Desert rodent communities are model systems for understanding the effects of environmental severity and pulsed resources on community assembly, structure, and dynamics; however, the unique characteristics of arid ecosystems require long-term studies to reveal patterns and underlying mechanisms. The 3 papers in this Special Feature demonstrate the novel insights that can be gained from long-term (>10 years) ecological research on desert rodent communities.

Population fluctuations of 13 rodent species in 5 habitats were monitored for 16 years in the central Negev Desert, Israel. Analysis of factors affecting population dynamics of 9 common and abundant species demonstrated that densities of most rodent species in the late summer, after the end of reproduction, were determined mainly by total precipitation during the previous rainy season. Rodent densities in the winter, before the reproductive season, were determined mainly by their densities in the previous (late summer) season. Rodent populations in dry river beds (wadi) demonstrated either no or negative correlations with total rainfall, suggesting episodes of population crash due to flash winter flooding. For all species occurring in more than 1 habitat, densities, at least in some habitats, were correlated with their contemporary densities in other habitats. For these species, processes of density-dependent habitat selection were indicated using isodars analysis. Generally, population dynamics of desert rodents were determined by the complex interactions of extrinsic (rainfall) and intrinsic mechanisms but were modified by density-dependent habitat selection.

Arid systems are characterized by spatiotemporal variability in resources and, as such, make ideal systems for examining the role of resource limitation in the long-term dynamics of populations. Using 28 years of data, we examine the long-term relationships of 3 guilds of desert rodent consumers with precipitation and primary productivity in a changing environment. Lags in rodent response to precipitation increased with increasing trophic level over the entire time series, consistent with resource limitation. However, we found that consumer–resource dynamics are complex and variable through time. Precipitation exhibited increasing influence on both primary producers and consumers in this system over time. Experimental evidence suggests that reorganization of community composition, coincident with environmental change, likely explains some of the increasing influence of precipitation. Additional, indirect evidence suggests some role for increasing shrub density and changing precipitation regimes. Results from our long-term study demonstrate that the global phenomena of changing precipitation regimes, increasing frequency of extreme climatic events, and shrub encroachment are likely to have strong, interactive impacts in reorganizing ecological communities, with significant consequences for ecosystem dynamics.

We monitored populations of 2 species of desert rodents, the sandy inland mouse (Pseudomys hermannsburgensis) and spinifex hopping-mouse (Notomys alexis), over 18 years in the Simpson Desert, central Australia. Populations fluctuated synchronously from very low numbers, or “busts,” during prolonged dry periods to high numbers, or “booms,” after heavy rainfall 3 times over the study period. On the basis of observations that food resources expand after rainfall, we predicted that rodents would show increased rates of recapture, fidelity to burrows, and burrow sharing during population increase (boom) phases compared with decline or bust phases, and also reduce their movements and foraging activity in open habitats during population booms. The behavior of both species was similar but not as we had anticipated. Burrow fidelity and numbers of animals per burrow were roughly 2-fold higher during both the population increase and decrease phases as compared with the population-low phase, whereas rates of movement were reduced by about half. As revealed by giving-up density trials, animals foraged less at experimental food patches during population increase and decrease phases than during busts, and also foraged less in open than in covered habitats. Recaptures of N. alexis were similar across all population phases, whereas P. hermannsburgensis was recaptured more often when populations were decreasing than at other times. The results suggest that both species are dispersed and highly mobile during bust periods but sedentary and more social during population increases and collapses. These changes in movements and social organization appear to be unusual in desert rodents, and we propose that future studies seek to identify the roles of food and other factors in driving them.

