investigate whether changes in active swimming effort and speed could be used as an alternative method of monitoring density variations throughout the foraging trip. In addition, we assessed consequences of density change on the swimming effort of individuals while diving and effects on dive duration. Both descent swimming speed and ascent swimming effort were found to be strongly correlated to descent rate during drift dives, enabling the fine-scale monitoring of seal density change over the whole trip. Negatively buoyant seals minimized swimming effort during descents, gliding down at slower speeds, and reduced their ascent swimming effort to maintain a nearly constant swimming speed as their buoyancy increased. One percent of seal density variation over time was found to induce a 20% variation in swimming effort during dives with direct consequences on dive duration.

The immunocompetence handicap hypothesis in two sexually dimorphic pinniped species--is there a sex difference in immunity during early development?

The 'immunocompetence handicap hypothesis' predicts that highly sexually dimorphic and polygynous species will exhibit sex differences in immunity. We tested this hypothesis in southern elephant and grey seals during their early development by measuring the following parameters: leucocyte counts, serum IgG levels, erythrocyte sedimentation rate and haematocrit. We failed to find any differences due to sex as assessed by the parameters investigated. Animals were sampled longitudinally during their development and there were significant age effects from birth to weaning in both species. Total and differential leucocyte counts and erythrocyte sedimentation rates increased just prior to weaning then decreased. Haematocrits declined whilst total circulating immunoglobulin G concentrations increased. Body temperatures remained constant throughout the postnatal period. Differences between the species were seen in total leucocyte counts and in polymorphonuclear cells and eosinophils. Southern elephant seals had higher concentrations than grey seals and total leucocyte counts in the former were among the highest reported for mammals.

Human disturbance, nursing behaviour, and lactational pup growth in a declining southern elephant seal (Mirounga leonina) population

We studied lactation behaviour in relation to pup growth in southern elephant seals (Mirounga leonina) at Macquarie Island, and compared harems in areas of high and low human presence to determine if there is an effect attributable to human activities, including scientific research. Pup weaning mass, a known correlate of first–year survival, was positively influenced by suckle bout durations during early and middle lactation and by maternal aggression during late lactation; no other behavioural variables were associated with weaning mass. In the area of high human presence, we observed from a distance the behaviour of mother–pup pairs directly before, during, and after visits to harems by other researchers. Alertness was raised threefold in the presence of people but quickly returned to predisturbance levels after their departure; there were no significant short-term effects on other behavioural variables. In the areas of high and low human presence, we observed the undisturbed behaviour of the seals in the absence of other people. No significant differences in any behavioural variables examined were found, indicating no long-term changes in behaviour resulting from human presence. Human disturbance therefore appears not to have significantly contributed to the population decline observed at Macquarie Island, but the conclusion requires caution given the fairly low power of our analyses.

Blood chemistry in southern elephant seal mothers and pups during lactation reveals no effect of handling

Serum clinical chemistry parameters were examined in lactating southern elephant seal Mirounga leonina mothers and their pups from the declining Macquarie Island population. There were significant changes in serum values from 2 to 21 days postpartum in both nursing mothers (increase: inorganic phosphate; decrease: creatinine, potassium, chloride, cholesterol, total protein, albumin, globulin, aspartate aminotransferase, creatine kinase) and suckling pups (increase: inorganic phosphate, globulin, cholesterol; decrease: albumin, alkaline phosphatase, gammaglutamyl transferase; increase followed by decrease: triglycerides, iron). We found no evidence that changes were due to chronic stress effects caused by repeated chemical immobilisations (mothers) or physical restraint (pups): at late lactation, clinical chemistry values were similar for mother-pup pairs of a control group (not handled previously), moderate treatment group (previously handled twice) and high treatment group (previously handled three to four times). We were not able to detect differences in clinical chemistry values between mother-pup pairs distributed over two areas differing in the frequency of human visits. The clinical chemistry values presented here can serve as reference ranges to allow future comparison with other southern elephant seal populations to investigate factors, e.g. food limitation, suspected to be involved in population declines.

