Apical Sperm Hook Morphology Is Linked to Sperm Swimming Performance and Sperm Aggregation in Peromyscus Mice.

Mammals exhibit a tremendous amount of variation in sperm morphology and despite the acknowledgement of sperm structural diversity across taxa, its functional significance remains poorly understood. Of particular interest is the sperm of rodents. While most Eutherian mammal spermatozoa are relatively simple cells with round or paddle-shaped heads, rodent sperm are often more complex and, in many species, display a striking apical hook. The function of the sperm hook remains largely unknown, but it has been hypothesized to have evolved as an adaptation to inter-male sperm competition and thus has been implicated in increased swimming efficiency or in the formation of collective sperm movements. Here we empirically test these hypotheses within a single lineage of Peromyscus rodents, in which closely related species naturally vary in their mating systems, sperm head shapes, and propensity to form sperm aggregates of varying sizes. We performed sperm morphological analyses as well as in vitro analyses of sperm aggregation and motility to examine whether the sperm hook (i) morphologically varies across these species and (ii) associates with sperm competition, aggregation, or motility. We demonstrate inter-specific variation in the sperm hook and then show that hook width negatively associates with sperm aggregation and sperm swimming speed, signifying that larger hooks may be a hindrance to sperm movement within this group of mice. Finally, we confirmed that the sperm hook hinders motility within a subset of Peromyscus leucopus mice that spontaneously produced sperm with no or highly abnormal hooks. Taken together, our findings suggest that any adaptive value of the sperm hook is likely associated with a function other than inter-male sperm competition, such as interaction with ova or cumulous cells during fertilization, or migration through the complex female reproductive tract.

Tail Length Evolution in Deer Mice: Linking Morphology, Behavior, and Function.

Determining how variation in morphology affects animal performance (and ultimately fitness) is key to understanding the complete process of evolutionary adaptation. Long tails have evolved many times in arboreal and semi-arboreal rodents; in deer mice, long tails have evolved repeatedly in populations occupying forested habitat even within a single species (Peromyscus maniculatus). Here, we use a combination of functional modeling, laboratory studies, and museum records to test hypotheses about the function of tail-length variation in deer mice. First, we use computational models, informed by museum records documenting natural variation in tail length, to test whether differences in tail morphology between forest and prairie subspecies can influence performance in behavioral contexts relevant for tail use. We find that the deer- mouse tail plays little role in statically adjusting center of mass or in correcting body pitch and yaw, but rather it can affect body roll during arboreal locomotion. In this context, we find that even intraspecific tail-length variation could result in substantial differences in how much body rotation results from equivalent tail motions (i.e., tail effectiveness), but the relationship between commonly-used metrics of tail-length variation and effectiveness is non-linear. We further test whether caudal vertebra length, number, and shape are associated with differences in how much the tail can bend to curve around narrow substrates (i.e., tail curvature) and find that, as predicted, the shape of the caudal vertebrae is associated with intervertebral bending angle across taxa. However, although forest and prairie mice typically differ in both the length and number of caudal vertebrae, we do not find evidence that this pattern is the result of a functional trade-off related to tail curvature. Together, these results highlight how even simple models can both generate and exclude hypotheses about the functional consequences of trait variation for organismal-level performance.

Long-Term Effects of Fatherhood on Morphology, Energetics, and Exercise Performance in California Mice (Peromyscus californicus).