Dietary plasticity and diet composition of Pallas's cat (Otocolobus manul) was quantified in central Mongolia. Diet of Pallas's cat was assessed by scat analyses, and prey surveys were used to estimate prey availability. Prey selection was calculated using multinomial likelihood ratio tests. Analysis of 146 scats identified 249 prey items. Pallas's cats ate a broad range of small mammals, insects, birds, reptiles, and carrion, but Daurian pikas (Ochotona dauurica) were the most frequently consumed prey. Multinomial likelihood ratio tests indicated nonrandom selection of prey species. Pikas were selected disproportionately to their availability, and other more numerous prey items were used less than expected, indicating feeding specialization. Specialization on pikas appears to optimize energy intake per unit foraging by Pallas's cats, because pikas are 2–4 times larger than other small mammal prey. We argue that pika control programs in China and Mongolia potentially threaten Pallas's cat populations, because no other prey species could fill the niche occupied by the pika in terms of size and year-round availability.

Despite the numerous studies carried out on the endangered European mink (Mustela lutreola) for conservation purposes, the reproductive biology of this species is largely unknown. In the wild reproductive parameters such as litter size are difficult to observe, particularly for cryptic species such as mustelids. We compared the reliability of nonstained and stained placental scar counts in farmed American mink (Neovison vison) with known litter sizes and then applied the best methodology to free-ranging related species—European mink, European polecat (Mustela putorius), and feral invasive American mink—for a comparative study of embryonic litter size in western Europe populations. The staining method allowed us to improve the detection of placental scars and to increase the reliability of the method by reducing the observer effect. Nevertheless, this analysis must be performed by 7 months postpartum, before the regeneration of uterine tissues. In free-ranging animals the mean embryonic litter size, estimated by stained placental scar counts and embryo counts, was significantly lower in European mink compared with polecats and American mink, and in polecats compared with American mink. Small litter sizes in European mink could be a factor limiting population growth rates in the species. Our results constitute a first step toward demographic analyses aimed at modeling the population dynamics of these species.

Migration can enhance survival and recruitment of mammals by increasing access to higher-quality forage or reducing predation risk, or both. We used telemetry locations collected from 140 adult female elk during 2000–2003 and 2007–2008 to identify factors influencing the migration of northern Yellowstone elk. Elk wintered in 2 semidistinct herd segments and migrated 10–140 km to at least 12 summer areas in Yellowstone National Park (YNP) and nearby areas of Montana. Spring migrations were delayed after winters with increased snow pack, with earlier migration in years with earlier vegetation green-up. Elk wintering at lower elevations outside YNP migrated an average of 13 days earlier than elk at higher elevations. The timing of autumn migrations varied annually, but elk left their summer ranges at about the same time regardless of elevation, wolf numbers, or distance to their wintering areas. Elk monitored for multiple years typically returned to the same summer (96% fidelity, n  =  52) and winter (61% fidelity, n  =  41) ranges. Elk that wintered at lower elevations in or near the northwestern portion of the park tended to summer in the western part of YNP (56%), and elk that wintered at higher elevations spent summer primarily in the eastern and northern parts of the park (82%). Elk did not grossly modify their migration timing, routes, or use areas after wolf restoration. Elk mortality was low during summer and migration (8 of 225 elk-summers). However, spatial segregation and differential mortality and recruitment between herd segments on the northern winter range apparently contributed to a higher proportion of the elk population wintering outside the northwestern portion of YNP and summering in the western portion of the park. This change could shift wolf spatial dynamics more outside YNP and increase the risk of transmission of brucellosis from elk to cattle north of the park.

Factors influencing the survival of pygmy rabbits (Brachylagus idahoensis) are poorly understood and have received minimal attention in previous studies. We investigated survival and causes of mortality of pygmy rabbits in southeastern Oregon and northwestern Nevada. We trapped 298 rabbits on 4 sites and fitted each with radiotransmitters. We used known-fate models in program MARK to estimate survival rates of radiomarked pygmy rabbits from September 2005 to August 2006. The best model, based on Akaike's information criterion, indicated that survival rates varied among study sites, sexes, and with monthly interval in a parallel pattern [model S (area * sex t)]. The estimated annual survival rate on the 4 study sites was notably low and varied from 0.003 (SE  =  0.003) to 0.173 (SE  =  0.066). Predation on radiomarked rabbits was high for both adult (88.6%) and juvenile (89.4%) rabbits. When the predator species could be determined, the most common predators of pygmy rabbits were coyotes (Canis latrans; 19.6%), avian predators (18.5%), and weasels (Mustela spp.; 9.8%). Population monitoring, which attempts to document local trends in pygmy rabbit abundance, should account for both the temporal and spatial variability of survival identified by this study.