Prenatal investment in the subantarctic fur seal, Arctocephalus tropicalis

We investigated prenatal investment in a large sexually dimorphic mammal, the subantarctic fur seal, Arctocephalus tropicalis, on Amsterdam Island in the Indian Ocean. Pups' sex ratio and body mass, body length, and body condition at birth were studied in relation to timing of birth and maternal characteristics (body length and body condition) during three consecutive breeding seasons. Pups' sex ratio did not differ from unity throughout the pupping period. The sex of the pup was related to neither maternal body length (i.e., maternal age) nor maternal body condition when mating occurred or at parturition (1 year later), which suggests that the sex ratio was not biased toward one sex during gestation. Newborn male pups were heavier and longer than female pups in all years. Longer mothers tended to arrive later in the season regardless of their body condition, and gave birth to heavier pups whatever the sex of the pup. Mothers in good condition gave birth to heavier male pups than mothers in poor condition, but no significant differences were found for female pups, suggesting that the costs of carrying male foetuses is higher than that of carrying female foetuses. Differences in allocation of maternal resources between male and female pups may be due to sex-related differences in body composition, since male pups were heavier than female pups for a given body length at birth. Thus, male and female foetuses may use maternal resources differently, with males growing in length whereas females appear to grow in body mass. The mothers we monitored over 2 consecutive years gave birth to pups that were similar in quality (in terms of birth mass) over years regardless of the sex of the previous pup and the mother's body length, suggesting that individual reproductive value is independent of maternal age. Furthermore, maternal body condition was not affected by the sex of the foetus, suggesting that there is no differential reproductive cost in carrying a male or a female foetus. Interannual differences in pup body size at birth suggest that environmental conditions such as prey availability during the last stages of gestation, and consequent maternal body condition, are important components of maternal investment in fur seals.

Individual mares bias investment in sons and daughters in relation to their condition

The Trivers-Willard hypothesis (TWH) predicts that a mother will treat a son or daughter differently depending on her ability to invest and the impact of her investment on offspring reproductive success. Although many studies have investigated the hypothesis, few have definitively supported or refuted it because of confounding factors or an inappropriate level of analysis. We studied maternal investment in sons and daughters in feral horses, Equus caballus, which meet the assumptions of the TWH with a minimum of confounding variables. Population level analyses revealed no differences in maternal behaviour towards sons and daughters. When we incorporated mare condition, we found that sons were more costly to mares in good condition, whereas daughters were more costly to mares in poor condition, although no differences in maternal behaviour were found. However, since the TWH makes predictions about individual mothers, we examined investment by mares who reared both a son and a daughter in different years of the study. Mares in good condition invested more in their sons in terms of maternal care patterns, costs to maternal body condition and costs to future reproduction. Conversely, mares in poor condition invested more in daughters. Therefore, with an appropriate level of analysis in a species in which confounding variables are minimal, the predictions of the Trivers-Willard hypothesis are supported. Copyright 2000 The Association for the Study of Animal Behaviour.

Environmental and physiological determinants of successful foraging by naive southern elephant seal pups during their first trip to sea

The ability to forage successfully during their first trip to sea is fundamental to the ultimate survival of newly weaned southern elephant seals (Mirounga leonina). However, there is considerable variation in the body mass and fat content of seal pups at weaning, which results in some individuals having larger energy and oxygen stores than others, which may confer advantages on them. The diving behaviour of 21 newly weaned seals was studied using satellite relayed data loggers. Seals were captured at Macquarie Island in December 1995 and 1996, approximately 4 weeks after weaning. Two groups of seals were specifically targeted: a heavy group from the top quartile of weaning masses (n = 6) and a light group from the lower quartile (n = 15). Most of the seals made dives in excess of 100 m depth and 5 min before final departure from the island. However, for the first 60-80 d, all of the seals exhibited behaviour quite distinct from the patterns reported for older conspecifics, and made relatively shallow (100 ± 39 m; mean ± SD) and short (5.7 ± 1.23 min) dives. During this time the seals spent 74.3 ± 12.6% of each day diving, and the depth of the dives did not follow any diurnal pattern. The diving behaviour of all seals changed abruptly when they started on their return to land. During this time their behaviour was more like that of adults: they made deeper (159 ± 9 m) and longer dives (9.01 ± 1.69 min) than previously, and the dives showed a strong diurnal pattern in depth. There is no obvious explanation for this change in behaviour, although its abrupt nature suggests that it is unlikely to have been due to physiological changes in the seals. The size of the seals at weaning was an important influence on diving behaviour. Heavy weaners made significantly deeper (130 ± 40 m) and longer dives (7.36 ± 0.55 min) than light weaners (88 ± 32 m and 5.04 ± 0.64 min, respectively). This indicates that smaller seals are constrained to some extent by their physiological capabilities, which perhaps requires some individuals to adopt different foraging strategies.

