Tree Squirrel Species Research Articles

Successful eradication of a suburban Pallas’s squirrel Callosciurus erythraeus (Pallas 1779) (Rodentia, Sciuridae) population in Flanders (northern Belgium)

Despite a growing catalogue of eradication projects, documented successful vertebrate eradications on the mainland remain scarce. Reporting on successful campaigns is crucial to counter pessimism on ambitious programmes to tackle invasive species and to allow conservation practitioners, wildlife managers and scientist to learn from previous experience. Moreover, there is a need for basic information on the effectiveness of control methods and management strategies that can be used. In this note we report on a successful low-tech eradication campaign of a local population of Pallas’s squirrel Callosciurus erythraeus, a species of tree squirrel with documented ecological and socio-economic impacts in its invasive range. The population was eradicated from a suburban park of about 15 ha using baited mesh wire life traps, in five consecutive capture campaigns between October 2005 and January 2011. Using maximum likelihood estimation from catch-effort data we calculated initial densities in the park at 3 squirrels ha−1. Although control started quickly and the extent of the invasion was limited, the campaign took over 5 years and required an estimated investment of over €200,000 including 1.5 years of post-eradication surveying. We provide basic data on the methods used to eradicate this invasive rodent. Critical success factors and possible improvements with respect to the specific context of this case are discussed. Adding this species to the list of species of EU concern currently under development could provide incentive to minimise impact of this tree squirrel at the continental scale.

DIVERSIFICATION AND GENE FLOW IN NASCENT LINEAGES OF ISLAND AND MAINLAND NORTH AMERICAN TREE SQUIRRELS (TAMIASCIURUS)

Pleistocene climate cycles and glaciations had profound impacts on taxon diversification in the Boreal Forest Biome. Using population genetic analyses with multilocus data, we examined diversification, isolation, and hybridization in two sibling species of tree squirrels (Tamiasciurus douglasii and Tamiasciurus hudsonicus) with special attention to the geographically and genetically enigmatic population of T. hudsonicus on Vancouver Island, Canada. The two species differentiated only about 500,000 years ago, in the Late Pleistocene. The island population is phylogenetically nested within T. hudsonicus according to our nuclear analysis but within T. douglasii according to mitochondrial DNA. This conflict is more likely due to historical hybridization than to incomplete lineage sorting, and it appears that bidirectional gene flow occurred between the island population and both species on the mainland. This interpretation of our genetic analyses is consistent with our bioclimatic modeling, which demonstrates that both species were able to occupy this region throughout the Late Pleistocene. The divergence of the island population 40,000 years ago suggests that tree squirrels persisted in a refugium on Vancouver Island at the last glacial maximum, 20,000 years ago. Our observations demonstrate how Pleistocene climate change and habitat shifts have created incipient divergence in the presence of gene flow. Sequence data have been archived in GenBank—accession numbers: KF882736–KF885216.

Nest tree and site selection of an introduced population of red-bellied squirrels (Sciurus aureogaster)

Nests play a crucial role in the life history of tree squirrels, and can be a critical component of their biology that enables them to be a successful invasive species. Red-bellied squirrels (Sciurus aureogaster) were introduced to Elliott Key, Florida, in the late 1930s, and spread to nearby islands. Red-bellied squirrels were believed extirpated by Hurricane Andrew in 1992, but were rediscovered on Elliott Key in 2005. In 2006 and 2007, we surveyed for squirrel nests and measured vegetation to evaluate forest characteristics associated with nest-site selection by squirrels, and compared these data to measurements collected at random locations. Squirrels placed nests in large trees with more canopy linkages in mixed-hardwood forest, and the nest trees were in areas with large trees, high tree density and canopy cover, and lower recent hurricane damage. Red-bellied squirrels selected characteristics of nest trees and forest structure in similar ways to individuals in their native range, and to other species of tree squirrels in general. Results from our research allowed land managers to assess possible management actions and provided important information for them to develop an effective management strategy. Park officials are currently working toward complete eradication of the introduced population of red-bellied squirrels from the Florida Keys. We suggest that behaviors of individuals in native ranges may elucidate patterns for individuals introduced to novel environments; however, we also caution that care must be taken in further extrapolation. Our findings emphasize the importance of understanding ecology of introduced species for effective management.

