African giant pouched rats (Cricetomys) are native to tropical Africa, where they range from Senegal and Gambia east across West Africa and the Congo Basin to the Indian Ocean coast of East Africa. Ecological niche models show that Cricetomys species differ in their invasive potential. Although neither of the presently recognized Cricetomys species appears to have genuinely broad distributional potential in North America, models predict that C. emini would have extremely restricted distributional potential, whereas C. gambianus would have a broader potential across the southeastern United States.

Modern demographic models in conservation biology incorporate demographic and environmental stochasticity. Variability in vital rates, relationships among vital rates, and relationships between vital rates and the environment can affect projections from demographic models. We used a 30-year data set to estimate monthly mass-specific survival in hispid cotton rats (Sigmodon hispidus). Survival probabilities were estimated for both sexes and 3 mass classes using a multistate model and program MARK. We tested for differences in monthly survival among 3 mass classes, relationships between survival and environmental variables, and autocorrelation in survival. We found lower survival in summer for the smallest and medium mass classes. Survival in the largest mass class was positively autocorrelated during February and March, indicating that winter survival in large cotton rats is characteristic of a season, but that other monthly survival rates could be treated as unrelated to each other or to environmental variables.

Vole population dynamics are well characterized in Europe, Fennoscandia, and Japan, but long-term studies of vole dynamics in North America are rare and much needed. Summer relative abundance of southern red-backed voles (Clethrionomys gapperi) fluctuated periodically during 22 years (1983–2004) at the Holt Research Forest, Maine, suggesting an outbreak or cyclic dynamic. The time series had a 4.4-year period, and 2nd-order lagged relative abundance was significant in an autoregressive model. This was consistent with vole series in Fennoscandia and Japan. Above-average white pine (Pinus strobus) seed crops preceded C. gapperi peaks in 4 of 5 peak phases. In a model including P. strobus seeds, both seeds and 2nd-order lagged abundance terms were significant. Relative importance of P. strobus seeds to this omnivore needs to be established. High and low phases of C. gapperi corresponded with those of sympatric white-footed mice (Peromyscus leucopus). The 2nd-order autoregressive process, 4.4-year periodicity, and synchrony with P. leucopus indicated the C. gapperi dynamic was cyclic. This was a unique finding for the northeastern United States.

The quality of the habitat patch in which individuals reside may influence demographic processes, thus affecting social organization. We manipulated the risk of predation and food availability to test the hypothesis that high patch quality decreased the propensity to disperse, increased the likelihood of social units becoming groups, and increased overall group size in prairie voles (Microtus ochrogaster). Prairie voles are socially monogamous, at least in part of their range, and they display varied social organization within a population, including groups (containing a breeding pair and at least 1 additional adult of either sex), male–female pairs, and single females. Our results indicated that the likelihood of dispersal from high-quality patches was significantly less than from low-quality patches. Dispersers also were significantly more likely to settle in similar or higher quality patches than the ones in which they were originally released. These patterns were primarily due to the dispersal of young males. Although the proportion of social units that were groups appeared to decrease with lower patch quality, the difference in social organization among patch types was not statistically significant. The total number of founding voles and founding males per social unit residing in the highest quality patches at the end of the study were significantly greater than in the lowest quality patches. Thus, under the conditions of our experiment, patch quality affected dispersal and group size but not the tendency to form groups.

Dead wood is important to small mammals and is hypothesized to be used as travel paths. We evaluated the likelihood of different a priori models regarding sex- and season-specific differences and if quantity of wood in the environment influenced path selection of 41 Townsend's chipmunks (Tamias townsendii) in coniferous forests of western Oregon with the spool-and-line method using an information-theoretic approach. On average, 50% (SD = 4%) of the surficial portion of a chipmunk's path was associated with downed wood and 79% (SD = 10%) was on top of logs. Chipmunks disproportionately selected paths with downed wood relative to its availability and the model indicating that quantity of wood in the environment influenced path selection was 22.6 times more likely than the null model. At average wood densities (paths with 26% wood), a chipmunk was 3.0 times more likely to select locations with downed wood than locations without downed wood (95% confidence interval [95% CI] = 2.5–3.5). Furthermore, chipmunks selected wood that averaged 1.2 times larger in diameter than randomly available wood (95% CI = 1.1–1.3). Our findings document that Townsend's chipmunks preferentially use downed wood and we hypothesize that downed wood may influence fitness or survival of individual chipmunks.