Variation in milk production and lactation performance in grey seals and consequences for pup growth and weaning characteristics

Phocid seals are one of the few groups of mammals capable of sustaining the energetic demands of lactation entirely through body nutrient stores while fasting. Lactation performance of the female in turn influences the rate and pattern of pup growth. We examined variation in and patterns of milk composition and production, maternal energy output, and pup growth and energy deposition over the entire lactation period in 18 grey seal mother-pup pairs using hydrogen isotope (3H2O and D2O) dilution. Milk composition was independent of maternal mass and nutrient stores, indicating dependence on other physiological and genetic factors. Heavier females lactated longer (r2=0.653, P<0.001), had higher total milk outputs (r2=0.652, P<0.001), and produced larger pups at weaning (r2=0.417, P=0.005). While fatter females lactated for longer periods of time (r2=0.595, P<0.001), females with a larger lean body mass at parturition produced more milk (r2=0.579, P<0.001). Total milk energy output was the strongest predictor of pup weaning mass, which, along with the pup's efficiency of energy storage, accounted for 91% of the variation in weaning mass. Nevertheless, there was sufficient plasticity in milk composition and energy output that some smaller females produced relatively large pups. Few females appeared to deplete body nutrients to the point where it might limit the duration of lactation.

Northern fur seal young: interrelationships among birth size, growth, and survival

We investigated size at birth, growth, and early survival of northern fur seals (Callorhinus ursinus) from birth to weaning at Bering Island, Russia, over 8 breeding seasons from 1982 to 1989. One thousand and thirteen fur seals (565 males and 448 females) were measured in a longitudinal study and an additional 2697 animals were measured at birth. At birth, female pups were about 10% lighter and about 3% shorter than male pups. The coefficients of variation of mass (12.5 and 12.8%) and length (4.5 and 4.7%) were similar for the two sexes. We partitioned the lactation period into four time periods: 1, the perinatal period (ages 0-10 days); 2, the early development period (ages 11-40 days); 3, the period of intensive molting (ages 41-80 days); and 4, the preweaning period (ages 81-140 days). We investigated four measures of growth: absolute growth in mass (AGM) in grams per day, absolute growth in length (AGL) in millimetres per day, relative growth in mass (RGM) as a percentage per day, and relative growth in length (RGL) as a percentage per day. For both sexes, AGM was highest during period 4 (mean = 124.8 g/day, SE = 7.4 g/day, and mean = 109.6 g/day, SE = 6.8 g/day for males and females, respectively) and AGL was highest during period 2 (mean = 3.74 mm/day, SE = 0.18 mm/day, and mean = 3.42 mm/day, SE = 0.21 mm/day for males and females, respectively). RGM (mean = 1.06%, SE = 0.09%, and mean = 1.02%, SE = 0.11% for males and females, respectively) and RGL (mean = 0.53%, SE = 0.03%, and mean = 0.50%, SE = 0.03% for males and females, respectively) were highest during period 2 for both sexes. For both sexes, growth rates were slowest during the molting period. Sexual differences were detected in AGM in period 4 and for the combined data over periods 1-3. Sexual differences in AGL were detected for the combined data over periods 1, 3, and 4 only. No sexual differences in relative growth were found. Subsequent growth in mass and length was correlated with birth size. We found the greatest annual variation during the periods when growth was fastest. Condition indices were calculated using the allometric relationship between length and mass separately for neonates and pups older than 5 days. The condition indices at birth varied significantly annually. The condition index was lowest during the molting period (3). Animals that survived for at least 40 days were larger at birth and had a higher condition index than those that did not survive. In years of moderate or high pup survival rates, survival rates were higher in animals born later in the breeding season.

