Male White-footed Mice Research Articles

Photoperiod Alters Visceral Adiposity and Molecular Circadian Rhythms in White‐footed Mice at Thermoneutrality

The length of daylight (photoperiod) and environmental temperature fluctuate seasonally and act as cues for physiology. How photoperiod per se impacts adipose physiology is essential to consider as misalignment of circadian rhythms is suggested to promote obesity. Photoperiod alters fat deposition when temperature simultaneously acts as a cue. However, whether photoperiod regulates adipose physiology independent of temperature is unknown, as ambient temperatures below thermoneutrality are typically used. We compared the effects of 4 weeks of long (LD - 16hrs light:8hrs dark) and short (SD – 8hrs light:16hrs dark) photoperiod exposure at thermoneutrality (27°C) to isolate photoperiod from temperature cues in photosensitive F1 generation, male white-footed mice (Peromyscus leucopus). Short photoperiod selectively lowered visceral white adipose tissue (WAT) (vWAT) mass (p<0.05 vs LD), with no effect on subcutaneous WAT or interscapular brown adipose tissue mass or bodyweight. Calorie consumption and 24-hour feeding behaviour were not affected by photoperiod, although short photoperiod mice ate almost exclusively in the dark (SD ate 8.5% of daily intake in 8hr light phase vs. LD ate 26% of intake in 16hr light phase (p<0.0001)). In vWAT, short photoperiod increased Adrβ3, Lpl and Nr1d1 mRNA expression (p<0.05 vs LD), with no change in Ucp1, Pgc1a or Hsl. Circadian Per1 and Per2 gene expression aligned with the onset of dark and thus were misaligned between photoperiods (p<0.05), while Bmal1 and Clock were also misaligned. Thus, photoperiod alone is a strong enough cue to alter visceral fat accumulation and alter clock gene misalignment, without temperature-induced thermogenesis. These findings have important implications for understanding obesity in humans in whom day light is artificially extended while environmental temperatures are kept consistent.

A single testosterone pulse rapidly reduces urinary marking behaviour in subordinate, but not dominant, white-footed mice

Androgenic hormones can rapidly influence how animals behave, although the dynamics of these effects remain elusive. In particular, we know little about whether androgens rapidly impact behavioural strategizing or whether such effects vary among individuals of the same species. Here, we examine these issues by testing how transient testosterone (T) surges, which normally occur in response to social stimuli, rapidly alter the urinary marking behaviour of socially subordinate and dominant male white-footed mice, Peromyscus leucopus. Marking behaviour in mice is important in reproductive competition and advertisement; thus, how an individual marks its environment with urine likely reflects the strategy it uses to find a mate. Moreover, urine-marking patterns are indicative of social status and territorial propensity. Our results show that a single T pulse, within minutes, suppresses all measures of urinary marking behaviour in subordinate mice, including amounts of perimeter (territorial) marking and total (status-related) marking. These effects are not observed in dominant mice. The data collectively suggest that androgens rapidly change behaviour tightly associated with reproductive strategy and decision making. Moreover, these effects depend on an individual's social status, which suggests that there is intraspecific variation in the way that androgens affect marking behaviour on short timescales. This study fills a gap in our understanding about the plasticity of rapid androgenic effects on behaviour, and how these responses can influence adaptive reproductive tactics.

Photoperiodic regulation of hippocampal neurogenesis in adult male white‐footed mice (Peromyscus leucopus)

