Papers in this Special Feature were presented at a symposium on the social biology of rodents that was held in June 2001 at the 81st Annual Meeting of the American Society of Mammalogists. Our decision to host a symposium on this topic resulted from our realization that although rodents have played a vital role in research on social behavior, no recent summaries of the social biology of these animals were available. Given the number of biological disciplines, research strategies, and species that are relevant to this topic, a comprehensive review of rodent social biology was not possible. Instead, in structuring the symposium, we chose to focus on a subset of behavioral issues for which studies of rodents currently are providing exciting new insights. Topics selected for inclusion—communication, kin recognition, philopatry, and sociality—are timely and are of considerable interest to biologists studying a wide array of animal taxa. Thus, papers presented in the symposium reflect recent advances not only in our knowledge of rodent social biology but also in our conceptual understanding of animal social behavior.

In many species of rodents, sense of smell is the most important source of information about the social and nonsocial world. I selectively review the literature on chemical communication in this group, primarily as studied in the laboratory. I start with a discussion of the term “pheromone,” the types of chemical signals, and the mechanisms underlying responses to chemical signals. I discuss the chemical complexity of odors in rodents and the significance of multiple sources of odors. Efforts to chemically identify signals are reviewed briefly; some progress, especially with domestic house mice (Mus musculus domesticus), has been made, but new collaborations between chemists and biologists are needed. Three areas of current research are reviewed in detail: the role of the major histocompatibility complex in the production of body odors and the role of these odors in mate choice, the use of odors in kin recognition, and the functions of scent overmarking.

Significant advances have been made in understanding kin recognition as it pertains to nepotism (preferential treatment of kin) and mate choice (optimization of inbreeding and outbreeding). Yet complementary knowledge about how animals discriminate conspecifics on the basis of genetic relatedness remains unclear for most species. Because of the diversity of their scent sources and highly developed olfactory systems, rodents present a unique opportunity for examining chemical communication and kin recognition as a function of sociality. I review general processes of kin recognition and summarize mechanisms of recognition used by rodents. As a case study, I also examine recognition systems of ground squirrels, relating odor production and perception to differences in patterns of nepotism. Belding's ground squirrels (Spermophilus beldingi) produce ≥2 odors (from oral and dorsal glands) that correlate with relatedness (kin labels), and they are able to use these odors to make precise discriminations among their unfamiliar relatives. Thus, S. beldingi can recognize their distant female kin and male kin, even though these kin are not treated nepotistically (e.g., through cooperative territory defense or alarm call production). Furthermore, S. beldingi use these kin labels and recognition abilities to interact differentially with conspecifics as a function of relatedness. In ground squirrels and in other rodents, components of the kin recognition process (production of recognition cues, discrimination of these cues, and differential treatment of conspecifics) have evolved differentially among even closely related species. Kin recognition abilities in the absence of nepotism might reflect selection for inbreeding avoidance mechanisms, in addition to sex-biased natal dispersal. Together, data and the review demonstrate that recognition abilities cannot be predicted on the basis of sociality alone and suggest that comparative analyses, by multiple assays of discrimination, might be necessary to understand variation in the function of kin recognition within and across species.

The costs and benefits of philopatry (remaining at the natal nest) have been discussed in numerous papers. Nevertheless, there is still debate about the relative importance of factors that favor philopatry, which can result in the formation of social groups. The decision to remain at the natal nest can be examined by using models such as the delayed-dispersal threshold model, which takes into account risks of dispersal, probability of becoming established on a suitable territory, and probability of finding a mate. These factors, in turn, are influenced by ecological variables such as distribution of critical resources and population density. The often-cited conclusion from this and similar models is that ecological or social constraints promote philopatry whereas relaxation of the constraints result in dispersal. More recent theoretical approaches have included not only some of these ecological factors but also life-history traits (e.g., survival and age of maturation). Some of the latter models suggest that external constraints alone are inadequate to explain interspecific variation in group living. I review existing data to evaluate the relative importance of variables proposed to influence philopatry in rodents and argue that future studies may benefit from a broad approach that encompasses life-history and ecological factors, such as adult survival and territory quality.

