An increase in estradiol facilitates the onset of paternal behavior in the dwarf hamster (Phodopus campbelli)

In the dwarf hamster (Phodopus campbelli), activational effects of testosterone (T) and estradiol (E₂) in the regulation of paternal behavior have been repeatedly rejected because peripheral concentrations of E₂ do not change across the reproductive cycle of males. Further, castration no affected paternal behavior despite that both T and E₂ concentrations decreased significantly. However, the role of these hormones has not been evaluated in models of castration and hormonal replacement in virgin males. Here, we analysed the effects of E₂ and T in paternal behavior in virgin male dwarf hamster (Phodopus campbelli). Thirty paternal (PAT) males were bilaterally castrated; of them, 10 were implanted with T, 10 with E₂ and 10 males received no treatment. Other 10 PAT males underwent sham-castration. Seventeen aggressive (AGG) males were also bilaterally castrated; of these, 10 AGG received E₂ replacement, 7 were not treated. Other 7 AGG males were submitted to sham-castration. Following treatments, paternal behavior tests were conducted again. T and E₂ levels in plasma were quantified by radioimmunoassay (RIA). The results showed that the treatments did not affect the paternal behavior of males that were initially paternal. Neither castration nor sham-castration surgery affected the behavior of AGG males. However, when these males were treated with E₂ and the concentrations of this hormone increase significantly they became paternal. Our data suggest that an increase in E₂ levels shifted infanticidal behavior to paternal behavior in dwarf hamster.

Post-castration retention of reproductive behavior and olfactory preferences in male Siberian hamsters: role of prior experience

Reproductive behavior of virtually all adult male rodents is dependent on concurrent availability of gonadal steroids. The ejaculatory reflex is incompatible with long-term absence of testicular steroids and typically disappears within 3 weeks after castration. Male Siberian hamsters are an exception to this rule; mating culminating in the ejaculatory reflex occurs as many as 6 months after castration (persistent copulation). The emergence of persistent copulation many weeks after gonadectomy is here shown not to require repeated post-castration sexual experience. Preoperative sexual experience, on the other hand, significantly increases the percent of males that copulate after gonadectomy, but is not required for the emergence of this trait in 25% of males. Castration prior to puberty prevents persistent copulation in all individuals in adulthood. Persistent copulators, unlike males that cease mating activity after castration, prefer the odors of estrous over non-estrous females when tested 4 months after castration and 7 weeks after the last mating test. Neural circuits of persistent copulators retain the ability to mediate male sex behavior and preferences for female odors in the complete absence of gonadal steroids; they are influenced by preoperative sexual experience and organizational effects of gonadal hormones at the time of puberty.

In uniparentalPhodopus sungorus, new mothers, and fathers present during the birth of their offspring, are the only hamsters that readily consume fresh placenta

Placentophagia is common among parturient female mammals but non-parturient females generally refuse placenta. Biparental male dwarf hamsters (Phodopus campbelli) readily consume placenta. The present study quantified placentophagia and liver acceptance in the closely related Siberian hamster P. sungorus in which males do not participate in the birth and are not responsive to a displaced pup. Sexually naïve P. sungorus males and females refused both placenta and liver (all groups <10%). Reproductive females specifically consumed placenta on the day before (G17), and the day of, parturition (G18) (>80%). Males rejected both tissues on G17 and accepted placenta soon after the birth (G18) (80%) only if they were present during the birth. Palatability of the placenta was not responsible for the species difference as P. campbelli accepted P. sungorus placenta. Results are consistent with a neophobic reaction to both placenta (conspecific or heterospecific) and liver as P. sungorus also rejected P. campbelli placenta.

Estradiol and progesterone in paternal and non-paternal hamsters (Phodopus) becoming fathers: conflict with hypothesized roles

Phodopus campbelli has an extensive paternal behavior repertoire whereas the closely-related Phodopus sungorus is not paternally responsive to a displaced pup. For the first time in a naturally paternal mammal, male estradiol and progesterone were determined during two critical phases: (1) the transition from sexually naive male to paired, expectant father that occurs in the absence of stimuli from pups (sexually naive males, paired males on G8, G12, G15, or G17 of the 18-day gestation) and (2) after pup stimuli became available to the males (paired males on days L1, L3, L5, or L12 of pup development). Hormone concentrations in naive males and between G17 and L1 (as stimuli from the birth and the pups became available to males) were also compared. Paternal responsiveness was tested on L3-L5 and confirmed species differences. Hormone concentrations in naive males were similar in the two species and males of both species had estradiol concentrations as high as fertile adult females. However, in direct contrast to predictions, estradiol concentrations were stable in P. campbelli males but increased before the birth, fell across the birth, and increased over pup development in P. sungorus males. Progesterone concentrations in P. campbelli males increased from G17 to L1 whereas a decrease had been predicted. Testosterone dynamics were consistent with previous studies. Either hormonal facilitation of paternal behavior is a hyper-variable trait that has evolved differently in different species, or, more probably, peripheral hormone concentrations are inadequate to explain the role of sex steroid hormones in paternal behavior.