Female mobility and the mating system of the banner-tailed kangaroo rat (Dipodomys spectabilis)

Like some species of pika (Ochotona) and tree squirrel (Tamiasciurus), the banner-tailed kangaroo rat (Dipodomys spectabilis) is a solitary, philopatric larder-hoarder that practices unisexual territoriality. Seeking to understand how this unusual combination of traits might influence the mating system, we used systematic trapping at mounds (burrows) and spool-and-line tracking to examine male and female visits to each other. Where genetic data exist, unisexual larder-hoarders such as D. spectabilis have been found to be polygynandrous. We asked whether the fact that females sometimes mate with more than 1 male is a passive side effect of imperfect mate defense by males, or whether females actively engage in behaviors that encourage multiple mating. During the nonbreeding season, adults of both sexes were rarely captured at mounds other than their own. In contrast, during the breeding season females as well as males were significantly more likely to be captured away from their home mounds, most often at mounds belonging to adults of the opposite sex. Spool-and-line tracking during the breeding season confirmed these results. Consistent with the expectation that they engage in competitive mate searching, adult males visited adult females' mounds significantly more than expected by chance and without regard to age, size, or relatedness. More surprisingly, adult females exhibited similar behavior, expanding their home ranges and disproportionately visiting the mounds of large adult males during the nights preceding estrus. Increased (and apparently directed) female mobility during the days preceding estrus may serve as a means of shaping the pool of competing mates and is worthy of more attention in this and other, ecologically similar species.

Sciurillus pusillus(Rodentia: Sciuridae)

Sciurillus pusillus (E. Geoffroy Saint-Hilaire, 1803) is a sciurid commonly called the Neotropical pygmy squirrel. S. pusillus is an exceptionally small species of tree squirrel that appears pale gray due to a grizzled pattern of light hair tips on the dorsal pelage. Ranging from Amazonian Peru and Brazil northeast to the Guianas with a patchy distribution, S. pusillus is a denizen of the upper canopy of lowland rain forest. This species is listed as “Data Deficient” by the International Union for Conservation of Nature and Natural Resources.

Feeding behavior and activity patterns of Amazon red squirrels

Foraging sites are important for the survival of animals. Individuals cue on characteristics of the forest that offer enough food resources and also provide safety. During June and July of 2009 and 2010, we studied northern and southern Amazon red squirrels ( Sciurus igniventris , S. spadiceus ) to determine what forest characteristics were associated with feeding sites. We examined habitat use at three levels: vegetation community where feeding sign was located, site characteristics of the forest immediately surrounding the feeding sign, and the tree exhibiting feeding sign. We measured the site characteristics inside a 10-m radius circular plot, the physical characteristics of the tree exhibiting feeding sign, and the same variables at random locations for comparison. Because there is lack of knowledge about these squirrels, we also conducted focal observations to study their behavior. Squirrels use mainly high and low restinga and selected Astrocaryum and Attalea palm trees that were taller and larger compared with random locations. Squirrels used all vertical strata of the forest, and the main behaviors observed were travel and forage. Behaviors occurred similarly across the day but differed by vertical strata. Although squirrels used vegetation communities different than available and selected for tree characteristics, site characteristics did not appear to be important in contrast with other tree squirrel species.

Late Miocene Sciuridae (Mammalia, Rodentia) from Anatolia, Turkey

ABSTRACT Isolated cheek teeth of Sciuridae (Rodentia, Mammalia) from nine late Miocene localities in central Anatolia (Turkey) are described. The teeth represent at least 12 different species, five of which belong to the ground squirrel genus Tamias, two to the ground squirrel genus Spermophilinus, one to the flying squirrel genus Hylopetes, and two to the flying squirrel genus Pliopetaurista. One species, Tamias anatoliensis (type locality Altıntaş 1), is new. An unknown genus and species of giant tree or ground squirrel is represented by one tooth. Two teeth probably form the oldest record in western Eurasia of the tree squirrel genus Sciurus. Seven of the localities that yielded Sciuridae are lacustrine deposits, two are karst fissure fills. Their estimated ages range from MN 9 to MN 13. The majority of late Miocene Sciuridae from Anatolia show affinities with European sciurids of the same period.

