Endocrine correlates of hibernation-independent gonadal recrudescence and the limited late-winter breeding season in woodchucks, Marmota monax

The reproductive endocrinology and behavior of Vancouver Island marmot (Marmota vancouverensis)

The Vancouver Island marmot (Marmota vancouverensis; VIM) is one of North America's most endangered species with fewer than 150 individuals remaining in the wild. A captive breeding program was established across four facilities in Canada as an insurance population and source of animals for reintroduction to the wild. The purpose of this study was to gather information about the basic reproductive biology and behavior of this species, which is essential to improve captive breeding programs. Regular fecal samples were obtained from adult female (n = 14) and male (n = 10) marmots, 2 years of age and older, over 1-3 breeding seasons (2-3 months duration posthibernation) for steroid hormone analysis. Enzyme immunoassays were validated for quantifying fecal testosterone metabolite concentrations for males, and fecal estrogen and progesterone metabolite concentrations for females. Results indicated that fecal progesterone metabolite concentrations can be used to monitor ovulation and pregnancy. Behavioral monitoring through infrared video surveillance was conducted in four breeding pairs over a 2-year period (n = 7 behavioral profiles). Breeding behaviors correlated strongly with changes in reproductive endocrine profiles. A high frequency of play behavior or "wrestling" was observed in conjunction with breeding activity before an elevation in progesterone metabolite concentrations. Impending parturition was associated with increased aggression and exclusion of the male from the maternal nestbox as well as an increase in nesting activity. Observational data combined with hormonal analysis suggest that female VIMs are induced ovulators and that multiple breeding attempts may be required for ovulation and conception. Gestation appears to be approximately 34 days from peak breeding activity (32 days from estimated ovulation). Fecal testosterone concentrations suggest that testicular activity is seasonal with the reproductive activity occurring immediately posthibernation. Monitoring breeding behavior is a useful means of indicating estrus, conception and pregnancy, which can also be supported by the hormonal analysis of daily fecal samples of individual animals.

Maintenance of biological rhythms during hibernation in Eastern woodchucks (Marmota monax)

We undertook a study to determine presence of circadian rhythms during woodchuck hibernation using continuously monitored body temperatures. Males had shorter torpor and longer euthermic periods than females. Circular statistics revealed a significant mean vector for males entering into torpor (10:21 h), but not for females. No significant mean vector was found for male or female arousal from torpor. A contingency test was applied to the torpor bout durations. All 7 males tested had significant tau's between 24 and 26 h, while 6 of the 13 females tested had significant tau's with a range of 22-27 h. These results implicate a free-running circadian clock during torpor bouts. Overall, the data support the existence of biological rhythms during hibernation in woodchucks, especially for males during arousals. Since entries into torpor appear to be synchronized for males, arousal periods may be used to resynchronize their circadian system. The persistence of biological rhythms during hibernation may help to insure successful mating in the spring after emergence.

Refractoriness to short day lengths augments tonic and gonadotrophin-releasing hormone-stimulated lutenising hormone secretion

Siberian hamsters (Phodopus sungorus) undergo reproductive involution following exposure to short winter day lengths. Following approximately 20 weeks of exposure to short day (SD) lengths, hamsters become refractory to the inhibitory effects of SD, and reproductive competence is restored in anticipation of spring. The extent to which changes in gonadal steroid-dependent and -independent regulation of gonadotrophin secretion participate in this vernal reactivation of the gonads is not known. This experiment tested whether tonic and gonadotrophin-releasing hormone (GnRH)-stimulated regulation of lutenising hormone (LH) secretion differs between photoresponsive and photorefractory Siberian hamsters. Male hamsters born into long day (LD) lengths were castrated or subjected to a sham-castration surgery at 17 days of age, implanted s.c. with blank or testosterone-filled capsules, and housed in LD or SD thereafter. Baseline LH and LH responses to GnRH (200 ng/kg, s.c) were measured at 14 (photoresponsive) and 40 (photorefractory) weeks of age. Despite lower circulating testosterone concentrations in gonadally regressed SD hamsters on week 14, tonic LH concentrations were comparable among all groups of gonad-intact hamsters on weeks 14 and 40; however, week 14 SD hamsters exhibited significantly higher GnRH-stimulated LH responses. Tonic LH concentrations were indistinguishable among all groups of castrated hamsters bearing empty implants on week 14, but prolonged exposure to LD led to a decrease in resting LH, whereas prolonged exposure to SD resulted in an increase in LH. In castrated hamsters bearing testosterone implants, baseline LH concentrations were comparable in all groups, but GnRH treatment resulted in significantly higher LH concentrations in photorefractory (week 40, SD) hamsters relative to all other groups. The data suggest that the development of photorefractoriness in Siberian hamsters is characterised by enhanced gonadal hormone-independent stimulation of LH secretion, and diminished sensitivity to inhibitory negative-feedback effects of testosterone on LH secretion. Decreases in responsiveness of gonadotrophin secretion to gonadal hormone negative feedback may contribute to the process of photorefractoriness and assist in maintaining the growth of reproductive organs during the process of gonadal recrudescence.

Mammalian hibernation: cellular and molecular responses to depressed metabolism and low temperature

Mammalian hibernators undergo a remarkable phenotypic switch that involves profound changes in physiology, morphology, and behavior in response to periods of unfavorable environmental conditions. The ability to hibernate is found throughout the class Mammalia and appears to involve differential expression of genes common to all mammals, rather than the induction of novel gene products unique to the hibernating state. The hibernation season is characterized by extended bouts of torpor, during which minimal body temperature (Tb) can fall as low as -2.9 degrees C and metabolism can be reduced to 1% of euthermic rates. Many global biochemical and physiological processes exploit low temperatures to lower reaction rates but retain the ability to resume full activity upon rewarming. Other critical functions must continue at physiologically relevant levels during torpor and be precisely regulated even at Tb values near 0 degrees C. Research using new tools of molecular and cellular biology is beginning to reveal how hibernators survive repeated cycles of torpor and arousal during the hibernation season. Comprehensive approaches that exploit advances in genomic and proteomic technologies are needed to further define the differentially expressed genes that distinguish the summer euthermic from winter hibernating states. Detailed understanding of hibernation from the molecular to organismal levels should enable the translation of this information to the development of a variety of hypothermic and hypometabolic strategies to improve outcomes for human and animal health.

Seasonal changes in serum leptin, food intake, and body weight in photoentrained woodchucks

Male woodchucks (Marmota monax) were maintained in northern vs. southern hemisphere photoperiods, provided feed and water ad libitum, and evaluated every 2 wk for 23 mo for body weight, absolute and relative food intake, body temperature, serum testosterone, and serum concentrations of leptin measured using an anti-mouse leptin enzyme-linked immunoassay. During late spring and summer, body weight increased 56 +/- 4% above winter nadirs, and during the autumn and early winter weights decreased 27 to 43% below midsummer maxima. Serum leptin initially increased during increases in body weight, in the late spring, reached peak values (490 +/- 32 pg/ml) in summer during the initial decline in body weight, and later decreased along with body weight to reach basal values (20 +/- 5 pg/ml) in late winter. Spontaneous declines in food intakes in summer began 2-6 wk before resulting declines in body weight and occurred during increases in leptin >100 pg/ml. The rate of decline in food intakes was greatest when serum leptin was at or near peak values. Food intake increased in late winter when leptin was low and 7-10 wk before resulting increases in body weight. Testis recrudescence occurred when leptin was declining to near basal levels. The results suggest that leptin is involved in the hormonal regulation of the circannual cycle in the drive for voluntary food intake in this species.