Three species of Mexican deer mice of the Peromyscus mexicanus species group (P. grandis, P. guatemalensis, and P. zarhynchus) were characterized morphologically and genetically to test hypothesized concepts of species limits. We investigated if previously proposed phenetic relationships among these 3 taxa were supported by morphometric and genetic data. Analyses of nongeographic and geographic variation for individuals from 36 localities in Guatemala and southeastern Mexico were conducted to assay morphologic and geographic boundaries. In addition, 35 mitochondrial cytochrome-b gene sequences were analyzed using maximum-parsimony and Bayesian inference methods to determine relationships among the 3 taxa. This study based on comparisons to type specimens provided support for the presence of 3 morphologically and genetically distinct units. Our analyses suggest that P. grandis and P. guatemalensis are more closely related to each other than either is to P. zarhynchus, rejecting existing hypotheses that suggest that P. zarhynchus and P. grandis are phenetically more similar. The results of this study depict relationships among other members of the P. mexicanus group and patterns of speciation and biogeography and allow identification of regionally important phylogeographic units in Mesoamerica.

A collection of about 500 molars from 5 Pleistocene localities on the island of Bonaire (off the coast of Venezuela), previously identified as Thomasomys sp., is reidentified as representing a new genus and 2 new species of oryzomyine rodents based on comparative examination and phylogenetic analysis of morphological characters. The material from 1 of the 5 localities is distinguished by its smaller size and several discrete characters; a larger species is represented in the other 4 localities. In addition, a single edentulous dentary represents a different species that is described as an indeterminate genus and species of Sigmodontinae.

Quaternary climate fluctuations and topographical variation in the Pacific Northwest region of North America have interacted to affect the historical biogeography of biota in this region. High-elevation mammals have unique diversification patterns due to their isolation on mountaintops and potential for population growth and range expansion in lowland refugia that are available during glacial periods. We examined the phylogeographic structure, dates of lineage diversification, and historical demography of western heather vole (Phenacomys intermedius) populations across several mountain ranges in the Pacific Northwest. Our analysis of sequence variation in the mitochondrial control region using both maximum-likelihood and Bayesian methods identified 3 major geographically distinct lineages: an Oregon and California lineage, Washington lineage, and Northern and Interior lineage. Our estimate of divergence times using a Bayesian relaxed molecular-clock method revealed that diversification among these major lineages began ∼1.8 million years ago (mya) in the early Pleistocene with the split of the Oregon and California lineage followed by the split of the Washington lineage and the Northern and Interior lineage ∼1.5 mya. All 3 clades remain allopatric, suggesting that they did not share a common refugium during cold climatic intervals of the Pleistocene. Further diversification within each major clade occurred in the middle Pleistocene when populations in isolated mountain ranges became distinct. Demographic estimates from Bayesian skyline plots indicate that each of the 3 major clades has experienced population decline since the early Holocene, possibly due to the redistribution of populations into higher-elevation habitats that became restricted to mountaintops following continental and alpine deglaciation.

We studied den use and den-habitat selection by the Prince of Wales Island flying squirrel (Glaucomys sabrinus griseifrons) at multiple spatial scales in fragmented temperate rain-forest habitats because of the role dens play in the distribution, reproduction, and population density of this endemic subspecies. We observed differences in spatial patterns associated with den use between juveniles and adults: juvenile core denning areas were almost an order of magnitude larger than those of adults, and juveniles used about one-half the number of dens per month as adults. Female juveniles exhibited both the largest mean and maximum movements between consecutive dens among all age and sex classes. At the microhabitat scale of den selection snags were not selected over live trees, but flying squirrels primarily used cavities in snags and live trees. Flying squirrels also selected dens in the largest diameter live trees and snags, in snags with intermediate levels of decay, in live trees with more conks and visible bole entries, and in western hemlock (Tsuga heterophylla). At the broader scale of den selection flying squirrels chose den locations in neighborhoods with higher-volume forests and lower levels of fragmentation, although not lower absolute amounts of edge, than was available across the landscape. Our results suggest that extensive modification of landscapes from clear-cut logging and the creation of an early-seral matrix appeared to influence spatial patterns of den use in flying squirrels and den selection at the broader scale. When compared to patterns in a more-intact landscape, den selection at the microhabitat scale did not relate to differences in landscape context, suggesting additional factors might play an important role in den use across the region.