Several behavioral studies of large, gregarious, and sexually dimorphic macropods have shown that males form dominance hierarchies and large males have the highest reproductive success. The bridled nailtail wallaby (Onychogalea fraenata) is a smaller and strongly sexually dimorphic macropod, but is also highly solitary and males do not form dominance hierarchies that are maintained temporally or spatially. Genetic studies of paternity have shown that large males are the most reproductively successful and only one-quarter of males sire offspring at any one time. The aim of this study was to investigate the tactics that males adopt to secure access to females at the time of estrus and to investigate whether females can influence which males have access to them. This study was conducted using 2 wild, free-ranging populations of bridled nailtail wallabies. Females in estrus were located and observed, and the total number of males present, the relative weight rank of each male, and interactions between individuals were recorded. Females showed a preference for large males and incited male–male competition when the group of males present was large. Unlike other dimorphic macropods, fights among males were rare and were restricted to males of similar size. Large males gained access to females by guarding and following them closely and threatening other males who attempted to gain access. Smaller males spent less time with females, suggesting that small males may leave multimale groups in an attempt to locate unguarded females. Given the solitary nature of this species and the lack of a stable dominance hierarchy to influence male reproductive success, mate searching and mate guarding may be important male reproductive tactics in this species.

Movements, distribution, and roosting requirements of most species of temperate-zone bats in autumn are poorly understood. We conducted the 1st radiotelemetry study of autumn migrations and prehibernation roost selection of bats in western North America. Big brown bats (Eptesicus fuscus, n = 55) in the Poudre River watershed, Colorado, moved from low-elevation summer ranges to high-elevation locations in autumn, where they roosted in rock crevices during the period leading up to winter hibernation. We characterized rock crevices used as roosts in autumn at these higher elevations at microhabitat and landscape scales. We used logistic regression combined with an information theoretic approach to determine which variables were most important in roost selection. At the microhabitat scale, autumn roosts were higher to the ground above and below the exit point and were in deeper crevices that had more constant temperatures than randomly selected crevices. At the landscape scale, aspect of the hillside was important, with autumn roosts typically facing north–northwest. Autumn roosts fell into 2 categories: those used for a few days (transient roosts) and those used for ≥7 days and presumed to be hibernacula. Temperature regimes in the presumed hibernacula appear to provide optimal conditions for use of winter torpor, whereas transient roosts may offer passive rewarming and energy savings for bats still active in early autumn. Elevational segregation of sexes also was documented in our region, with a preponderance of females found at lower elevations and males at higher elevations in summer. Sex ratios at higher elevations became even in autumn. Use of short elevational migrations and selection of hibernation sites in rock crevices may be a common overwintering strategy of insectivorous bats of western North America.

Fisher (Martes pennanti) populations may respond numerically to fluctuating hare populations, although uncertainty remains about this contention. We analyzed livetrapping data from Ontario (448,654 trap nights) that contained trend information for fishers, lagomorphs (snowshoe hares [Lepus americanus] and eastern cottontails [Sylvilagus floridanus]), and North American porcupines (Erethizon dorsatum) for 1995–2004. The data encompassed a synchronous fluctuation of lagomorphs and porcupines. Fisher populations appeared to have a delayed, positive numerical response with a lag of 2 years to changing lagomorph abundance, but they also appeared to switch to an unmeasured, alternate prey during the lagomorph low. This finding suggests that fishers are generalist predators that should have a stabilizing effect on the predator–prey system.

Introduced lake trout (Salvelinus namaycush) threaten the native cutthroat trout (Oncorhynchus clarki bouvieri) population in Yellowstone Lake, Yellowstone National Park, Wyoming. A reduction in availability of cutthroat trout may affect fish predators negatively, especially during spawning season. In this study we examined the importance of spawning cutthroat trout to a fish specialist—the North American river otter (Lontra canadensis). We measured scent-marking rates at 87 otter latrine sites on Yellowstone Lake and its tributary streams throughout the cutthroat trout spawning period and identified prey in scats collected at these sites. Based on fecal deposition rates, otters were more active on spawning streams and less active on Yellowstone Lake during the height of cutthroat trout spawning, with a return to elevated activity on the lake after spawning had ended. Cutthroat trout appeared to be the most common prey item in otter scats throughout the study, based on 515 samples identified to the family level and 110 samples analyzed to the species level. Overall, trout occurred in 72% and longnose suckers (Catostomus catostomus) in 43% of otter feces. Suckers were more prevalent than trout in otter scats only on tributary streams, toward the end of cutthroat trout spawning. Introduced lake trout, which inhabit deep water and are largely inaccessible to otters, occurred in less than 5% of otter scats. River otters in the Yellowstone Lake ecosystem appear to be relatively restricted in their diet and are heavily dependent on cutthroat trout. Our findings suggest that continued declines in the cutthroat trout population could negatively impact otters, potentially disrupting their role in linking aquatic and terrestrial ecosystems in and around Yellowstone Lake.