Social facilitation of wound healing

It is well documented that psychological stress impairs wound healing in humans and rodents. However, most research effort into influences on wound healing has focused on factors that compromise, rather than promote, healing. In the present study, we determined if positive social interaction, which influences hypothalamic-pituitary-adrenal (HPA) axis activity in social rodents, promotes wound healing. Siberian hamsters received a cutaneous wound and then were exposed to immobilization stress. Stress increased cortisol concentrations and impaired wound healing in isolated, but not socially housed, hamsters. Removal of endogenous cortisol via adrenalectomy eliminated the effects of stress on wound healing in isolated hamsters. Treatment of isolated hamsters with oxytocin (OT), a hormone released during social contact and associated with social bonding, also blocked stress-induced increases in cortisol concentrations and facilitated wound healing. In contrast, treating socially housed hamsters with an OT antagonist delayed wound healing. Taken together, these data suggest that social interactions buffer against stress and promote wound healing through a mechanism that involves OT-induced suppression of the HPA axis. The data imply that social isolation impairs wound healing, whereas OT treatment may ameliorate some effects of social isolation on health.

Huddling, locomotor, and nest-building behaviors of furred and furless Siberian hamsters

Rodents living in the cold employ both behavioral and physiological mechanisms to achieve thermoregulation. We examined the impact of fur loss on behavioral thermoregulation in cold-challenged Siberian hamsters (Phodopus sungorus). Intact female hamsters exposed to an ambient temperature (T(a)) of 5 degrees C increased their general locomotor activity by 50% relative to animals maintained at 23 degrees C. At both T(a)'s, fur removal resulted in substantial increases in daily food intake (37% and 22% at 5 and 23 degrees C, respectively) but did not affect the amount of locomotor activity; increased food intake after fur loss evidently is not caused by increases in locomotor activity. Furred hamsters housed in groups of three at 5 degrees C consumed 16% less food per day than did singly housed individuals. Fur removal resulted in identical 39% increases in food intake in group- or singly housed animals. Energy savings that accrued from huddling were identical in furred and furless animals; this behavior conserves energy even in the absence of an insulative pelage. The availability of nesting material resulted in an 18% reduction in food consumption in intact animals kept at 5 degrees C. The increase in food intake produced by fur removal was attenuated by approximately 80% when furless animals had access to nesting material. Huddling and nest-building behaviors each ameliorate energetic challenges posed by absence of fur; animals that concurrently employ several modes of thermoregulation realize substantial energy savings in the cold.

Predator odour and its impact on male fertility and reproduction in Phodopus campbelli hamsters

This study investigated the influence of cat urine odour in suppressing development and fertility in Campbell's hamster males. Exposure to this odour from postnatal day 11 until day 45 (sexual maturation) resulted in reduced sex organ weights, reduced testosterone levels and in an increase in abnormalities of the synaptonemal complex in both sex chromosomes and autosomes. Subsequent breeding experiments revealed a significant decrease in litter size. All these data indicate a severe effect of predator odour on the breeding success of potential prey species. It is assumed that these effects are caused by the sulphurous compounds in the urine; however, the underlying mechanisms are not yet known.

Diet self-selection and food hoarding after food deprivation by Siberian hamsters

Siberian hamsters (Phodopus sungorus) increase food hoarding, but not food intake, after a fast. Because the physiological mechanisms underlying these changes in food hoarding are virtually unknown, we sought insight into these mechanisms by allowing hamsters to self-select their diet from food sources varying in macronutrient composition and caloric density ("dietary wisdom"). Therefore, the effects of food deprivation length on diet self-selection were tested in adult female hamsters after adaptation to three composite diets: sunflower seeds (SS), pellet chow (PC), and rabbit chow (RC). One group initially was fasted for 32 h, the other for 56 h, and then each was refed. The remaining nonexperienced fast was instated after prefast body mass, food intake, and hoarding were recovered. Food hoarding, but not food intake, was increased regardless of fast length or sequence; moreover, the largest increase in food hoarding was on the first day of refeeding and was primarily reflected as increased SS hoarding. When the longer fast occurred first body mass loss was greater and the increased food hoard size was maintained for more days than when the longer fast came second. The order of food intake and hoarding preferences was not changed after a fast (SS > PC > RC), but the degree of food hoarding preference for SS was exaggerated. Collectively, these results support the notion that food hoarding increases with decreases in lipid stores, and show that when internal lipid stores are decreased, external lipid stores are preferentially increased.