Photoperiodic organisms monitor environmental day length to engage in seasonally appropriate adaptions in physiology and behavior. Among these adaptations are changes in brain volume and neurogenesis, which have been well described in multiple species of birds, yet few studies have described such changes in the brains of adult mammals. White-footed mice (Peromyscus leucopus) are an excellent species in which to investigate the effects of day length on adult hippocampal neurogenesis, as males, in addition to having reduced hippocampal volume in short days (SD) with concomitant impairments in hippocampus-mediated behaviors, have photoperiod-dependent changes in olfactory bulb neurogenesis. We performed the current experiment to assess the effects of photoperiod on hippocampal neurogenesis longitudinally, using the thymidine analog bromodeoxyuridine at multiple time points across 10weeks of SD exposure. Compared with counterparts held in long day (LD) lengths, across the first 8weeks of SD exposure hippocampal neurogenesis was reduced. However, at 10weeks in SD lengths neurogenic levels in the hippocampus were elevated above those levels in mice held in LD lengths. The current findings are consistent with the natural photoperiodic cycle of hippocampal function in male white-footed mice, and may help to inform research on photoperiodic plasticity in neurogenesis and provide insight into how the complex interplay among the environment, genes and adaptive responses to changing day lengths affects brain structure, function and behavior at multiple levels.

Social and nutritional stressors: agents for altered immune function in white-footed mice (Peromyscusleucopus)

As habitats become more fragmented, population densities and diets of wildlife can change dramatically, contributing to increased stress and incidence of infections and disease. To better understand effects of human disturbance on immunocompetence of wild mammals, we studied individual and combined effects of social and nutritional stress on the health of wild-captured adult male white-footed mice (Peromyscus leucopus (Rafinesque, 1818)), a species that commonly occurs in close proximity to humans. Paired mice had reduced body mass and circulating monocytes, higher serum corticosterone level, and a significantly weaker humoral immune response compared with mice housed individually. Mice fed a 5% protein diet had reduced body mass and circulating monocytes, but no differences in humoral or cellular immune responses compared with mice fed a 30% protein diet. The only interactive effect of the two stresses on immune-related parameters was on mass of the spleen. We hypothesize that reduced humoral immunocompetence in response to acute and chronic social stress likely contributes to increases in disease transmission in high-density populations associated with fragments of habitat. In addition, anthropogenic impacts that limit food availability may be of greater importance to immune function than impacts on food quality.

Photoperiod Mediated Changes in Olfactory Bulb Neurogenesis and Olfactory Behavior in Male White-Footed Mice (Peromyscus leucopus)

Brain plasticity, in relation to new adult mammalian neurons generated in the subgranular zone of the hippocampus, has been well described. However, the functional outcome of new adult olfactory neurons born in the subventricular zone of the lateral ventricles is not clearly defined, as manipulating neurogenesis through various methods has given inconsistent and conflicting results in lab mice. Several small rodent species, including Peromyscus leucopus, display seasonal (photoperiodic) brain plasticity in brain volume, hippocampal function, and hippocampus-dependent behaviors; plasticity in the olfactory system of photoperiodic rodents remains largely uninvestigated. We exposed adult male P. leucopus to long day lengths (LD) and short day lengths (SD) for 10 to 15 weeks and then examined olfactory bulb cell proliferation and survival using the thymidine analog BrdU, olfactory bulb granule cell morphology using Golgi-Cox staining, and behavioral investigation of same-sex conspecific urine. SD mice did not differ from LD counterparts in granular cell morphology of the dendrites or in dendritic spine density. Although there were no differences due to photoperiod in habituation to water odor, SD mice rapidly habituated to male urine, whereas LD mice did not. In addition, short day induced changes in olfactory behavior were associated with increased neurogenesis in the caudal plexiform and granule cell layers of the olfactory bulb, an area known to preferentially respond to water-soluble odorants. Taken together, these data demonstrate that photoperiod, without altering olfactory bulb neuronal morphology, alters olfactory bulb neurogenesis and olfactory behavior in Peromyscus leucopus.