Interannual variation in birth mass and postnatal growth rate of Juan Fernández fur seals

The objectives of this study were to establish body mass at birth, postnatal growth rate, and the factors that influence these parameters for the Juan Fernández fur seal, Arctocephalus philippii. Females of this species have an unusual attendance pattern in which foraging trips and shore visits last, on average, 12.3 and 5.3 days, respectively. Pup mass was obtained from cohorts born during the reproductive seasons in 1988 through 1992. Birth masses of male and female pups were significantly different, averaging 6.1 and 5.5 kg, respectively (F = 13.2, P < 0.0003, n = 238). Birth masses also differed among cohorts, being lowest in 1992 and highest in 1990. During the first 2 months of life, male and female pups grew at the same rate (79 ± 61.5 g · day-1 (mean ± SD); F[1] = 0.03, P = 0.8562). Interannual differences in growth rate during the first month were significant (F[4] = 8.14, P < 0.0001), as was the interaction between month and year effects (F[2] = 6.81, P = 0.0012). Growth rates for the 1990 cohort were lower than those in all other years except 1992. Birth masses and postnatal growth rates of Juan Fernández fur seal pups are comparable to those of other otariid species.

Reproductive effort of male hooded seals (Cystophora cristata): estimates from mass loss

This study investigated mass loss, body composition, and behaviour patterns of male hooded seals during the reproductive season. During the 6 years of study (between 1989 and 1995), 139 records of male mass were obtained that involved 115 individuals. Body masses of males ranged from 147 to 434 kg. Mean mass at first capture was 312.5 ± 53.0 kg (N = 119). Year, date of sampling, and age all significantly influenced mass. Nineteen males were recaptured at least twice during a single season. Mean rate of mass loss among these individuals was 2.5 ± 1.1 kg/day (range 0.7–4.6 kg/day). Body composition early in the breeding season, measured using tritiated water (N = 6), produced mean estimates of 51.6 ± 1.6% water, 29.3 ± 2.4% fat, 16.9 ± 0.7% protein, and 1.9 ± 0.2% ash. Time–depth recorders attached to three males indicated that they spent 84.7 ± 15.4% of their time hauled out on the surface of the ice during the breeding season. Each of these males was on the ice for a few days; they then spent a few hours at sea before returning to the ice surface. Mean dive depth was only 14.1 ± 3.2 m (maximum 66 m) and mean dive duration was only 1.7 ± 0.3 min (maximum 28 min). Extrapolating mean daily rates of body mass loss to encompass a 2.5-week breeding season, males would lose an average of 44 kg, which represents 14% of their mean body mass. Compared with values for males of other phocid species this value is conservative. It appears that the short breeding season among hooded seals is energetically advantageous for both sexes.

Seasonal variation in vitamin D metabolites in southern elephant seal (Mirounga leonina) females at south Georgia

Southern elephant seals spend two periods on land each year, during breeding and moult, exposed to intensive UV radiation. The time between periods on land are spent at sea, with little exposure to the sun. A study of serum 25-OH-D3 and 1,25(OH)2-D3 on southern elephant seals was carried out at South Georgia. Samples were collected on four different occasions: early and late breeding, and early and late moult. The levels of 25-OH-D3 increased when seals were on land, and decreased when at sea. Two annual peaks of 25-OH-D3 were found, both of which immediately followed periods of intensive exposure of UV radiation. 1,25(OH)2-D3 levels showed a seasonal variation, but no significant changes while being on land were detected. The diving behaviour at sea for southern elephant seals and no detectable change in 25-OH-D3 indicates that the seals feed on prey containing vitamin D.