Southern flying squirrels (Glaucomys volans) face a dilemma: winter aggregation is beneficial for thermoregulation but costly due to nest mates pilfering stored food in the home area and the tendency for groups to attract predators. Living with kin in winter aggregations may mitigate these deleterious effects because if an individual dies, its stored food can be beneficial to relatives, thereby increasing inclusive fitness. Southern flying squirrels from 7 populations and a captive colony were genotyped at 6 microsatellite loci. We calculated group mean relatedness and dyad relatedness within groups. In the wild, winter populations were found to be more highly related than expected by chance. Fifty-seven percent of animals were associated with a highly related individual in their winter aggregation. We show that southern flying squirrels have a preference for relatives as winter nest mates.

Direct observation of mating behavior in the Japanese giant flying squirrel (Petaurista leucogenys) in a temperate mixed forest, central Japan, was carried out for 976 nights over an 8-year period (1983–1991). A total of 153 mating dates was observed involving 35 different adult females. Two mating seasons were apparent, 1 in early summer from mid-May to mid-June (range 36 days) followed by 1 in winter from mid-November to late January (74 days). Each female came into estrus twice a year and mated on 1 night in each mating season. Mothers with nursing young came into estrus relatively later within a mating season. No difference was observed in the proportion of mothers mating in each season, and the mild winter conditions enabled reproductive investment by females during winter. Each reproductive bout (from conception to weaning) required 4–6 months, and mature females were engaged in reproduction all year. The biannual reproductive strategy is thought to be compensated for by a small litter (1 or 2 young). The initiation of feeding by the recently weaned offspring coincided with the months of highest food availability in spring and autumn.

In recent years molecular data have been used increasingly to estimate phylogenies and aid in species delimitation. We generated and analyzed sequence data for spiny pocket mice (Heteromys) for the mitochondrial gene cytochrome b (1,140 base pairs [bp]) and 2 nuclear gene segments, MYH6 (252 bp) and EN2 (189 bp). We used maximum-parsimony, maximum-likelihood, and Bayesian optimality criteria to estimate relationships among species and provide a framework for using a species-delimitation method to investigate the possibility of multiple species within the widespread H. desmarestianus group. We recovered several well-supported clades within this complex, including H. goldmani, H. oresterus, and H. nubicolens. Incorporating karyotype, allozyme, and morphological data from earlier studies, we found sufficient supporting evidence to justify maintaining H. goldmani, H. oresterus, and H. nubicolens as species and identifying 4 additional clades as candidate species. We present a revised taxonomic arrangement within the genus. The subgenus Xylomys should be retained and be composed of H. nelsoni. The subgenus Heteromys should be divided into 3 species groups: the H. anomalus group (H. anomalus and H. australis) together with H. catopterius, H. oasicus, and H. teleus (insertae sedis); the H. gaumeri group (H. gaumeri); and H. desmarestianus group (H. desmarestianus, H. goldmani, H. oresterus, H. nubicolens, and the 4 candidate species).