In male mammals that provide care for their offspring, fatherhood can lead to changes in behavioral, morphological, and physiological traits, some of which might constitute trade-offs. However, relatively little is known about these changes, especially across multiple reproductive bouts, which are expected to magnify differences between fathers and nonreproductive males. We evaluated consequences of fatherhood in the monogamous, biparental California mouse (Peromsycus californicus) across seven consecutive reproductive bouts. We compared breeding adult males (housed with sham-ovariectomized females) with two control groups: nonbreeding males (housed with ovariectomized females treated with estrogen and progesterone to induce estrous behavior) and virgin males (housed with untreated ovariectomized females). At five time points (before pairing, early postpartum of the first litter, late postpartum of the second litter, early postpartum of the sixth litter, and late postpartum of the seventh litter or comparable time points for nonbreeding and virgin males), we measured males' body composition, hematocrit, predatory aggression, resting metabolic rate, maximal oxygen consumption (V˙O2 max⁡), grip strength, and sprint speed. We also weighed organs at the final time point. We predicted that fathers would have lower relative body fat and lower performance abilities compared with control groups and that these effects would become more pronounced with increasing parity. Contrary to predictions, breeding and control males differed in surprisingly few measures, and the number and magnitude of differences did not increase with parity. Thus, our expectations regarding trade-offs were not met. As reported in studies of single reproductive events, these results suggest that fatherhood has few costs in this species when housed under standard laboratory conditions, even across multiple reproductive bouts.

Acclimatization of low altitude-bred deer mice ( Peromyscus maniculatus) to high altitude.

A colony of deer mice subspecies ( Peromyscus maniculatus sonoriensis) native to high altitude (HA) has been maintained at sea level for 18-20 generations and remains genetically unchanged. To determine if these animals retain responsiveness to hypoxia, one group (9-11 wk old) was acclimated to HA (3,800 m) for 8 wk. Age-matched control animals were acclimated to a lower altitude (LA; 252 m). Maximal O2 uptake (V̇o2max) was measured at the respective altitudes. On a separate day, lung volume, diffusing capacity for carbon monoxide (DLCO), and pulmonary blood flow were measured under anesthesia using a rebreathing technique at two inspired O2 tensions. The HA-acclimated deer mice maintained a normal V̇o2max relative to LA baseline. Compared with LA control mice, antemortem lung volume was larger in HA mice in a manner dependent on alveolar O2 tension. Systemic hematocrit, pulmonary blood flow, and standardized DLCO did not differ significantly between groups. HA mice showed a higher postmortem alveolar-capillary hematocrit, larger alveolar ducts, and smaller distal conducting structures. In HA mice, absolute volumes of alveolar type I epithelia and endothelia were higher whereas that of interstitia was lower than in LA mice. These structural changes occurred without a net increase in whole-lung septal tissue-capillary volumes or surface areas. Thus, deer mice bred and raised to adulthood at LA retain phenotypic plasticity and adapt to HA without a decrement in V̇o2max via structural (enlarged airspaces, alveolar septal remodeling) and nonstructural (lung expansion under hypoxia) mechanisms and without an increase in systemic hematocrit or compensatory lung growth. NEW & NOTEWORTHY Deer mice ( Peromyscus maniculatus) are robust and very active mammals that are found across the North American continent. They are also highly adaptable to extreme environments. When introduced to high altitude they retain remarkable adaptive ability to the low-oxygen environment via lung expansion and remodeling of existing lung structure, thereby maintaining normal aerobic capacity without generating more red blood cells or additional lung tissue.

Transplacental transmission of tick-borne Babesia microti in its natural host Peromyscus leucopus.

BACKGROUND: Babesia microti is an emerging tick-borne pathogen and the causative agent of human babesiosis. Mathematical modeling of the reproductive rate of B. microti indicates that it cannot persist in nature by horizontal tick-host transmission alone. We hypothesized that transplacental transmission in the reservoir population contributes to B. microti persistence and emergence in North American rodent populations. METHODS: Peromyscus leucopus were collected from Connecticut and Block Island, Rhode Island and analyzed using a highly specific quantitative PCR (qPCR) assay for infection with B. microti. RESULTS: In April, 100% (n = 103) of mice were infected with B. microti. Females exhibited significantly higher parasitemia than their offspring (P < 0.0001) and transplacental transmission was observed in 74.2% of embryos (n = 89). Transplacental transmission of B. microti is thus a viable and potentially important infectious pathway in naturally infected rodent species and should be considered in future theoretical and empirical studies. CONCLUSIONS: To our knowledge, this study is the first to report transplacental transmission of B. microti occurring in its natural reservoir host, P. leucopus, in the United States and the only study that provides a quantitative estimate of parasitemia. This vector-independent pathway could contribute to the increased geographic range of B. microti or increase its abundance in endemic areas.