The round-tailed muskrat (Neofiber alleni) is a species of conservation concern that represents a monotypic genus with a restricted geographic distribution. Few data exist on space use to assist conservation efforts for this enigmatic species. We radiotracked 23 round-tailed muskrats in isolated freshwater wetlands in central Florida. Muskrats used lodges for diurnal rest sites when wetlands were inundated but switched to burrows during a seasonal dry period. Individuals shifted use among 10–15 rest sites within their home ranges. Body mass affected patterns of rest-site use; heavier individuals used fewer rest sites and were more likely to have a preferred site compared to lighter individuals. Estimates of home-range size averaged 1,263–2,071 m² and were at least an order of magnitude greater than existing estimates but less than expected given the body size of the species. Males may sometimes occupy nonoverlapping home ranges as predicted by theory. Small wetlands can support subpopulations of the round-tailed muskrat, which is a dispersal-limited species that exhibits metapopulation dynamics among wetlands.

Knowledge of foraging ecology of endangered mammals is often based on limited data because of logistical constraints of accessing animals, their stomach contents, or fecal samples. Here we use a stable isotope approach to examine feeding habits of a rare mammal, gaining insights over a greater temporal scale than a traditional fecal analysis would allow, and ameliorating some of the constraints of reduced sample sizes that can limit studies of mammalian foraging ecology. We focus on the endangered pygmy raccoon (Procyon pygmaeus), an endemic species from Cozumel Island, Mexico. Raccoons are thought to be omnivorous based on studies in the temperate zone, yet few dietary analyses have been conducted on raccoons in the tropics. Using hair and blood samples obtained from trapping over 3 years (2001–2003) and at 3 localities, the feeding habits of this species were examined based on the isotopic ratios of nitrogen (δ¹⁵N) and carbon (δ¹³C). Feces also were collected to supplement and compare to isotopic data. Both isotopic data and scat analyses suggest an omnivorous diet specialized on crabs, which constituted >50% of the diet, followed by fruits and insects. Hair and blood samples did not significantly differ in carbon or nitrogen isotopic ratios and we found no age- or sex-related variation. Although we observed subtle spatial and temporal variation in diet, both stable isotope and fecal analyses emphasize the dominance of crabs across these scales.

We investigated growth and determinants of adult female body size in brown bears (Ursus arctos) in 2 study areas in Sweden. Scandinavian female brown bears reached 90% of their asymptotic size at 4.1–4.7 years. Four factors were considered in our analysis of the determinants of adult female size: annual food conditions, population density, multilocus heterozygosity, and yearling body size. Our results suggest that individual body size variation of female brown bears is negatively related to density-dependent factors and positively correlated to density-independent fluctuations in the environment. Density-dependent factors may operate by increasing competition for food, resulting in a decrease in body size. Food resources for brown bears in Sweden fluctuate annually in the boreal forest and influence individual body size. Multilocus heterozygosity and yearling body size were not important in explaining adult size, because initially smaller individuals show compensatory growth when experiencing good food conditions.

Palms (Arecaceae) are a dominant element within the neotropical plant community and because they substantially contribute to the overall and year-round fruit availability they are considered a key resource for frugivores, particularly for peccaries. Similarly, peccaries (Tayassuidae) are a dominant element within the neotropical mammal community. Their evolution of a strong mastication apparatus, unique interlocking canines, patterns of movement, and foraging ecology are viewed as adaptations to exploit hard seeds, particulary palm seeds. But how strong are the interactions between peccaries and palms, and what are the ecological ramifications? This review synthesizes over 76 papers, published between 1917 and 2004, which revealed that peccaries consumed fruits from 46 palm species, 73% of whose seeds were destroyed after ingestion. Furthermore, peccaries disperse palm seeds; eat flowers, seedlings, and roots; and trample seedlings. Thus, peccaries affect the spatiotemporal distribution and demography of palms. Local extinction of peccaries resulted in dramatic changes in the forest ecology. New conservation strategies are required to protect peccaries and prevent negative cascading effects.