Associations between Ixodes scapularis ticks and small mammal hosts in a newly endemic zone in southeastern Canada: Implications for Borrelia burgdorferi transmission

Immature Ixodes scapularis infestation and Borrelia burgdorferi infection of wild small mammals were studied from June to October in 2007 and from May to October in 2008 at 71 study sites in a zone where I. scapularis populations and environmental Lyme disease risk are emerging in southwestern Quebec. Seasonal host-seeking activity of immature I. scapularis was similar to patterns reported previously in Canada and the USA: nymphal activity peaked in spring while larval activity peaked in late summer. Synchronous activity of nymphs with some larvae was observed in late spring, which could favour establishment of B. burgdorferi strains that cause short-lived infections in their hosts. White-footed mice ( Peromyscus leucopus), deer mice ( P. maniculatus), chipmunks ( Tamias striatus), and red squirrels ( Tamiasciurus hudsonicus) carried 92.0% of the larvae and 94.2% of the nymphs collected. Adult male white-footed mice carried significantly larger numbers of both larval and nymphal I. scapularis than other species and classes of small mammals (different demographic groups or physiological status: age, sex, sexual activity). We conclude that seasonality and host association were comparable to previous studies in North America, even in the context of a newly endemic pattern of low infection prevalence and low densities of host-seeking and feeding I. scapularis in southwestern Quebec. Our studies suggest that B. burgdorferi transmission cycles are focused on adult male mice (which carried 35% of all feeding ticks collected in the study), so control methods targeting this class of hosts may be particularly effective. However, our study also suggested that habitats containing a diverse host structure may dilute transmission cycles by partitioning of nymphal and larval ticks on different host species.

Species differences in the winner effect disappear in response to post-victory testosterone manipulations

Evolutionary processes can interact with the mechanisms of steroid hormone action to drive interspecific variation in behavioural output, yet the exact nature of these interactions is poorly understood. To investigate this issue, we compare the endocrine machinery underlying the winner effect (an ability to increase winning behaviour in response to past victories) in two closely related species of Peromyscus mice. Typically, after winning a fight, California mice (Peromyscus californicus) experience a testosterone (T) surge that helps enhance their future winning behaviour, whereas white-footed mice (Peromyscus leucopus) experience neither a T surge nor a change in subsequent winning behaviour. However, our results indicate that when the post-victory T response of male white-footed mice is phenotypically engineered to resemble that of California mice, individuals are capable of developing a strong and lasting winner effect. Moreover, this 'induced' winner effect in white-footed mice qualitatively matches the winner effect that develops naturally in California mice. Taken together, these findings suggest that white-footed mice have the physiological machinery necessary to form a robust winner effect comparable to that formed by California mice, but are unable to endogenously activate this machinery after achieving winning experiences. We speculate that evolutionary processes, like selection, operate on the physiological substrates that govern post-victory T release to guide divergence in the winner effect between these two species.

Photoperiod-mediated impairment of long-term potention and learning and memory in male white-footed mice

Adult mammalian brains are capable of some structural plasticity. Although the basic cellular mechanisms underlying learning and memory are being revealed, extrinsic factors contributing to this plasticity remain unspecified. White-footed mice (Peromyscus leucopus) are particularly well suited to investigate brain plasticity because they show marked seasonal changes in structure and function of the hippocampus induced by a distinct environmental signal, viz., photoperiod (i.e. the number of hours of light/day). Compared to animals maintained in 16 h of light/day, exposure to 8 h of light/day for 10 weeks induces several phenotypic changes in P. leucopus, including reduction in brain mass and hippocampal volume. To investigate the functional consequences of reduced hippocampal size, we examined the effects of photoperiod on spatial learning and memory in the Barnes maze, and on long-term potentiation (LTP) in the hippocampus, a leading candidate for a synaptic mechanism underlying spatial learning and memory in rodents. Exposure to short days for 10 weeks decreased LTP in the Schaffer collateral–CA1 pathway of the hippocampus and impaired spatial learning and memory ability in the Barnes maze. Taken together, these results demonstrate a functional change in the hippocampus in male white-footed mice induced by day length.