Control of breathing and ventilatory acclimatization to hypoxia in deer mice native to high altitudes.

AIM: We compared the control of breathing and heart rate by hypoxia between high- and low-altitude populations of Peromyscus mice, to help elucidate the physiological specializations that help high-altitude natives cope with O2 limitation. METHODS: Deer mice (Peromyscus maniculatus) native to high altitude and congeneric mice native to low altitude (Peromyscus leucopus) were bred in captivity at sea level. The F1 progeny of each population were raised to adulthood and then acclimated to normoxia or hypobaric hypoxia (12 kPa, simulating hypoxia at ~4300 m) for 5 months. Responses to acute hypoxia were then measured during stepwise reductions in inspired O2 fraction. RESULTS: Lowlanders exhibited ventilatory acclimatization to hypoxia (VAH), in which hypoxia acclimation enhanced the hypoxic ventilatory response, made breathing pattern more effective (higher tidal volumes and lower breathing frequencies at a given total ventilation), increased arterial O2 saturation and heart rate during acute hypoxia, augmented respiratory water loss and led to significant growth of the carotid body. In contrast, highlanders did not exhibit VAH - exhibiting a fixed increase in breathing that was similar to hypoxia-acclimated lowlanders - and they maintained even higher arterial O2 saturations in hypoxia. However, the carotid bodies of highlanders were not enlarged by hypoxia acclimation and were similar in size to those of normoxic lowlanders. Highlanders also maintained consistently higher heart rates than lowlanders during acute hypoxia. CONCLUSIONS: Our results suggest that highland deer mice have evolved high rates of alveolar ventilation and respiratory O2 uptake without the significant enlargement of the carotid bodies that is typical of VAH in lowlanders, possibly to adjust the hypoxic chemoreflex for life in high-altitude hypoxia.

Metabolic and affective consequences of fatherhood in male California mice.

Physiological and affective condition can be modulated by the social environment and parental state in mammals. However, in species in which males assist with rearing offspring, the metabolic and affective effects of pair bonding and fatherhood on males have rarely been explored. In this study we tested the hypothesis that fathers, like mothers, experience energetic costs as well as behavioral and affective changes (e.g., depression, anxiety) associated with parenthood. We tested this hypothesis in the monogamous, biparental California mouse (Peromyscus californicus). Food intake, blood glucose and lipid levels, blood insulin and leptin levels, body composition, pain sensitivity, and depression-like behavior were compared in males from three reproductive groups: virgin males (VM, housed with another male), non-breeding males (NB, housed with a tubally ligated female), and breeding males (BM, housed with a female and their first litter). We found statistically significant (P<0.007, when modified for Adaptive False Discovery Rate) or nominally significant (0.007

The ultimate and proximate mechanisms driving the evolution of long tails in forest deer mice.

Understanding both the role of selection in driving phenotypic change and its underlying genetic basis remain major challenges in evolutionary biology. Here, we use modern tools to revisit a classic system of local adaptation in the North American deer mouse, Peromyscus maniculatus, which occupies two main habitat types: prairie and forest. Using historical collections, we find that forest-dwelling mice have longer tails than those from nonforested habitat, even when we account for individual and population relatedness. Using genome-wide SNP data, we show that mice from forested habitats in the eastern and western parts of their range form separate clades, suggesting that increased tail length evolved independently. We find that forest mice in the east and west have both more and longer caudal vertebrae, but not trunk vertebrae, than nearby prairie forms. By intercrossing prairie and forest mice, we show that the number and length of caudal vertebrae are not correlated in this recombinant population, indicating that variation in these traits is controlled by separate genetic loci. Together, these results demonstrate convergent evolution of the long-tailed forest phenotype through two distinct genetic mechanisms, affecting number and length of vertebrae, and suggest that these morphological changes-either independently or together-are adaptive.