Group living is an important component of the social biology of numerous rodent species. For the past 2 decades, efforts to understand the ecological factors associated with group living in rodents have focused on subterranean taxa, in particular African mole-rats of the family Bathyergidae. Comparative analyses of the habitats occupied by solitary and social bathyergids suggest that group living occurs when the combined effects of sporadic rainfall, hard soils, and patchily distributed food resources render energetic costs of burrow excavation prohibitive for lone individuals. To determine whether these variables were associated with group living in other subterranean taxa, we characterized ecological differences between a solitary and a social member of a phylogenetically independent lineage of subterranean rodents, the Ctenomyidae. Specifically, we quantified soil conditions and spatial distributions of food resources for 1 population each of the solitary Patagonian tuco-tuco (Ctenomys haigi) and the group-living colonial tuco-tuco (C. sociabilis), both of which occur in the Limay Valley of southwestern Argentina. Our analyses revealed that while members of both species occurred in relatively mesic mallin habitats, only C. haigi also occurred in arid steppe habitat. No differences in spatial distributions of food resources were found between study sites. Significant differences in soil conditions, however, were detected; soils from steppe habitat were significantly harder to penetrate than soils from mallin areas, suggesting that costs of burrow excavation may be higher for the solitary C. haigi. Thus, data from ctenomyids in the Limay Valley are not consistent with explanations for group living developed for bathyergid mole-rats. Although ecological comparisons of ctenomyids over larger spatial scales are required, our data suggest that interactions between ecology and group living in subterranean rodents may be more diverse than previously realized.

I identify 4 topics for future research. 1) Our knowledge of rodents from the developing world is limited, yet recent evidence suggests that many species are highly social. I concentrate on South American rodents to demonstrate the wealth of information available when we study these taxa and incorporate the findings into our current paradigms of social evolution. 2) Relatively little is known about proximate mechanisms that govern social systems. Research on the neuroendocrine basis of pair-bond formation and paternal care has provided valuable insights, but many questions remain. 3) Mounting evidence indicates that, in addition to genetics, social environment can play a major role in behavioral and physiological development. Studies on effects of early social environment would contribute to our understanding of ontogeny of individual differences in social species. 4) Recent evidence suggests a significant geographic variation in social characteristics of some rodent species. The extent and ecological correlates of such intraspecific variation merit further investigation.

Movements, ranging behavior, and social organization of the ricefield rat (Rattus argentiventer) were studied by radiotracking in a rice field in West Java, Indonesia. Home ranges were estimated by the minimum convex polygon method and were found to be significantly larger in the nonbreeding season than in the breeding season. During the breeding season, males had larger home ranges than did females (3.20–3.24 ha, as compared to 2.51–2.34 ha), but no sexual differences were found in the nonbreeding season. Overlap between home ranges occurred during the breeding season, especially for males. During the nonbreeding season, rats appeared to be nomadic, and nests seemed to be distributed at random within ranges. Males and females were never found to share nests during the nonbreeding season. Habitat use was investigated by recording percentage occupancy of the following habitat elements: rice paddies, banks, and vegetation. Totals of 63% and 60% of radiofixes were made in paddies during the rainy and dry seasons, respectively. Paddies were probably used for feeding, nest construction, and protection from predation.

Although multiple paternity has been observed in various groups of mammals with litter sizes >1, it has not been documented in wild ungulate populations. With the use of DNA microsatellites, we investigated multiple paternity in pronghorn (Antilocapra americana) at the National Bison Range. Females in this population twin every year, and we were able to assign paternity to 25 sets of twins sired in 1999 and 2000, eleven (44%) of which were fathered by different males. Complex female mate choice is the most likely explanation for the observed pattern; however, sperm competition might contribute if a female has mated multiple times. This study represents the 1st reported example of multiple paternity in a natural ungulate population and indicates that some aspects of mammalian mating systems still are not fully understood.

Feasibility of assigning parentage using variable microsatellite loci was assessed for 2 species of prairie dogs. Parentage was determined from 7 microsatellite loci for 46% of juveniles born during 1994 in a colony of Gunnison's prairie dogs (Cynomys gunnisoni), and for 53% and for 45% of juveniles born during 1996 and 1997, respectively, in a colony of Utah prairie dogs (C. parvidens). Frequency of multiple paternity estimated for Gunnison's (77%) and Utah (71% and 90%) prairie dogs was greater than that detected previously for black-tailed prairie dogs (5%–10%) but within the range reported for other ground-dwelling squirrels. Of the 84 adult females and 33 adult males present during 1994 in the colony of Gunnison's prairie dogs, 75 (89%) and 22 (67%), respectively, produced weaned offspring. Breeding success for Utah prairie dogs was relatively low in 1996 (45% for females and 32% for males) but increased in 1997 (80% for females and 81% for males).