Enrichment and photoperiod interact to affect spatial learning and hippocampal dendritic morphology in white‐footed mice (Peromyscus leucopus)

In seasonally changing environments, individuals must coordinate endogenous processes with ambient conditions. Winter is a challenging time to survive and reproduce. In order to anticipate decreased food availability and low temperatures in winter, many rodents use decreasing day lengths as a precise temporal cue. Short day lengths alter several adaptations, including reproduction, immune function, aggressive behavior and spatial learning in non-tropical rodents. Specifically, short days impair spatial learning in white-footed mice (Peromyscus leucopus) and alter dendritic complexity in the hippocampus. The goal of the current study was to determine whether short days constrain neural plasticity. If short days limit the capacity for plasticity, then environmental enrichment, a manipulation that induces morphological changes, should alter dendritic morphology in long, but not short, days. Male white-footed mice were assigned to long (16 : 8 LD) or short (8 : 16 LD) photoperiod in either enriched or standard cages. Enrichment consisted of a large cage, cage mates, Habitrail tubes, a nest box and a running wheel. Mice were tested in the Morris water maze. Reproductive tissues were collected and weighed; brains were processed for dendritic morphology. Short days impaired spatial learning. Short days also reduced spine density on apical dendrites within the CA3 region of the hippocampus. However, enrichment prevented short-day-induced deficits in learning and also increased hippocampal spine density in the CA1 and CA3 regions in short-day mice. These results suggest that day length and other non-photic environmental factors interact to regulate dendritic morphology, and that short photoperiods do not constrain the capacity for functional neural plasticity.

Short Days Increase Hypothalamic-Pituitary-Adrenal Axis Responsiveness

Individuals dramatically alter physiology and behavior to adapt to seasonal changes in their environment. To cope with winter stressors such as reduced food availability and low temperatures, central stress responses are presumably modulated at the level of the hypothalamic-pituitary-adrenal (HPA) axis, but the details remain unspecified. We examined the effects of long or short photoperiods (day lengths) on corticosterone responses to restraint, HPA negative feedback sensitivity, glucocorticoid receptor gene expression in the hippocampus, the role of corticosterone in spatial learning, and corticosterone responses to stressors associated with the spatial water maze task in adult male white-footed mice (Peromyscus leucopus). Short days increased corticosterone responses to restraint, increased hippocampal glucocorticoid receptor expression, enhanced corticosterone negative feedback on the HPA axis, and increased sensitivity to dexamethasone suppression of corticosterone. Although spatial learning and memory performance (via water maze) of all mice was impaired after pharmacological corticosterone inhibition, both water maze exposure and treatment injections alone were sufficient to increase short-day, but not long-day, corticosterone concentrations. Thus, the effects of corticosterone on spatial learning in these mice may be complicated by photoperiodic differences in stressor response to the learning task itself. Overall, these results suggest that photoperiod-evoked modification of the HPA axis and its potential behavioral consequences may be adaptive for winter survival.

Testosterone and photoperiod interact to affect spatial learning and memory in adult male white‐footed mice (Peromyscus leucopus)

Gonadal hormones affect spatial learning and memory in mammals and circulating gonadal hormone concentrations fluctuate by season. Most nontropical rodents are spring/summer breeders and males display higher testosterone concentrations during the breeding season compared with the nonbreeding season (fall/winter). Seasonal patterns of testosterone concentration (as well as many other seasonal modifications of physiology, morphology, and behaviour) are induced by manipulation of photoperiod (day length; i.e. short or long days) in the laboratory. Coincident with reducing testosterone concentration, short days also impair spatial learning and memory performance in male white-footed mice (Peromyscus leucopus) compared with long days. We hypothesized that short-day-induced reduction of testosterone concentrations inhibits spatial learning and memory performance compared with long days. Adult male white-footed mice were maintained in long (16 h light/day) or short (8 h light/day) days for 14 weeks following sham-castration, castration plus saline implant, or castration plus testosterone implant treatment. Spatial learning and memory was assessed using a water maze, and photoperiod-evoked changes in gene expression of sex steroid receptors within the hippocampus were also examined. Castrated, short-day mice with testosterone replacement displayed enhanced water maze performance compared with other short-day mice, but no differences among testosterone treatments were observed in long-day mice. Photoperiod did not affect hippocampal androgen, oestrogen alpha, or oestrogen beta receptor gene expression. These results suggest that photoperiod modulates the effects of testosterone on spatial learning performance by mechanisms indirect of the hippocampus.