Cranial morphological variation in Peromyscus maniculatus over nearly a century of environmental change in three areas of California.

Determining how species respond to prolonged environmental change is critical to understanding both their evolutionary biology and their conservation needs. In general, organisms can respond to changing environmental conditions by moving, by adapting in situ, or by going locally or globally extinct. Morphological changes, whether plastic or adaptive, are one way that species may respond in situ to local environmental change. Because cranial morphology is influenced by selective pressures arising from an organism's abiotic and biotic environments, including aspects of thermal physiology, diet, and sensory ecology, studies of cranial morphology may generate important insights into how species are responding to environmental change. To assess potential response of deer mice (Peromyscus maniculatus) to changing conditions in the Sierra Nevada Mountains of California, we quantified cranial variation in museum specimens of this species collected approximately 100 years apart. Specifically, we examined how cranial morphology varies in three populations of this geographically widespread, ecological generalist over elevation and time. Our analyses indicate that cranial morphology does not differ with elevation within either modern or historical samples but does vary between time periods, suggesting that in situ responses to environmental change have occurred. Contrary to predictions based on Bergmann's rule, we found no consistent relationship between body size and either elevation or time, suggesting that morphological differences detected between historic and modern specimens are specific to factors influencing cranial structure. Collectively, these analyses demonstrate the potential importance of in situ changes in morphology as a response to changing environmental conditions.

Genetic variation in total number and locations of GnRH neurons identified using in situ hybridization in a wild-source population.

The evolution of brain function in the regulation of physiology may depend in part upon the numbers and locations of neurons. Wild populations of rodents contain natural genetic variation in the inhibition of reproduction by winter-like short photoperiod, and it has been hypothesized that this functional variation might be due in part to heritable variation in the numbers or location of gonadotropin releasing hormone (GnRH) neurons. A naturally variable wild-source population of white-footed mice was used to develop lines artificially selected for or against mature gonads in short, winter-like photoperiods. We compared a selection line that is reproductively inhibited in short photoperiod (Responsive) to a line that is weakly inhibited by short photoperiod (Nonresponsive) for differences in counts of neurons identified using in situ hybridization for GnRH mRNA. There was no effect of photoperiod, but there were 60% more GnRH neurons in total in the Nonresponsive selection line than the Responsive selection line. The lines differed specifically in numbers of GnRH neurons in more anterior regions, whereas numbers of GnRH neurons in posterior areas were not statistically different between lines. We compare these results to those of an earlier study that used immunohistochemical labeling for GnRH neurons. The results are consistent with the hypothesis that the selection lines and natural source population contain significant genetic variation in the number and location of GnRH neurons. The variation in GnRH neurons may contribute to functional variation in fertility that occurs in short photoperiods in the laboratory and in the wild source population in winter.

The relationship between cardiopulmonary size and aerobic performance in adult deer mice at high altitude.

Deer mice (Peromyscus maniculatus sonoriensis) populations in the White Mountains of Eastern California are found across a substantial range of partial pressures of oxygen (PO2). Reduction in PO2 at high altitude can have a negative impact on aerobic performance. We studied plastic changes in organ mass and volume involved in aerobic respiration in response to acclimation to high altitude, and how those changes are matched with aerobic performance measured by VO2,max. Adult deer mice born and raised at 340 m were acclimated at either 340 or 3800 m for a period of 9 weeks. Lung volume increased by 9% in mice acclimated to high altitude. VO2,max was also significantly higher under hypoxic conditions after high altitude acclimation compared with controls. Body mass-corrected residuals of VO2,max were significantly correlated with an index of cardiopulmonary size (summed standardized residuals of lung volume and heart mass) under both hypoxic and normoxic conditions. These data show that phenotypic plasticity in lung volume and heart mass plays an important role in maintaining aerobic performance under hypoxic conditions, and accounts for up to 55% of the variance in aerobic performance.