Body mass is sexually dimorphic and varies seasonally for all 5 species of prairie dogs (Sciuridae: Cynomys), as shown by data from live individuals over a period of 28 years (1974–2001; n = 16,447 body masses). Sexual dimorphism (i.e., body mass of males as percentage of body mass of females) during the breeding season is 105% for black-tailed prairie dogs, 127% for Utah prairie dogs, 131% for Gunnison's prairie dogs, and 136% for white-tailed prairie dogs. Sexual dimorphism is minimal at the end of the breeding season, when exhausted males are thin and early-breeding females are heavy with pregnancy. Sexual dimorphism is maximal at weaning, when rested, well-fed males are heavy and females are emaciated from lactation. The most likely ultimate causation for sexual dimorphism among prairie dogs is sexual selection.

Carnivores are ecologically diverse mammals. Dietary adaptations vary from strict herbivory to pure carnivory. These adaptations have evolved independently several times within families, so evolution within the order is iterative. I used 2 databases to examine whether ecological adaptations have an effect on longevity or spatial distributions of fossil carnivores. Miocene hypercarnivorous species had, on average, the widest spatial distributions. Their temporal distributions were not significantly longer than those of other carnivores. Body size also had a significant interaction with maximum spatial distribution. Large-bodied carnivores, regardless of feeding adaptations, had larger geographical ranges, but did not have greater longevity. However, when all the data were pooled, temporal distribution had a significant correlation with spatial distribution. The position of hypercarnivores at the top of the food chain and their specialized adaptations for meat eating probably increased their extinction risk to the same level of more narrowly distributed, less specialized carnivores.

Increases in mesopredator populations due to habitat fragmentation may have cascading effects on prey (e.g., songbirds) and may increase competitive interactions within the guild. We compared micro- and macrohabitat selection of the Virginia opossum (Didelphis virginiana) between areas with and without removal of the raccoon (Procyon lotor), a putative competitor, in a fragmented habitat in north-central Oklahoma. Live trapping conducted in 1998–2001 resulted in 482 total captures of opossums. Vegetation was sampled in summer (April–August) around trapsites associated with 235 of those captures. Ordination analyses indicated that opossums shifted microhabitat use toward eastern red cedar (Juniperus virginiana) forest when density of raccoons was reduced. Geographic Information Systems analyses at the habitat-patch scale revealed no differences in habitat selection after raccoon reduction. Niche breadths were high for both species, but niche overlap varied by time and treatment. Overlap of habitat patches between raccoons captured preremoval and opossums captured postremoval was high, suggesting a shift in habitat selection. The niche shift by opossums and change in niche overlap between raccoons and opossums supported our competition hypothesis. We acknowledge our lack of replication, but note that our work represents the 1st report of experimental manipulation coupled with work at multiple scales to examine competitive relationships between these key mesopredators.

Time series data on cyclic vole populations in 1971–1999 in northern Sweden show a long-term decline in numbers from the early 1970s to the late 1990s. We tested the destructive sampling hypothesis predicting that previously unsampled (new) plots in remote areas would yield higher density indices than the density level of permanent plots in the early 1970s. In autumn 1999 we sampled both permanent (treatment) plots and new (control) plots. Density was not higher on new than on permanent sampling plots for any of the predominant vole species, Clethrionomys glareolus, C. rufocanus, and Microtus agrestis. It appears unlikely that destructive sampling has caused the observed decline in vole numbers.

White-tailed deer, Odocoileus virginianus, is the most important game species in Venezuela. Some populations are currently threatened by overhunting and habitat loss, making it necessary for implementation of conservation programs. In this study, we employed molecular phylogenetics and population genetics principles to identify conservation units of Venezuelan populations of this species and to provide recommendations for its management. We analyzed DNA sequences—730 base pairs of the mitochondrial control region—in 26 individuals sampled from the 3 subspecies present in Venezuela. Results revealed moderate levels of genetic polymorphism. In addition, evidence of significant population structure was found. Phylogeographic analyses showed 4 lineages with the nominal subspecies O. v. gymnotis appearing to be polyphyletic. A remarkable divergence among haplotypes from Venezuela and North America was revealed in phylogenetic analyses, the former comprising a monophyletic group. The observed divergence among haplotypes from Venezuelan and North American populations was, in most cases, higher than that observed among the latter and haplotypes of O. hemionus (black-tailed deer). This result suggests that Venezuelan white-tailed deer could be considered an evolutionarily significant unit. We interpreted the results obtained within the context of climatic changes since Late Pleistocene. In addition to ecological and morphological evidence, our data suggest that O. v. margaritae and O. v. goudotii populations, the 2 subspecies considered endangered, are clearly differentiated and should be recognized as geminate evolutionary units and protected as distinct groups, even though there is no clear support for elevating these subspecies to species rank, as proposed recently.