We studied the feeding ecology of the thin-spined porcupine (Chaetomys subspinosus), a small arboreal rodent endemic to the Atlantic rain forest of Brazil and threatened with extinction. We captured and radiotracked 4 free-ranging individuals and collected 1,177 feeding records in 944 h of observation between April 2005 and September 2006. We found that the animal feeds exclusively on the leaves of woody trees, preferring young leaves. Diet composition and foraging pattern did not vary seasonally, although we observed a seasonal variation in the availability of young leaves. Annual diet comprised primarily (90%) 4 plant species, Albizia pedicellaris, Inga thibaudiana, Pera glabrata, and Tapirira guianensis. This small subset of plant species contains high levels of both leaf protein and fiber, and most preferred species have the capacity to establish symbiotic associations with nitrogen-fixing bacteria. A diet concentrated in a few taxonomically related, nitrogen-fixing species suggests that in addition to high protein levels other features linked to chemical tolerance and local abundance of food items also influence the use and selection of food by this small browser. We conclude that C. subspinosus is more folivorous than previously thought and that it is highly selective in food choice, as expected for a small arboreal mammalian folivore. Because the animals selected pioneers species that are locally abundant, widespread, and typical of early secondary forests, we suggest that food supply is not a limiting factor for the presence of thin-spined porcupines in small disturbed forest patches in the Atlantic forest.

Although mouse lemurs are solitary foragers, they are known to form sleeping associations. I examined several factors that could influence the choice to sleep communally and the composition of sleeping associations in Microcebus griseorufus, an inhabitant of the subarid spiny forest of southern Madagascar. These include the quantity and quality of available sleeping sites, socio-territoriality, predation risk, and thermoregulation. I radiotracked 26 individuals (12 males and 14 females) and recorded 222 uses of 151 sleeping sites. Mouse lemurs slept most often in tree forks and tangles of vegetation and preferred sleeping sites in Alluaudia spp. (36%) and Euphorbia spp. (30%), 2 very common tree genera in the spiny forest. Sleeping associations allowed the animals to use larger and more extensively overlapping home ranges and to have access to more food. Predation risk was much higher during nocturnal activity than during diurnal rest. Sleeping groups were small, usually pairs. Animals showed no signs of vigilance in sleeping sites and did not huddle consistently. Sleeping sites were chosen for their inaccessibility to predators and were well buffered against high and low ambient temperatures. Therefore, sleeping association was better explained by social territoriality than by predation pressure or thermoregulation.

To support the ossification of fetal and neonatal bones, reproductive females must transfer calcium to their offspring. The extent to which calcium is mobilized from maternal bone is related inversely to the amount of calcium ingested. Insectivorous bats consume a low-calcium diet, and thus, a reproductive female may experience a conflict over allocating dietary calcium to self-maintenance or her developing offspring. We tested the ability of big brown bats (Eptesicus fuscus) to use elevated concentrations of dietary calcium to determine if available dietary calcium limits the amount of calcium that mothers transfer to their offspring. During late pregnancy and early lactation captive bats were fed diets with calcium content 10 times greater than (high-calcium) or equivalent to (low-calcium) diets consumed by free-ranging individuals. We measured the calcium content of guano produced by mothers as an indicator of calcium absorption and the calcium content of their young as a measure of total calcium allocated to offspring development. Females on the high-calcium diet defecated 13 times more calcium than females on the low-calcium diet, and no difference in body concentrations of calcium existed between pups produced by females on low- and high-calcium diets. This suggests that limited calcium intake does not constrain calcium transfer to offspring in big brown bats. It is likely that the calcium demands of offspring production are met largely by mobilizing maternal skeletal calcium.

Protein is essential for growth and reproduction. We used nitrogen stable isotope analysis to reconstruct protein (i.e., nitrogen) dietary origin in 2 Antillean nectarivorous bats with contrasting morphology of the masticatory apparatus: the Greater Antillean long-tongued bat (Monophyllus redmani, Glossophaginae) and Poey's flower bat (Phyllonycteris poeyi, Phyllonycterinae). M. redmani has a masticatory apparatus that is more adapted to feed on flowers, whereas P. poeyi has a feeding structure that resembles the structure of frugivorous bats. M. redmani relied on a mixture of nitrogen derived from insects and plants, and P. poeyi depended more on plant nitrogen. Based on previous findings in fecal contents, we concluded that M. redmani obtained plant nitrogen mainly from pollen, and P. poeyi obtained it predominantly from fruits. Our study suggests that jaw morphology plays a major role in dietary diversification among nectarivorous bats to obtain protein.