Retinal cone photoreceptors of the deer mouse Peromyscus maniculatus: development, topography, opsin expression and spectral tuning.

A quantitative analysis of photoreceptor properties was performed in the retina of the nocturnal deer mouse, Peromyscus maniculatus, using pigmented (wildtype) and albino animals. The aim was to establish whether the deer mouse is a more suitable model species than the house mouse for photoreceptor studies, and whether oculocutaneous albinism affects its photoreceptor properties. In retinal flatmounts, cone photoreceptors were identified by opsin immunostaining, and their numbers, spectral types, and distributions across the retina were determined. Rod photoreceptors were counted using differential interference contrast microscopy. Pigmented P. maniculatus have a rod-dominated retina with rod densities of about 450.000/mm(2) and cone densities of 3000-6500/mm(2). Two cone opsins, shortwave sensitive (S) and middle-to-longwave sensitive (M), are present and expressed in distinct cone types. Partial sequencing of the S opsin gene strongly supports UV sensitivity of the S cone visual pigment. The S cones constitute a 5-15% minority of the cones. Different from house mouse, S and M cone distributions do not have dorsoventral gradients, and coexpression of both opsins in single cones is exceptional (<2% of the cones). In albino P. maniculatus, rod densities are reduced by approximately 40% (270.000/mm(2)). Overall, cone density and the density of cones exclusively expressing S opsin are not significantly different from pigmented P. maniculatus. However, in albino retinas S opsin is coexpressed with M opsin in 60-90% of the cones and therefore the population of cones expressing only M opsin is significantly reduced to 5-25%. In conclusion, deer mouse cone properties largely conform to the general mammalian pattern, hence the deer mouse may be better suited than the house mouse for the study of certain basic cone properties, including the effects of albinism on cone opsin expression.

Effects of inbreeding on skeletal size and fluctuating asymmetry of Peromyscus polionotus mice.

Measurements of size and asymmetry in morphology might provide early indications of damaging effects of inbreeding or other genetic changes in conservation breeding programs. We examined the effects of inbreeding on size and fluctuating asymmetry (FA) in skull and limb bone measurements in experimental populations of three subspecies of Peromyscus polionotus mice that had previously been shown to suffer significant reductions in reproductive success when inbred. Inbreeding caused significant depression in mean size in two of the subspecies (P. p. rhoadsi and P. p. subgriseus), but the effects were smaller in the third (P. p. leucocephalus). Inbreeding caused an increase in FA of just one of eight bilateral traits in one subspecies (P. p. rhoadsi). Inbreeding depression in size was more easily detected than the effects of inbreeding on FA. FA may be much less sensitive to inbreeding and other stresses than are more direct measures of fitness such as reproductive output and body mass growth rate. Given the large sample sizes and statistical complexity required to assess changes to typically very small levels of FA in captive populations, FA will not likely provide a useful measure of inbreeding depression in captive populations.

Monogamous and promiscuous rodent species exhibit discrete variation in the size of the medial prefrontal cortex.

Limbic-associated cortical areas, such as the medial prefrontal and retrosplenial cortex (mPFC and RS, respectively), are involved in the processing of emotion, motivation, and various aspects of working memory and have been implicated in mating behavior. To determine whether the independent evolution of mating systems is associated with a convergence in cortical mechanisms, we compared the size of mPFC and RS between the monogamous prairie vole (Microtus ochrogaster) and the promiscuous meadow vole (Microtus pennsylvanicus), and between the monogamous California mouse (Peromyscus californicus) and the promiscuous white-footed mouse (Peromyscus leucopus). For both promiscuous mice and voles, the mPFC occupied a significantly larger percentage of total cortex than in the monogamous species. No significant differences were observed for the RS or overall cortex size with respect to mating system, supporting the convergent evolution of mPFC size, specifically. Individual differences in the mating behavior of male prairie voles (wandering versus pair-bonding), presumably facultative tactics, were not reflected in the relative size of the mPFC, which is likely a heritable trait. Given the importance of the mPFC for complex working memory, particularly object-place and temporal order memory, we hypothesize that the relatively greater size of the mPFC in promiscuous species reflects a greater need to remember multiple individuals and the times and locations in which they have been encountered in the home range.