We documented cover characteristics at white-tailed deer (Odocoileus virginianus dakotensis) bedding and feeding sites on winter and summer ranges in the central Black Hills, South Dakota and Wyoming. Radiolocations of female (2,592; n = 73) and male (573; n = 12) deer were compared with 1,087 random locations. Characteristics of cover recorded at deer and random locations included basal area, point-centered quarter distance, density of tall shrubs and saplings, and available hiding (horizontal) cover. On winter range, females selected areas with lower levels of hiding cover than males, whereas habitats selected on summer range were similar. On winter and summer ranges, females bedded in areas with greater hiding cover than feeding or random sites. Three principal components (interpreted as hiding cover, thermal cover, and radiation cover) differed with respect to season and explained 78.5% of the variation in cover characteristics. In winter, males had higher scores for hiding cover, while females had higher scores for radiation cover. Conversely, during summer, females had higher scores for hiding and thermal cover. Results were compared to the reproductive-strategy (RSH) and predator-avoidance (PAH) hypotheses, which attempt to explain sexual segregation in ungulates. Univariate results indicated the sexes occupied habitats with similar cover characteristics on summer range but not on winter range. Therefore, we rejected the RSH for summer range but were unable to reject the RSH for winter range. Also, hiding cover was of greater importance to males on winter range and females on summer range. Based on these results, we rejected the PAH for winter range but were unable to reject the PAH for summer range.

We assessed seasonal variation in food habits of Indian giant flying squirrels (Petaurista philippensis) and food availability at Fushan Experimental Forest in Taiwan. Between July 1997 and December 1998, giant flying squirrels consumed at least 79 species-specific parts belonging to 30 plant species from 19 families. Castanopsis cuspidata was eaten most often (24.95% of annual diet), followed by Ficus superba (14.67%) and Glochidion acuminatum (12.18%). P. philippensis fed mainly on young leaves (27.7% of annual diet) and mature leaves (24.0%). Leaf parts (including buds, petioles, young leaves, and mature leaves) constituted 74.0% of the annual diet. Monthly and annual diet variation was obvious, and this was related to availability of species-specific plant parts. P. philippensis fed on mature leaves and petioles in winter; buds, young leaves, and fruit in spring; and young leaves and fruit in summer and autumn. They preferred buds, young leaves, flowers, and fruit to mature leaves and petioles. However, mature leaves and petioles seemed to be relied on when more favorable food items were scarce.

Live-trapping and radiotracking of American mink (Mustela vison) on 24 km of the River Thames, United Kingdom, revealed range sizes (male: 6.8 km, female: 2.7 km), numbers of other mink found in the range of each male (0.98 males, 1.95 females) and each female (1.18 males, 0.32 females), and range overlaps between dyads of males (88.2%), a male and a female (male: 32.6%, female: 69.1%), and females (66.3%). We used these data to estimate the potential energetic burden of conspecifics on each other. As a monthly average, cohabiting males and other females take an estimated 30% and 9%, respectively, of the total energy consumption by mink within the range of each female. Similarly, cohabiting males and females consume an estimated 21% and 40%, respectively, of the energy in each male's range. Intraspecific resource depression may force both sexes, especially males, to maintain larger ranges. We suggest that, in the case of American mink, intraspecific prey-resource competition affects the range sizes of both sexes.