Brazilian free-tailed bats (Tadarida brasiliensis) emerge from cave roosts in dense columns in which adjacent bats are separated by only small distances. We describe and quantify variation in the structure of echolocation calls produced by these emerging bats and determine if call structure changes in relation to the rate of emergence measured using thermal infrared imaging. We recorded emergence calls at 2 roosts, 1 housing approximately 200,000 bats and the other approximately 17,000 bats. We found that Brazilian free-tailed bats emit distinct frequency-modulated (FM_start) and constant frequency (CF_start) calls during emergence that are significantly different from echolocation calls they emit while foraging. We propose that these calls provide different information for orientation within the emergence column. CF_start calls are very similar to social calls used by Brazilian free-tailed bats, suggesting 2 potential functions for this call type. The structure of both the FM_start and CF_start calls were not related to the number of bats emerging from a roost, although significant structural differences existed between sites. The differences between sites could be associated with the spacing of bats during emergence, because bats appeared to form tighter columns at the larger roost colony compared to the smaller colony.

The genus Myotis has undergone significant taxonomic revision since the advent of DNA sequencing techniques. Prior morphological examination of Myotis has indicated as many as 4 subgenera correlated with foraging strategies. Recent studies using mitochondrial DNA (mtDNA) sequence data have questioned the validity of these subgenera and have indicated that several taxa may require reevaluation as to their position within Vespertilionidae. Nevertheless, no study has used large-scale nuclear DNA sequencing to examine relationships within Myotis. We generated 4,656 base pairs (bp) of nuclear intron (PRKC1, STAT5A, and THY) and exon (APOB, DMP1, and RAG2) sequence data in addition to 2,866 bp of mtDNA sequence data to test previously hypothesized subgeneric groupings of Myotis. We included 21 species of Myotis from all morphological subgenera previously suggested, representatives of all subfamilies and tribes currently recognized in Vespertilionidae, and multiple representatives of all other families currently included in the superfamily Vespertilionoidea. We also included a representative of the rare African genus Cistugo, because significant doubt exists about its familial position. Our phylogenetic analyses did not support the morphologically defined Myotis subgenera and confirm that morphological similarities among Myotis are the result of convergent evolution. Divergence estimates derived from the total data set were concordant with previous studies, suggesting a middle Miocene trans-Beringian dispersal from Asia colonized North America, with subsequent South American colonization and diversification prior to the formation of the Isthmus of Panama 3–4 million years ago. Myotis latirostris fell outside of Myotis, and the high genetic distance separating it from other Myotis suggested that M. latirostris represented a distinct genus. The genus Cistugo, previously a subgenus within Myotis, fell basal to all vespertilionids, with a high genetic distance separating it from Vespertilionidae. We conclude that Cistugo should constitute a distinct family within Vespertilionoidea.

Shrew abundance has been linked to the presence of coarse woody debris (CWD), especially downed logs, in many regions in the United States. We investigated the importance of CWD to shrew communities in managed upland pine stands in the southeastern United States Coastal Plain. Using a randomized complete block design, 1 of the following treatments was assigned to twelve 9.3-ha plots: removal (n  =  3; all downed CWD ≥10 cm in diameter and ≥60 cm long removed), downed (n  =  3; 5-fold increase in volume of downed CWD), snag (n  =  3; 10-fold increase in volume of standing dead CWD), and control (n  =  3; unmanipulated). Shrews (Blarina carolinensis, Sorex longirostris, and Cryptotis parva) were captured over 7 seasons from January 2007 to August 2008 using drift-fence pitfall trapping arrays within treatment plots. Topographic variables were measured and included as treatment covariates. More captures of B. carolinensis were made in the downed treatment compared to removal, and captures of S. longirostris were greater in downed and snag compared to removal. Captures of C. parva did not differ among treatments. Captures of S. longirostris were positively correlated with slope. Our results suggest that abundance of 2 of the 3 common shrew species of the southeastern Coastal Plain examined in our study is influenced by the presence of CWD.