The effects of diet composition on body fat and hepatic steatosis in an animal (Peromyscus californicus) model of the metabolic syndrome.

The objective of this research was to determine body composition, total fat content, fat distribution, and serum leptin concentration in hyperlipidemic (high responder, HR) and normolipidemic (low responder, LR) California mice (Peromyscus californicus). In our initial experiments, we sought to determine whether differences in regional fat storage were associated with hyperlipidemia in this species. To further characterize the hepatic steatosis in the mice, we performed 2 additional experiments by using a diet containing 45% of energy as fat. The body fat content of mice fed a low fat-diet (12.3% energy as fat) was higher than that of mice fed a moderate-fat diet (25.8% energy as fat). Total body fat did not differ between HR and LR mice. There was no significant difference between intraabdominal, gonadal, or inguinal fat pad weights. Liver weights of HR mice fed the moderate-fat diet were higher than those of LR mice fed the same diet, and the moderate-fat diet was associated with nonalcoholic fatty liver (NAFL). Mice fed the 45% diet had higher histologic score for steatosis but very little inflammatory response. Chemical analysis indicated increased lipid in the livers of mice fed the high-fat diet compared with those fed the low-fat diet. HR and LR mice had similar serum leptin concentrations. California mice develop NAFL without excess fat accumulation elsewhere. NAFL was influenced by genetic and dietary factors. These mice may be a naturally occuring model of partial lipodystrophy.

Paternal experience suppresses adult neurogenesis without altering hippocampal function in Peromyscus californicus.

Paternal care is rare among mammals, occurring in ≈6% of species. California mice (Peromyscus californicus) are unusual; fathers participate extensively in raising their young and display the same components of parental care as mothers, with the exception of nursing. Parenting is a complex experience, having stressful and enriching aspects. The hippocampus is sensitive to experience and responds to both stress and environmental enrichment with changes in structure and function. In rats, where females care exclusively for offspring, parenting is associated with suppressed hippocampal adult neurogenesis. Since this effect has been causally linked to lactation, it is unlikely that fathers would show a similar change. To investigate this issue, we examined adult neurogenesis in the hippocampus of California mouse fathers compared to males without pups and observed reduced adult neurogenesis. Similar effects were found in California mouse mothers. Next, we investigated whether behaviors linked to the hippocampus, namely, object recognition and novelty-suppressed feeding, were altered in fathers, and observed no substantial changes. During caregiving, suppressed adult neurogenesis does not appear to be related to changes in behaviors associated with the hippocampus, although it is possible that there are other effects on hippocampal function.

Cranial shape varies along an elevation gradient in Gambel's white-footed mouse (Peromyscus maniculatus gambelii) in the Grinnell Resurvey Yosemite transect.