West Nile virus associations in wild mammals: a synthesis

Exposures to West Nile virus (WNV) have been documented in a variety of wild mammals in both the New and Old Worlds. This review tabulates at least 100 mammal species with evidence of WNV exposure. Many of these exposures were detected in free-ranging mammals, while several were noted in captive individuals. In addition to exposures, this review discusses experimental infections in terms of the potential for reservoir competence of select wild mammal species. Overall, few experimental infections have been conducted on wild mammals. As such, the role of most wild mammals as potential amplifying hosts for WNV is, to date, uncertain. In most instances, experimental infections of wild mammals with WNV have resulted in no or low-level viremia. Some recent studies have indicated that certain species of tree squirrels (Sciurus spp.), eastern chipmunks (Tamias striatus), and eastern cottontail rabbits (Sylvilagus floridanus) develop viremia sufficient for infecting some mosquito species. Certain mammalian species, such as tree squirrels, mesopredators, and deer have been suggested as useful species for WNV surveillance. In this review article, the information pertaining to wild mammal associations with WNV is synthesized.

Callosciurus squirrels: worldwide introductions, ecological impacts and recommendations to prevent the establishment of new invasive populations

Abstract Rodents are traded as pet species, a practice that frequently results in new introduced populations. This is particularly true for tree squirrels where, often, only a few founders can establish viable colonies. Here, we review the worldwide introductions, ecology and impacts of two tree squirrel species, Callosciurus erythraeus and Callosciurus finlaysonii, and discuss the elements of a strategy to reduce squirrel introductions and settlements. C. erythraeus has established viable populations in Argentina, France, The Netherlands, Hong Kong and Japan. An introduction to Belgium may have been stopped successfully. C. finlaysonii has been introduced to Italy, Singapore and Japan. After 1950, the mean number of introduction events was one every two years. The most evident damage caused by these species is bark stripping that can be severe and may significantly impact trees and timber plantations. Data on negative impacts to native species are reported but have not yet been formally quantified. Both squirrel species carried with them parasites from the native range into the new habitats, leading to the introduction of other species. The ability of tree squirrels to establish themselves successfully, often from only a few founders, combined with their human appeal make them high‐risk species, and the pet trade should be considered as a high‐risk pathway for new introductions. A proactive approach to preventing new introductions should therefore include trade restrictions, and should be combined with public education initiatives at national and European scales. Tree squirrels represent an ‘alien species conundrum’. Experience from the UK and Italy has shown that if action is delayed until introductions are recognized as a problem, it is generally too late to control populations effectively, due to logistic, legal or economic reasons, or due to lack of public support. In the case of new populations, a rapid response mechanism is therefore critical. Once established, populations may become invasive and difficult or impossible to control.

The Narrow Niche hypothesis: Gray squirrels shed new light on primate origins

Current hypotheses for primate origins propose that nails and primate-like grasping hands and feet were important early adaptations for feeding in fine branches. Comparative research in this area has focused on instances of convergence in extant animals, showing that species with primate-like morphology feed predominantly from terminal branches. Little has been done to test whether animals without primate-like morphology engage in similar behavior. We tested the fine-branch niche hypothesis for primate origins by observing branch use in Eastern gray squirrels, Sciurus carolinensis, a species lacking primate grasping adaptations that has been understudied in the context of primate origins. We hypothesized that because gray squirrels lack primate-like grasping adaptations, they would avoid feeding and foraging in terminal branches. Instantaneous focal animal sampling was used to examine the locomotor and postural behaviors used while feeding and foraging. Our results demonstrate habitual and effective usage of terminal branches by gray squirrels while feeding and foraging, primarily on tree seeds (e.g., oak, maple, and elm). Discriminant function analysis indicates that gray squirrels feed and forage like primates, unlike some other tree squirrel species. Given the absence of primate-like features in gray squirrels, we suggest that although selection for fine-branch foraging may be a necessary condition for primate origins, it is not sufficient. We propose an alternative model of primate origins. The Narrow Niche hypothesis suggests that the primate morphological suite evolved not only from selection pressure for fine branch use, but also from a lack of engagement in other activities.