Milk consumption and growth efficiency in Antarctic fur seal (Arctocephalus gazella) pups

The body composition, milk consumption, and growth efficiency of Antarctic fur seal (Arctocephalus gazella) pups were measured over three consecutive lactation periods (1990–1992) at Bird Island, South Georgia. The body composition of pups differed between the sexes; whereas for any given age females were lighter than males, they had proportionately higher body lipid reserves than males. Milk consumption was determined on 388 occasions in 177 pups. Mean estimates of milk consumption ranged from 2.5 to 3.2 kg (42–53 MJ) during the 6-day perinatal period and from 2.9 to 3.6 kg (49–68 MJ) during the 1- to 2-day maternal attendance periods (feeding bouts). There were no differences in milk consumption between the sexes in any year. Both per-bout and per-day milk consumption increased steadily with age before decreasing significantly in the last 30–40 days of lactation. Per-bout milk consumption was positively related to the duration of the maternal foraging trip and attendance cycle, and both per-bout and per-day milk consumption were related to pup mass. Pups of both sexes consume the same amount of milk, but males direct more of their milk consumption into lean tissue growth than females, which accumulate greater adipose stores. Therefore, mass and mass changes may not be appropriate parameters for investigating differential maternal investment between the sexes in otariid pups.

Prenatal and postnatal transfer of fatty acids from mother to pup in the hooded seal

Unlike most mammals, hooded seal (Cystophora cristata) pups are born with a substantial layer of adipose tissue. Subsequently, during the brief lactation period of only 4 days, fasting mothers mobilize enormous amounts of lipid from blubber and secrete milk (60% fat) at rates of 10 kg.day-1. Pups gain 7 kg.day-1 due primarily to the deposition of fat in blubber. We measured blubber content and fatty acid composition of blubber and milk in hooded seal mother-pup pairs at birth and over the 4-day lactation period to examine the nature and source of fetal lipids, the incorporation of maternal blubber fatty acids into milk lipid, and patterns of fatty acid deposition in suckling young. The fatty acid composition of the blubber of the newborn was notably different from that of its mother. Fetal deposition was likely due to a combination of both fetal synthesis and direct placental transfer of maternal circulating fatty acids. The blubber of the newborn was characterized by high levels (> 90% of total fatty acids) of saturated and monounsaturated fatty acids of primarily endogenous origin. In particular, the fetus appeared to have high delta-9 desaturase activity as evidenced by the large amounts of 14:1n-5 (4.2%) and 16:1n-7 (37.0%) in newborn blubber compared to maternal blubber (0.2% and 14.1%, respectively). Nevertheless, essential and long-chain polyunsaturated fatty acids of the n-3 and n-6 families, which could only have originated by direct transfer from the mother, comprised > 7% of pup blubber fatty acids and indicated greater rates of placental transfer than found in humans. In hooded seal mothers, rapid lipid transfer during the brief lactation period appeared to be facilitated by direct incorporation of mobilized fatty acids into milk. Although some differences in proportions of specific fatty acids were found between milk and maternal blubber, most of these differences declined over the course of lactation. However, selective mobilization of 20:5n-3 from maternal blubber into milk was apparent throughout lactation and resulted in elevated levels in pup blubber at weaning compared to maternal blubber. Ingested fatty acids were deposited directly and without modification into the blubber of pups, and by 4 days the fatty acid composition of pup blubber was virtually identical to that of the milk consumed.

The adaptation of milk secretion to the constraints of fasting in bears, seals, and baleen whales

Although lactation is accompanied by increased nutrient demands for milk synthesis, many species of bears, true seals, and baleen whales fast for much or all of lactation. Large body mass in these species confers the advantage of greater stores of fat and protein relative to rates of milk production. Given the constraints on substrate availability during fasting, the milks of fasting mammals are predicted to be low in carbohydrate, protein, and water and to be high in fat. The milks of bears, true seals, and baleen whales conform to this prediction. Mammals that lactate while fasting may lose up to 40% of initial BW. The production of milk entails the export of up to one-third of body fat and 15% of body protein in the dormant black bear and in several seal species, which greatly depletes maternal resources and may represent a physiological threshold, because higher protein and fat outputs have only been measured in species that start feeding. The low K:Na ratio of seal and whale milks and the low Ca:casein and inverse Ca:P ratios in seal milks are unusual and warrant further study.