Positive effects of testosterone and immunochallenge on energy allocation to reproductive organs

A number of studies have suggested the incompatibility of simultaneous increases in immune and reproductive functions. Other research has indicated that immune responses may be modulated depending on the relative benefits of increased survival and prospects for current and future reproduction. We tested the hypothesis that energy allocation to reproductive and other organ systems is not affected by testosterone level and energy expenditure on immune functions. Adult male white-footed mice (Peromyscus leucopus) with or without elevated testosterone levels and with or without immunochallenges were tested. Testosterone treatment was associated with reduced humoral immune response indicating immunosuppressive effects, reduced masses of gastrointestinal organs, reduced corticosterone level, increased kidney and seminal vesicle masses, and increased hematocrit. Immunochallenge was associated with increased resting metabolic rate and testes and seminal vesicle masses. Reproductive organ masses were greatest in immunochallenged mice with exogenous testosterone. Simultaneous increases in energy allocation to immune and reproductive structures may be an adaptive response that would enhance survival and current prospects for reproduction.

Short Photoperiods Impair Spatial Learning and Alter Hippocampal Dendritic Morphology in Adult Male White-Footed Mice (Peromyscus leucopus)

Although seasonal changes in brain morphology and function are well established in songbirds, seasonal plasticity of brain structure and function remain less well documented in mammals. Nontropical animals display many adaptations to reduce energy use to survive winter, including cessation of reproductive activities. Because of the high energetic costs of brain tissue, we hypothesized that male white-footed mice (Peromyscus leucopus) would reduce brain size in response to short days as well as regress their reproductive systems. Because short days may decrease hippocampal volume and impair spatial learning and memory in rodents and because of the potential for seasonal plasticity in the hippocampus, we hypothesized that photoperiod alters hippocampal morphology to affect spatial learning and memory. Mice housed in either long or short days for 10 weeks were examined for performance in a water maze; brains were then removed and weighed, and hippocampal volumes were determined. We also measured dendritic morphology and spine density in the CA1, CA3, and dentate gyrus. Short days decreased brain mass and hippocampal volume compared with long days. Short days also impaired long-term spatial learning and memory relative to long days but did not affect sensory discrimination or other types of memory. Short days decreased apical (stratum lacunosum-moleculare) CA1 spine density, as well as increased basilar (stratum oriens) CA3 spine density. Results from this study suggest that photoperiod alters brain size and morphology, as well as cognitive function. Understanding the mechanisms mediating these photoperiod-induced alterations may provide insight for treatment of seasonal cognitive and affective disorders.

Social environment modulates photoperiodic immune and reproductive responses in adult male white-footed mice(Peromyscus leucopus)

Social cues may interact with photoperiod to regulate seasonal adaptations in photoperiod-responsive rodents. Specifically, photoperiod-induced adjustments (e.g., reproduction and immune function) may differ among individuals in heterosexual pairs, same-sex pairs, or isolation. Heterosexual cues may be more influential, based on their potential fitness value, than same-sex cues or no social cues. The present study examined the effects of pair (with a male or female) or individual housing on reproductive and immune responses in male white-footed mice (Peromyscus leucopus) maintained in long or short photoperiods. Female pairing did not affect reproductive responses in short-day males. In long days, however, the presence of a female increased both testosterone concentrations and testes mass compared with individually housed and male-paired mice, respectively. Short-day, individually housed males enhanced delayed-type hypersensitivity (DTH) responses compared with single-housed mice in long days, but all paired groups decreased DTH responses regardless of photoperiod. The lack of enhanced DTH response in male mice paired with females coincided with reduced circulating corticosterone concentrations in both photoperiod treatments. Together, these results suggest that social environment may have important modulatory effects on photoperiod-regulated immune responses in male white-footed mice.