Systematic Review of Endemic Sulawesi Squirrels (Rodentia, Sciuridae), with Descriptions of New Species of Associated Sucking Lice (Insecta, Anoplura), and Phylogenetic and Zoogeographic Assessments of Sciurid Lice

Analyses of fur color patterns, morphometric data derived from external, cranial, and dental dimensions, and distributions of collection sites for voucher specimens form the basis for a taxonomic revision of Sulawesi's endemic squirrel fauna. Eight species of tree squirrels in Rubrisciurus and Prosciurillus and two species of ground squirrels in Hyosciurus are recognized. All are diurnal and inhabit primary forest formations. Diet consists of fruit, nuts, seeds, and arthropods. Rubrisciurus rubriventer, the largest in body size, forages on the ground and in the lower canopy layer, is found throughout Sulawesi where primary forest persists, and occurs through an altitudinal range embracing tropical lowland evergreen and lower montane rain forests; it is absent from upper montane rain forest. Five species of arboreal squirrels comprise the Prosciurillus leucomus group, a cluster of species occupying the upper forest canopy: P. leucomus, known only from lowland and montane habitats in the northern peninsula and one offshore island; P. alstoni, recorded from lowland tropical evergreen rain forest in the eastern section of Sulawesi's central core, the east-central and southeastern arms, and two southeastern islands; P. weberi, represented by a few specimens from the coastal lowlands of the southern core of Sulawesi; P. topapuensis, endemic to the western mountain block in Sulawesi's central core and occurring along an altitudinal gradient from lowland evergreen rain forest to upper montane rain forest; and P. rosenbergii, the only species of squirrel collected on islands in the Sangihe Archipelago north of the northeastern tip of the northern peninsula. The Prosciurillus murinus group contains two species of small body size: P. murinus, found throughout Sulawesi and in all forest formations, from the coastal lowlands to mountaintops, and a forager in the lower canopy layers; and P. abstrusus, known only from montane forest habitats on Pegunungan Mekongga in the southeastern peninsula. Of two species of ground squirrels, Hyosciurus heinrichi occupies montane forest habitats in the western mountain block of Sulawesi's central core. It is altitudinally parapatric to H. ileile, which inhabits lowland evergreen and lower montane rain forests in the western mountain block and northeastern lowlands of central Sulawesi, and montane forest on the northern peninsula.A slightly revised classification of Sciuridae is provided in which a new tribe, Exilisciurini, is proposed for the Bornean and Philippine Exilisciurus. Previously published results of morphological and molecular analyses point to Rubrisciurus, Prosciurillus, and Hyosciurus as a monophyletic cluster, the Hyosciurina, nested within a larger clade, the Nannosciurini, which along with Exilisciurini n. tribe and Funambulini, comprise the Nannosciurinae, one of the three subfamilies constituting Sciuridae, and one that contains most of the Indomalayan genera. The present diversity of species endemic to Sulawesi was derived from an ancient lineage that crossed a sea barrier from the Sunda Shelf to Sulawesi during the late Miocene.Eight new species of hoplopleurid sucking lice (Insecta, Anoplura) are described as parasitizing 8 of the 10 species of squirrels endemic to Sulawesi: Hoplopleura rubrisciuri from Rubrisciurus rubriventer, Hoplopleura leucomus from Prosciurillus leucomus, Hoplopleura alstoni from Prosciurillus alstoni, Hoplopleura topapuensis from Prosciurillus topapuensis, Hoplopleura murinus from Prosciurillus murinus, Hoplopleura abstrusus from Prosciurillus abstrusus, Hoplopleura heinrichi from Hyosciurus heinrichi, and Hoplopleura ileile from Hyosciurus ileile. Examples of Prosciurillus weberi and P. rosenbergii were surveyed but no lice were recovered. A phylogenetic analysis based on cladistic principles for six species of Sulawesian squirrel lice for which both sexes were available is presented and the results discussed with respect to host relationships. These new dat