Habitat use by members of a wild population of American mink (Mustela vison) was evaluated by continuous monitoring of individuals that were live trapped and radiotracked year round in the Upper Thames region, United Kingdom. Spatially lagged autoregressive models were used to investigate the relationship between population abundance and measured habitat variables. Resident mink were found in places characterized by rich tree cover, plenty of scrub, rank grasses, and especially abundant rabbits, and they avoided open habitat characterized by farming activities. These trends were not detected, however, in either transient adults or juveniles. The presence of the opposite sex did not appear to influence the presence of resident mink of the other sex. The single most important feature influencing the presence of resident mink was the size of rabbit warrens. Warrens were, overall, the most important den sites for mink, especially for breeding females. Because the distribution of rabbit warrens seemed to be strongly affected by riverside farmland management, this might eventually determine the distribution and local population growth of feral mink in the Upper Thames region.

Tracking tubes offer an efficient alternative to live-trapping for studies that require very large sample sizes. However, it is necessary to know how small mammals change in their tendency to enter tracking tubes with time. We measured this change in response in a region of boreal forest in northern Nova Scotia, Canada, in 1994–1998. Over 4 weeks, small mammals increased in their tendency to enter tubes, with no difference in response among species. Thus, when designing studies using tracking tubes, one needs to adjust for differences in duration—one cannot simply calculate a tracks/tube-night measure. We show how to statistically remove the effect of duration.

To understand the importance of plasticity of central processing organs (i.e., stomach and intestinal organs) to animals in the wild, field studies of changes in organs and their relationships with variable environmental conditions and energy demands are needed. We determined relationships among diet quality, indicators of energy demand, sizes of central processing organs, and rate of nutrient uptake in free-living white-footed mice (Peromyscus leucopus) in western Kentucky. Moderate winters and dry summers characterized the study site. In contrast to studies conducted in more extreme environments, we found limited differences in the sizes and function of central processing organs among seasons. Masses, but not lengths of intestinal organs or the rate of glucose uptake, differed among seasons. Heart and spleen masses, but not masses of the lungs, kidneys, or liver, also differed among seasons. Differences in the amount of food ingested and percentage of protein in the diet were likely contributors to seasonal differences in intestinal organ masses. Lactation was associated with heavier masses of all central processing organs and increased rate of glucose uptake, but not with heavier masses of vital organs. Greater size and function of intestinal organs during lactation was attributed primarily to high energy demand and secondarily to diet. We postulated that large changes in size and function of central processing organs in free-living mammals occur during intense, but not moderate, changes in energy demand and diet composition and are dependent on the type of energy demand incurred.

We examined the reliability of using nitrogen concentration of stomach contents from hispid cotton rats (Sigmodon hispidus) as an index of dietary nitrogen. Stomach contents of hispid cotton rats fed pelleted diets varying in nitrogen concentration were analyzed for stomach nitrogen. Regression analysis revealed a positive linear relationship between stomach and dietary nitrogen, but the relationship was not 1:1. Thus, inverse estimation of the regression equation can be used to adjust for a lack of a 1:1 ratio to obtain more reliable and accurate estimates of diet quality. Although we expected this relationship to be robust in its application to field studies, the pelleted diet model consistently underestimated dietary nitrogen during model evaluation experiments with natural forages. We conclude that the applicability of using nitrogen concentration of the stomach contents of cotton rats as an index to dietary nitrogen is dependent on the level of accuracy and precision required in estimating nitrogen concentration of foods consumed.

We compared energy intake of reproductive and nonreproductive female Brandt's voles (Microtus brandtii). Daily energy intake of pregnant voles was significantly higher than that of nonreproductive voles. Metabolizable energy intake increased after parturition and reached a maximum at the end of lactation (334.23 kJ/day), 323% of typical nonreproductive requirements. Moreover, pregnant females had significantly higher lipid mass than controls. During lactation lipid contents were lower than in controls. Over the course of lactation females decreased by 17.6% in body mass, suggesting that females have depleted their body reserve. Use of energy reserves and maximum metabolizable energy intake both were dependent on litter size. Our results indicate that Brandt's voles meet most of their energy demands for lactation by increasing food intake but also supplement that energy by using body reserves even before food intake is maximized.

Vocal individuality has been found in a number canid species. This natural variation can have applications in several aspects of species conservation, from behavioral studies to estimating population density or abundance. The swift fox (Vulpes velox) is a North American canid listed as endangered in Canada and extirpated, endangered, or threatened in parts of the United States. The barking sequence is a long-range vocalization in the species' vocal repertoire. It consists of a series of barks and is most common during the mating season. We analyzed barking sequences recorded in a standardized context from 20 captive individuals (3 females and 17 males) housed in large, single-pair enclosures at a swift fox breeding facility. Using a discriminant function analysis with 7 temporal and spectral variables measured on barking sequences, we were able to correctly classify 99% of sequences to the correct individual. The most important discriminating variable was the mean spacing of barks in a barking sequence. Potential applications of such vocal individuality are discussed.