Effect of Fluorescent Powder Marking of Females on Mate Choice by Male White-Footed Mice (Peromyscus leucopus)

Although fluorescent powder marking is a well established and common marking technique used by small-mammal ecologists, few studies have commented on how it might affect behavior. Potential effects on behavior are particularly relevant in studies that use fluorescent powder marking as a method of quantifying mating behavior. We performed a laboratory experiment using white-footed mice (Peromyscus leucopus) to test the null hypothesis that the presence of fluorescent powder has no significant effect on mate choice. We tested this hypothesis by establishing choice between a pair of females by a male and determining if the choice changed when one of the females was powdered. The presence of fluorescent powder on females did not alter mate choice by males after a choice had been established. Our results demonstrate that using fluorescent powder on females should not alter male mating activity.

Vasopressin and Aggression in Cross-Fostered California Mice (Peromyscus californicus) and White-Footed Mice (Peromyscus leucopus)

To examine how developmental experiences alter neural pathways associated with adult social behavior, we cross-fostered pups between the more aggressive and monogamous California mouse (Peromyscus californicus) and the less aggressive and polygamous white-footed mouse (P. leucopus). Cross-fostered males became more like their foster parents when tested as adults. Male white-footed mice became more aggressive only in an aggression test in a neutral arena, whereas the territorial California mice became less aggressive in resident–intruder aggression test, as measured by attack latency. Only the species that displayed a change in resident–intruder aggression showed a change in arginine vasopressin (AVP) levels: cross-fostered California mice had significantly lower levels of AVP-immunoreactive (AVP-ir) staining than controls in the bed nucleus of the stria terminalis (BNST) and the supraoptic nucleus (SON) and a nonsignificant trend toward lower levels in the medial amygdala (MA). Neither species showed changes in AVP-ir staining in a control area, the paraventricular nucleus (PVN). The changes in AVP-ir staining in the BNST and SON may not be caused by stress because cross-fostering was not associated with changes in adult plasma concentrations of two steroid hormones, corticosterone and testosterone, that have been associated with stress-related alterations in AVP pathways. These results suggest that manipulating the early parental environment can directly alter both a neurotransmitter system and species-typical patterns of social behavior, but that these effects may vary between species and under different social contexts.

Short photoperiods evoke testicular apoptosis in white-footed mice (Peromyscus leucopus).

Many small, nontropical mammals stop breeding during winter. Chronic exposure of males to short days (<12.5 h light/day) causes the testes to atrophy and both steroidogenesis and gametogenesis to decrease. Male white-footed mice (Peromyscus leucopus) exposed to inhibitory short day lengths provide a natural animal model to study the cellular mechanisms regulating testicular regression. In the present study, the possible role of apoptosis was assessed during naturally occurring, short day-induced gonadal regression in white-footed mice by in situ terminal transferase-mediated end labeling (TUNEL), quantitative DNA 3'-end-labeling autoradiography (laddering) of DNA fragments, and quantification of Fas protein expression, an early initiator of apoptosis. Sexually mature male mice were exposed to short (8 h of light, 16 h of darkness) or long (16 h of light, 8 h of darkness) day lengths for 2, 4, 6, 8, or 10 weeks; gonads were then removed and processed for detection of apoptotic activity. In common with previous studies, the first significant reduction in relative testis mass was observed at week 10 of short day exposure. A 2- to 3-fold increase in apoptotic (TUNEL-positive) germ cells per seminiferous tubule was observed in the testes of mice exposed to short days for 4, 6, 8, or 10 weeks compared with the testes of long day animals. The extent of 3'-end labeling of low mol wt DNA increased with 4-8 weeks of short day exposure. Western blot analysis revealed an up-regulation of the Fas protein in the testes of short day males at 4, 8, and 10 weeks. Fas staining was primarily localized to spermatocytes and spermatids. Plasma testosterone concentrations decreased in short compared with long day animals after 6, 8, or 10 weeks. The increase in TUNEL positive-labeled germ cells, testicular DNA fragmentation, and up-regulation of the Fas protein before short day reductions of testis mass and function suggest that apoptosis is important for the mediation of photoperiod-induced testicular regression in white-footed mice.