Sound Communication among African Rainforest Squirrels

Sound signals of 9 species of rainforest tree squirrels were studied in the field and in captivity at Makokou, Gaboon. The signals can be classified into 7 behavioral categories: nestling ticking and isolation, distress, contact-seeking, defensive, low intensity alarm, and high intensity alarm. On the basis of physical structure, sounds are further classified into 12 basic types and their variants. A given species has a repertoire of about 7 of these sound types. Review of the literature on sciurid calls suggests that most species emit sounds under the same set of behavioral contexts described for the African squirrels, and that many of the basic structural sound types are widespread within the same contexts throughout the group. In comparison to temperate squirrels, some calls of rainforest species show a lowering of frequency and increase in length. These features are apparently adaptations to the dense vegetation and high background noise of the rainforest environment.

Morphological Diversity Despite Isometric Scaling of Lever Arms

In the absence of a substantial functional shift, morphological evolution is usually expected to follow an allometric trajectory, however, studies of tree squirrel jaws have found isometry across most of their size range. This isometry appears to reflect the integration of a small number of lever arm lengths that are critical for generating bite force. To test whether this integration constrains only the ratios of these lengths, or jaw shape in general, we analyzed jaw shapes and a set of lengths comparable to those used in previous studies for 23 species of sciurine tree squirrels (Sciurus, Tamiasciurus and Microsciurus), a lineage that is both functionally uniform and spans a large size range. We found that the measured lengths were highly correlated and isometric with respect to each other, but negatively allometric with respect to jaw size. Shape differences are generally small, but shape diversity was still greater than the diversity of mechanical advantages (input lever lengths scaled by output lever length). In addition, phylogenetic analyses demonstrated that only a minute fraction of shape evolution is correlated with size evolution. This contrast between the diversity of shape and the stability of proportions among a suite of functionally relevant lengths suggests that constraints on those lengths and the associated mechanical parameters have little or no ability to restrict changes in other aspects of jaw form.

Flexibility of cue use in the fox squirrel (Sciurus niger)

Recent work on captive flying squirrels has demonstrated a novel degree of flexibility in the use of different orientation cues. In the present study, we examine to what extent this flexibility is present in a free-ranging population of another tree squirrel species, the fox squirrel. We trained squirrels to a rewarded location within a square array of four feeders and then tested them on transformations of the array that either pitted two cue types against one cue type, the majority tests, or all cue types against each other, the forced-hierarchy test. In Experiment 1, squirrels reoriented to the two-cue-type location in all majority tests and to the location indicated by the visual features of the feeders in the forced-hierarchy test. This preference for visual features runs contrary to previous studies that report the use of spatial cues over visual features in food-storing species. In Experiments 2-5 we tested squirrels with different trial orders (Experiments 2 and 3), a different apparatus (Experiment 4) and at different times of the year (Experiment 5) to determine why these squirrels had chosen to orient using visual features in the first experiment. Like captive flying squirrels, free-ranging fox squirrels showed a large degree of flexibility in their use of cues. Furthermore, their cue use appeared to be sensitive both to changes in the test apparatus and the season in which we tested. Altogether our results suggest that the study of free-ranging animals over a variety of conditions is necessary for understanding spatial cognition.