Species that exist in naturally fragmented subpopulations can maintain long-term viability through interpopulation connectivity and recolonization of suitable habitat. We used radiotelemetry to study movements of 3 herds of bighorn sheep (Ovis canadensis) that recently colonized previously unoccupied parts of western Montana. These herds also provided a unique opportunity to compare resource-selection patterns in newly colonized habitats, and we used logistic regression in a global information system framework to generate predictive models for females in each herd. We detected relatively long (19- to 33-km) extra–home range movements by males in all 3 herds, and connectivity with nearby bighorn and domestic sheep herds. An information-theoretic approach to model selection revealed greater differences in resource selection among herds than anticipated. Initial evaluation of resource-selection models by resubstituting data showed excellent predictive accuracy (P ≤ 0.002), but testing models across sites gave mixed results, and in many cases, poor fit (0.001 ≤ P ≤ 0.960). High vagility of males and variability in resource selection by females suggests increased potential for future recolonization and connectivity.

Fire and bison (Bison bison) are thought to be historically responsible for shaping prairie vegetation in North America. Interactions between temporal–spatial distributions of bison and prescribed burning protocols are important in current restoration of tallgrass prairies. We examined dynamics of bison distribution in a patch-burned tallgrass prairie in the south-central United States relative to bison group size and composition, and burn age and temporal distribution. Bison formed larger mixed groups during summer and smaller sexually segregated groups the rest of the year, and bison selected dormant-season burn patches in the 1st postfire growing season most often during spring and summer. Large bison herds selecting recently burned areas resulted in seasonally variable and concentrated grazing pressure that may substantially alter site-specific vegetation. These dynamics must be considered when reintroducing bison and fire into tallgrass prairie because variable outcomes of floral richness and structural complexity are likely depending on temporal–spatial distribution of bison.

The morphological identification of cryptic rodent species has historically been problematic. At best, many cryptic species have been identified by chromosomal differences. However, to study the life histories of such rodent species, there is a need for a molecular technique for cryptic species identification that does not involve destructive sampling. In this manuscript we examine mitochondrial DNA (mtDNA) cytochrome-b genetic variation in 2 cryptic murid rodent species, the red veld rat (Aethomys chrysophilus) and the Tete veld rat (A. ineptus) from southern Africa. Phylogenetic and phylogeographic analyses of these sequences showed reciprocal monophyly between populations of the 2 species in southern Africa, but no support for monophyly of A. chrysophilus from southern and eastern Africa. This suggests that the analysis of mtDNA can be used to distinguish these 2 sister species in southern Africa. However, these results need to be investigated further by DNA analyses of type specimens, topotypical material, or both from adjacent localities.

Few studies have examined thalamic development in the embryonic mammalian brain. For a better understanding of the development of the thalamus, the time sequence of the location of neurons in thalamic nuclei was examined in a developing marsupial embryo and fetus. To accomplish this, uptake of tritiated thymidine was examined in the gray short-tailed opossum (Monodelphis domestica) from 3 days before birth through 16 days after birth. Total neurogenesis was optimal 4 days after birth. Plots of the labeled cells revealed that thalamic and cortical development are almost concurrent, similar to the time course of eutherian mammals. A consistent time gradient of proliferation in dorsal thalamus was not found. Rather, sectors of the dorsal thalamus showed significant differences regardless of day of development. The dorsal sector showed more neurogenesis than the ventral sector, caudal more than rostral, and lateral more than the medial sector. An analysis of individual nuclei of dorsal thalamus showed no significant differences in amount of neurogenesis with day of injection.

In northern Greece, European ground squirrels or sousliks (Spermophilus citellus) construct complex burrow systems by scratch-digging behavior. The present study investigated the presence of anatomical characters related to digging in the forelimb of S. citellus. The forelimb of 3 preserved specimens was dissected and several qualitative and quantitative variables on selected muscles were collected. In addition, selected osteological variables and indices were calculated in a sample of 207 sciurid postcrania representing 14 burrowing and nonburrowing genera. Both analyses showed that the forelimb of S. citellus was characterized by enlarged and powerful shoulder retractors, well-developed arm retractors with distal insertions upon a relatively robust humerus, enlarged elbow extensors associated with a long olecranon, and dominant pronators and carpal and digital flexors. Similar morphology is also encountered in other semifossorial mammals, indicating significant adaptations to scratch-digging behavior. However, the characters examined designate a more compromised morphology, associated with the generalized postcranium of sciurids. On the other hand, S. citellus exhibits a more specialized forelimb morphology, compared to that of other marmotines, for a semifossorial way of life, in association with the subgeneric derived morphology, lack of social interactions, and exploitation of a habitat with harder soils.