Environmental variation over a species's range creates differing pressures to which organisms must adjust in order to survive. Taxa can respond to these pressures at population and individual levels, leading to localized phenotypic differentiation. Assessing the spatial distribution of phenotypic variation can illuminate how dramatically varying environmental factors shape phenotypes and may forecast a taxon's ability to adapt should conditions change. We characterized morphological variation along a transect sampled in the Grinnell Resurvey project to determine whether Gambel's white-footed mouse (Peromyscus maniculatus gambelii), a generalist taxon inhabiting the full elevational range of habitats in Yosemite National Park and surrounding areas, has responded morphologically to variation in its environment. We quantified variation in modern P. m. gambelii cranial shape using 2D generalized Procrustes analysis and Euclidean distance matrix-based geometric morphometrics. We performed multivariate regression of shape coordinates on elevation to test for environmental influences on shape within the principal geographic dimension of change along the transect. We observe a statistically significant correlation with shape on elevation for occlusal and lateral views of the cranium, explaining a small percentage of the overall variation in shape. Modern P. m. gambelii crania show a pattern of flexion in which the angle of the cranial base decreases at higher elevations. Results of EDMA parallel these findings, but highlight additional areas of the cranium that vary with elevation. Collectively, the patterns of variation detected suggest a biological response to the environment that warrants further study. This work lays the foundation for comparison with morphological data from historical specimens, which can address evolutionary scenarios generated from our findings, and for investigation of other taxa included in the resurvey project.

Cold acclimation in Peromyscus: individual variation and sex effects in maximum and daily metabolism, organ mass and body composition.

We studied metabolic and organ mass responses to thermal acclimation (7 weeks at 5 degrees C or 23 degrees C) in deer mice, Peromyscus maniculatus. Cold acclimation resulted in significantly higher maximal oxygen consumption in thermogenesis (V(O(2)max)) and daily mean oxygen consumption (V(O(2)mean)), an increase in the mass of most visceral organs, a lower absolute body fat and a marginally significant increase in hematocrit. The mass of digestive organs and body fat content differed significantly between sexes. Acclimation effects on fat content were more pronounced in females. Variation in heart and lung mass was positively correlated with V(O(2)max) and V(O(2)mean), while body fat content was negatively correlated with both traits. Nonetheless, a large fraction of the metabolic difference between cold- and warm-acclimated groups remained unexplained. Associations between traits at lower levels of biological organization measured here and whole-organism energetics remained consistent across acclimation temperatures, except for the correlation between kidney mass and V(O(2)mean), which was positive and significant in cold acclimation and negligible following warm acclimation. We conclude that: (1) V(O(2)max) and V(O(2)mean) share a common physiological basis that remains overall the same across acclimation regimes; (2) changes in these traits are associated primarily with changes in heart mass; and (3) male and female deer mice respond differently to thermal acclimation, possibly due to differences in reproductive allocation.

Gape and bite force in the rodents Onychomys leucogaster and Peromyscus maniculatus: does jaw-muscle anatomy predict performance?

Compared with the deer mouse, Peromyscus maniculatus, the grasshopper mouse, Onychomys leucogaster, exhibits modifications in its jaw-muscle architecture that promote wide gapes and large bite forces at wide gapes to prey upon large vertebrate prey. In this study, we determine whether jaw-muscle anatomy predicts gape and biting performance in O. leucogaster, and we also assess the influence of gape on bite force in the two species. Although O. leucogaster has an absolutely longer jaw, which facilitates larger gapes, maximum passive gape is similar in both species, averaging approximately 12.5 mm. Thus, when scaled to jaw length, O. leucogaster has a smaller maximum passive gape. These results suggest that predatory behaviors of O. leucogaster may not require remarkably large gapes. On the other hand, both absolute and relative bite forces exerted by O. leucogaster are significantly larger than those of P. maniculatus. The largest bite forces in both species occur at 5.0 mm of gape at the incisors, or 40% of maximum gape. Although bite force in both species decreases at larger gapes, O. leucogaster does maintain a larger percentage of maximum bite force at gapes larger than 40% of maximum passive gape. Therefore, although structural modifications in the masticatory apparatus of O. leucogaster may constrain gape, they may help to maintain bite force at large gapes. These results suggest that increases in gape differentially influence the length-tension properties of the jaw muscles in the two species. Finally, these results highlight the importance of considering the effect of muscle stretch on force production in comparative studies of bite force. As a first approximation, it appears that gapes of 40-50% of maximum gape in rodents optimizes bite force production at the incisors.

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.