The contemporary geographic distribution and interrelation of populations result from landscape structure, habitat specificity, and historical climate fluctuations. We examined the impact of Pleistocene environmental history on mitochondrial genealogies of the water shrew (Sorex palustris) in montane systems of western North America to test hypotheses concerning historical recolonization. Our main focus lies on the regional scale of the Pacific Northwest and the extent to which disjunct populations are associated with discrete, isolated mountains. We analyzed 87 specimens from the entire western portion of the species range to assess broadscale genealogical relationships. We then examined 61 individuals selected from 21 specific high-elevation sites in the Pacific Northwest to test whether these populations are structured in accord with distinct montane regions. Across western North America in general the relationship among S. palustris lineages is unresolved, suggesting a recent and rapid radiation from a common ancestral population. In the Pacific Northwest we found evidence of clades that are uniquely associated with individually isolated montane regions, clades that are distributed across several montane regions, and distinct clades that co-occur within the same montane region. We conclude that populations of S. palustris in western North America are derived from a single, southern, late-Pleistocene refugium. We also suggest that lineages shifted geographically by following the most northerly postglacial expansion of the boreal forest system.

This study assessed the occurrence of sexual size dimorphism (SSD) and sexual shape dimorphism (SShD) in the skull and mandible of representatives of most species within the 3 orders of living New World opossums, Didelphimorphia, Paucituberculata, and Microbiotheria, using geometric morphometrics. Centroid sizes and partial warps were extracted from landmarks set on images of the dorsal, ventral, and lateral views of the skull and lateral view of the mandible, and were compared between sexes to estimate SSD and SShD. Specimens totaling 2,932 from 71 species of Didelphidae, 5 species of Caenolestidae, and 1 species of Microbiotheriidae were analyzed. SSD was variable in the Didelphimorphia and the Paucituberculata and absent in Microbiotheria. Similar results were found for SShD, but SSD and SShD are not clearly coupled. I also evaluated the validity of Rensch's rule—the widely observed phenomenon of correlated increases in SSD and body size for male-biased sexual dimorphism, or correlated decreases in SSD in body size for female-biased sexual dimorphism—in the Didelphidae. Didelphids span 2 orders of magnitude in body size, and, when present, sexual dimorphism is male-biased. Regressions of SSD and SShD estimators onto size, using phylogenetic independent contrasts, showed either no significant relationship between SSD or SShD with size in any of the structures and views analyzed, or a trend contrary to Rensch's rule (smaller species more dimorphic, but with male-biased dimorphism). Lack of adherence to Rensch's rule in Didelphimorphia may relate to a lack of social interactions and male territoriality, usually associated with such a trend via sexual selection. If the trend contrary to Rensch's rule is real, an explanation may lie in the increasing amount of small-bodied species that recently have been found to be semelparous and thus subject to stronger selection for larger males.

Occurrence records represent the key to understanding species distribution patterns. I analyzed the historical and geographical occurrence of Dromiciops gliroides, a marsupial endemic to the temperate rain forests of southern Chile and Argentina and the sole living representative of the order Microbiotheria. Eighty-eight localities spanning 7°31′ latitude (∼850 km) and 3°13′ longitude (∼350 km) are known for the species and were taken from previous works and museum specimens and integrated in a geographic information system. Species distribution was analyzed in a historical, geographic, and biogeographic context, with the use of maps at different scales (region, subregion, province, ecoregion, and forest type). The software MaxEnt was used to generate 2 potential distribution models with environmental data. Generated models show a northern contraction and southern expansion in the species' potential distribution, with variation in habitat suitability throughout the current species range. Precipitation and some temperature-related variables influenced both generated models. The species' occurrence lies within the Andean region; Subantarctic and Patagonia subregions; and Maule, Valdivian, and Subandean Patagonia provinces. At a smaller scale, most of the localities occur within the Valdivian Temperate Forests ecoregion, and in the majority of the forest types described for it, implying broader ecological requirements than expected. Identification of critical areas through potential distribution modeling may have implications on species conservation and biogeographic patterns.