North American desert rodents in the family Heteromyidae live in an unpredictable environment characterized by extremes in temperature and food availability; therefore, the ability to hoard food is a vital adaptation. Although much laboratory research has investigated food-hoarding tactics of heteromyid rodents, data from natural systems are scarce. We used a combination of fluorescently labeled seeds and observations of focal individuals to evaluate food-hoarding behavior in wild Merriam's kangaroo rats (Dipodomys merriami) living in different competitive environments. There was considerable individual variation within populations in the tendency to larderhoard seeds in a burrow versus scatterhoard seeds in widely dispersed locations. However, Merriam's kangaroo rats living in a system where competitors were predominately conspecific scatterhoarded more than those living in a system where conspecifics were less abundant and heterospecific competitors also were present. Also, pilferage was more common between Merriam's kangaroo rats than across species. Comparisons of food hoarding between communities with different species composition indicate that intraspecific variation in behavior may be associated with variation in competitive environments.

Comprehensive morphometric analysis of osteological and necropsy specimens indicates that blunt heads and wide jaws, both of which create a more circular mouth opening and thus improve water flow for suction feeding, are common in Odontoceti and found in all families except freshwater river dolphins (Platanistoidea), which are exclusively long-snouted. Mandibular bluntness, here termed amblygnathy, correlates with dental reduction in odontocetes; there is a further association of reduced dentition with increased body length. Examination of quantitative data reveals that many odontocetes, especially globicephaline delphinids, have a blunter cranial profile (partly from facial musculature and other soft tissues) and fewer exposed teeth for grasping prey than is generally supposed, especially when the researcher relies solely on examination of skeletal materials. Numerous teeth present in cleaned skulls and jaws remain unerupted even in adults, as verified by necropsy tooth counts, rendering tooth counts from museum specimens unreliable indicators of in vivo conditions. Amblygnathy and smaller, rounder mouth openings correlate with other anatomical, ecological, and behavioral traits associated with suction feeding. It is likely that odontocete suction ingestion evolved independently in multiple lineages from use of suction to transport grasped prey in long-jawed ancestors, with consequent loss of the grasp and transport step as prey are sucked directly into the oral cavity or oropharynx.

Gestation and lactation are times when females mammals have high energy expenditures. There is a strong relationship between a female's physiological state and her food requirements and absorption of nutrients and energy needs. The aim of these morphometric studies on the alimentary canal in control (nonpregnant and nonlactating) and pregnant and lactating females of the common shrew (Sorex araneus) was a comparative analysis of changes in the mucosa of subsequent regions of the digestive tract. The physiological state of the female (during gestation and lactation) was reflected by the anatomic structure of some sections of the alimentary canal. The length of the esophagus increased by 24.3% in females in late gestation relative to the control group, but its length in lactating females was close to that of females in the early stage of gestation. The duodenum of females in late gestation or during lactation was 39.8% longer than in control females. Length of the mesenteric intestine in pregnant and lactating females also increased 12.1% on average compared with the control group. The greatest changes in the alimentary canal occurred in the mucous membrane of the small intestine in pregnant and lactating females. The longest villi were found in the duodenum and the proximal part of the mesenteric intestine in these groups. The effect of these changes is an increase of digestive and absorptive area of the duodenum and proximal and intermediate regions of the mesenteric intestine. The entire area of the mucous membrane of the small intestine was 49.1% greater in lactating females than in control females and those at the early stage of gestation, and 20.5% higher in comparison with females in late gestation.