Brief exposure to a natural predator, the short-tailed weasel, induces benzodiazepine-sensitive analgesia in white-footed mice

Exposure to a natural predator, the short-tailed weasel, Mustela erminea, elicited significant increases in the nociceptive responses of wild male white-footed mice, Peromyscus leucopus. A short (30 sec), ecologically relevant, nonvisual exposure to a weasel elicited a relatively brief (15 min) analgesia that was insensitive to the opiate antagonist, naloxone (1.0 mg/kg), and was blocked by either pre- or post-exposure injections of the benzodiazepine antagonist, Ro15–1788 (10 mg/kg), or agonist, diazepam (4.0 mg/kg). A 5 min exposure to the weasel elicited an analgesic response of longer duration (15–30 min) that was sensitive to both naloxone and the benzodiazepine agonist and antagonist. A 15 min exposure to the weasel induced a higher amplitude analgesia that was of relatively long duration (45 min), blocked by naloxone, and insensitive to the benzodiazepine manipulations. Exposures of 5 and 15 min to a nonpredator, the European rabbit, Oryctolagus cuniculus, elicited low amplitude, naloxone-reversible analgesic reponses that were unaffected by the benzodiazepine manipulations. Thirty-sec exposures to the rabbit had no significant effects on the nociceptive response of the rabbit. These results indicate that a brief, ecologically appropriate, exposure to a predator elicits a benzodiazepine-mediated analgesia while a more prolonged exposure to a predator induces opioid-mediated analgesia. These results show that the opioid and nonopioid distinction between the effects of long- and short-term laboratory stresses is also evident with a natural, ecologically relevant, stressor.

Photoperiodic regulation of reproductive development in male white-footed mice (Peromyscus leucopus) born at different phases of the breeding season.

Male white-footed mice were maintained from birth in chambers equipped with latitudinal timers programmed to simulate the natural progression of daylengths at 42 degrees N latitude. Mice were born into photoperiods starting on 1 April, 15 July, 15 September or 27 October. Testes and seminal vesicles of April- and July-born mice were mature at 60 days of age; puberty was delayed by several months in animals born into the September and October photoperiods but these mice became fecund at about 3.5 months of age, independent of current photoperiod. Therefore, increasing daylengths of late winter do not appear to synchronize the reproductive development of overwintering juveniles born at different times during the previous breeding season.

Sexual Maturation of Male and Female White-Footed Mice (Peromyscus leucopus noveboracensis): Influence of Physical or Urine Contact with Adults

Young white-footed mice were reared from weaning (21–24 days of age) in contact with individual adult males or their urine, individual adult females or their urine, or water. At 40 days of age the young animals were killed and their body and reproductive organ weights (testes and seminal vesicles or uteri and ovaries) were compared. Males reared in contact with adult females or their urine had developed larger testes and seminal vesicles than those in contact with water, adult males, or male urine (generally P < 0.05). Females in contact with adult males or male urine had significantly ( P < 0.05) smaller ovaries than those in contact with water, adult female urine, or adult females. Body weights of females in contact with males or male urine also averaged significantly less than those in contact with water. Additionally, females in contact with adult males or adult male urine had significantly fewer corpora lutea than those exposed to water.