Echolocation calls were recorded from Pteronotus quadridens flying in the field and in an enclosed space. In the field, search calls contained 1 or 2 harmonics. Patterns of call design show a segment of quasi–constant frequency (QCF_2nd-harmonic at 81–84 kHz), followed by a downward frequency-modulated (FM) component. The 2nd harmonic was always more intense than the 1st. Search, approach, and terminal phases of calls were described during hunting sequences of P. quadridens. The transition between call phases was characterized by monotonic variations in some acoustic parameters, including a decrease in call duration and an increase in repetition rate, bandwidth, and slope of the FM component. We also analyzed calls emitted by bats flying in confined spaces that consistently contained 3 harmonics, of which the 2nd harmonic contained the greatest energy. The values of call duration were shorter and bandwidth was higher than values characterizing calls emitted during the search phase in the field.

We studied the effects of prenatal protein malnutrition on visual discrimination learning in males of Petaurus breviceps using a Lashley box. Gliders were malnourished or adequately nourished during the prenatal period when their mothers received diets containing low (8%) or adequate (32%, controls) amounts of a protein (casein) throughout pregnancy. The mean weight of adequately nourished neonates was significantly higher (0.47 g ± 0.05 SE) than malnourished neonates (0.31 ± 0.02 g). Animals were first trained to discriminate between black or white stimulus cards affixed to 2 hinged doors. Second, cards containing horizontal or vertical stripes were presented. Finally, cards containing a circle or a square were presented. Gliders received food if they pushed through the door with the correct stimulus card and jumped onto a vertical platform. Malnourished (low-protein) subjects made significantly more errors on all tasks. Analyses of covariance indicated that differences in performance were not a function of body weight.

A new species of murine rodent is described from the Mt. Kitanglad Range of north-central Mindanao Island, Philippines. It is the 2nd known species of Limnomys, a genus endemic to Mindanao. The new species differs from Limnomys sibuanus in having a relatively longer tail, larger hind feet, softer pelage with grayish-white ventral coloration, larger skull with a longer rostrum, larger auditory bullae, larger molars, and other cranial and dental features. The new species apparently is nocturnal, omnivorous, and terrestrial-scansorial in habit. The 2 species of Limnomys have overlapping elevational ranges on Mt. Kitanglad and are known to occur syntopically at 1 locality. All 11 species of murines native to Mindanao occur in the Mt. Kitanglad range, and 2 are local endemics. The local occurrence of congeners in 4 genera suggests a repeated pattern of vicariance speciation and dispersal among highland areas of Mindanao.

Preble's meadow jumping mouse (Zapus hudsonius preblei), a federally listed threatened subspecies, and the western jumping mouse (Zapus princeps princeps) typically occur parapatrically but in some areas may be syntopic. Field differentiation between the taxa is difficult so we investigated the use of cranial characteristics as a basis for identification. We developed a discriminant function conducted on the means of repeated measurements to distinguish between the taxa from an initial sample of 105 specimens (n = 71 Z. p. princeps and n = 34 Z. h. preblei). We found that measurement error can contribute significantly to erroneous reclassification of specimens when only a single measurement set is used. Use of only presence or absence of the anterior median toothfold of M3 is not a reliable method for distinguishing between the subspecies. We used the discriminant function to identify 8 of 16 specimens collected in southeastern Wyoming as Z. h. preblei.

Eutherians give birth to relatively large infants. Females have larger pelves than males, though males typically have larger nonpelvic bones. Pelvic dimorphism is an adaptation for parturition. Metatherian females, who give birth to markedly small infants, should not show pelvic adaptations for parturition and should have smaller pelves than males. Adult females and males of the Virginia opossum, Didelphis virginiana, are compared. Males are significantly larger than females in absolute size for 14 of 16 pelvic and 8 nonpelvic measures. Sexes did not differ significantly for lower iliac length and posterior space of midplane. However, relative lengths for females are significantly larger than for males for these 2 measures and for the pubis. These pelvic dimorphisms in opossums cannot be adaptations for parturition. These dimorphisms result from asynchrony among skeletal elements in growth cessation and sexual differences in adulthood growth.