We assessed variation in length, width, density, and surface enlargement factor of papillae; rumen and intestinal digesta weight; intestinal length; and intestinal tissue weight of reproductive and nonreproductive female white-tailed (Odocoileus virginianus) and mule deer (O. hemionus) using similar burned and unburned habitat. Deer were collected from study areas in Custer and Pennington counties, South Dakota, in and adjacent to a wildfire burn. Length of papillae and enlargement factor of papillae surface of white-tailed deer and mule deer were greater in burned than unburned habitat, and dry weight of rumen digesta of white-tailed deer was greater in unburned than burned habitat. Papillae surface enlargement factor, papillae length and width, and dry weight of rumen and intestinal digesta were greater for lactating than nonlactating white-tailed deer. Papillae density, intestinal digesta dry weight, and intestinal length were greater in mule deer than white-tailed deer. The mosaic pattern of the burned habitat was beneficial at the mucosal level for white-tailed and mule deer within 3 years postfire. Enhanced papillae morphology of lactating compared with nonlactating and pregnant deer was attributed to high nutritional demands associated with reproductive status. Papillae density may be more important in mule deer because of feeding strategy (concentrate selector–intermediate feeder) compared to white-tailed deer (concentrate selector), whereas greater rumen digesta dry weight and intestinal length may be directly associated with size.

Aggregations by 3 species of dolphins (the bottlenose dolphin [Tursiops truncatus], the short-beaked common dolphin [Delphinus delphis], and the long-beaked common dolphin [Delphinus capensis]) and California sea lions (Zalophus californianus) were investigated in Santa Monica Bay, California. Groups were followed and observed during 201 boat-based surveys conducted in 1997–2001 documenting that sea lions were aggregated in 18.6% of the sightings with bottlenose dolphins (150 bottlenose dolphin sightings) and in 45.9% of the sightings with 1 of the 2 species of common dolphins (98 common dolphin sightings). Aggregations of bottlenose dolphins and sea lions were observed in inshore (<500 m from shore) and offshore (>500 m) waters, whereas common dolphins and sea lions were observed only in offshore waters. These aggregations were often recorded feeding near escarpments and submarine canyons, showing a striking preference for these bathymetric features. The results show that sea lions spend a significant amount of time following dolphins, sea lions initiate aggregation and departure from dolphin schools, these aggregations occur more often than is expected by chance, and no aggressive behavior between sea lions and dolphins was ever observed at or near the surface. I argue that sea lions may take advantage of the superior food-locating abilities of dolphins. This paper provides the 1st detailed description of mixed-species aggregations and habitat usage by 3 dolphin species and sea lions.

We used program CAPTURE to analyze 1,535 capture–recapture data sets from 33 species of small mammals for sources of variation in capture probabilities and to characterize species-specific responses to mark–recapture trapping. Program CAPTURE contains 8 models to account for all possible combinations of heterogeneity (h), behavioral response (b), and time (t) effects on capture probabilities. Model M_o (equal capture probabilities) was selected most often by CAPTURE as the most appropriate model for small sample sizes, reflecting the difficulty of detecting unequal probabilities of capture in small data sets. Heterogeneity in capture probabilities among individuals was evident in larger data sets for all species of small mammals examined. Strong behavioral responses were detected in several species. Sylvilagus, Tamias amoenus, Glaucomys sabrinus, Perognathus longimembris, and Microtus ochrogaster were trap shy, whereas Tamias striatus, Tamias townsendii, Spermophilus richardsonii, Perognathus parvus, Reithrodontomys megalotis, Peromyscus maniculatus, Peromyscus truei, Sigmodon hispidus, Microtus californicus, and Microtus pennsylvanicus were trap prone. Time effects were not apparent for most species except where they were accompanied with heterogeneity. Murid species had the highest estimated probabilities of capture, followed by heteromyids, sciurids, and leporids in decreasing order. Estimates of abundance from model M_h were significantly greater than those from models M_b, M_o, and M_t, which likely reflects the negative bias of these latter estimators.

Small mammals spend a majority of their lives in shelter sites such as belowground burrows. Understanding temporal patterns of burrow use would provide valuable information about the influence of physiological and environmental factors on activity patterns. To examine patterns of burrow use, we developed a system that automatically monitors activity of deer mice (Peromyscus maniculatus) at artificial burrows in tallgrass prairie. The automatic activity-monitoring system is composed of a passive integrated transponder (PIT)–tag transceiver that reads the identification tags of individuals and an infrared trail monitor that confirms movements in and out of artificial burrows. We PIT-tagged and monitored nightly activity of >90 deer mice on Konza Prairie Biological Station in northeastern Kansas from July 2003 to December 2004. The monitoring system allowed us to examine individual variation in activity relative to sex, parental attendance, and early exploration by young. We also discuss advantages and disadvantages of the system compared to other activity-monitoring techniques, so that researchers might develop similar systems for other study species.