Tree Squirrel Introduction: A Theoretical Approach with Population Viability Analysis

Reintroduction efforts require knowledge of how many animals are needed for successful establishment. Population viability analysis can be used to predict trajectories of introduced populations and tree squirrels provide an ideal model system to investigate this challenge. Conservation action is needed because more than 80% of species of tree squirrels are of precarious conservation status in some portion of their range. We combined data from closely related species of tree squirrels and used VORTEX to determine how many squirrels are needed to successfully establish populations of 6 species (Sciurus aberti, S. carolinensis, S. niger, S. granatensis, S. vulgaris, and Tamiasciurus hudsonicus). We ran multiple simulations to account for betweenpatch differences in breeding success (resource availability) and variation between years in different habitats. In the best-case scenarios, populations could be successfully established with fewer than 35 individuals for all species and as few as 15 for a subset of species. Empirical evidence from introductions of tree squirrels supports our simulation results, with 93% of populations of greater than 10 squirrels surviving more than 50 years. With relatively few individuals needed for establishing new squirrel populations, reintroductions are feasible and useful as a buffer for imperiled species.

ANNUAL CYCLES IN BODY MASS AND REPRODUCTION OF CHIRICAHUA FOX SQUIRRELS (SCIURUS NAYARITENSIS CHIRICAHUAE)

Abstract The Chiricahua fox squirrel (Sciurus nayaritensis chiricahuae) is an uncommon subspecies endemic to the Chiricahua Mountains of southeastern Arizona. We monitored annual cycles of body mass and reproduction to elucidate factors shaping the ecology of the sexes. Body mass of adult males did not fluctuate seasonally, although males tended to be heavier in winter. Body mass of adult females fluctuated seasonally, with lower body masses in summer than winter. Males and females did not differ in body mass in summer and fall, but females were heavier than males in winter and spring. Males with scrotal testes were found in the population during all seasons, but were especially prevalent in winter and spring. The majority of lactating females were present in spring and summer. Annual cycles in reproduction and body mass of Chiricahua fox squirrels are similar to more widespread species of tree squirrels, although males did not exhibit typical body mass fluctuations. Extreme spatial and temporal fluctuati...

The coexistence of Fox (Sciurus niger) and gray (S. caroliniensis) squirrels in the Chicago metropolitan area

Two species of tree squirrel inhabit the Chicago region, the fox (Sciurus niger) and gray (S. carolinien- sis) squirrel. Chicago residents submitted squirrel observations and associated landscape variables via a Website, allowing us to map squirrel distributions. Data were analyzed for patterns of correlation. At a smaller scale, we did a foot survey of fox and gray squirrels in the suburb of Oak Park, replicating an earlier study and comparing results. Gray squirrels were associated with densely populated areas, parks and campuses, fox squirrels with suburban areas. Compared to gray squirrels, fox squirrels were more likely to be observed in areas of high cat density. In the suburb of Oak Park, the current trend seems to be an extension of gray squirrel distribution and a decrease in fox squirrel distribution. Our study provides support for the idea that fox and gray squirrel coexistence is facilitated by a trade-off between managing the cost of predation and foraging efficiency, gray squirrels out-competing fox squirrels in areas of high food and low predator (or pet) density.

The innate basis of food-hoarding decisions in grey squirrels: evidence for behavioural adaptations to the oaks

Rodents selectively cache acorns of red oak species (subgenus Erythrobalanus; RO) over those of white oaks (subgenus Quercus; WO) because of perishability of early-germinating WO acorns. On occasion, at least three species of tree squirrels also will cache white oak acorns, but only after they excise acorn embryos, thereby preventing germination. Failure to observe embryo excision of acorns in several other mammal species led us to hypothesize that tree squirrels may have an innate tendency to perform embryo excision and possibly the differential caching of RO and WO acorns. To test this, we evaluated the hoarding decisions of eight captive-reared (naive) eastern grey squirrels, Sciurus carolinensis, with no previous experience with acorns and four wild-caught squirrels. Squirrels were presented individually with pairs of dormant RO and nondormant WO acorns (two species) and pairs of dormant and nondormant RO acorns over a 2-month period. Both naive and wild-caught squirrels selectively cached dormant RO acorns more often than those of one WO species (Quercus alba), but showed a much weaker response to other pairs of dormant and nondormant acorns. Naive squirrels also removed the radicles of germinating acorns, and on occasion, attempted to excise embryos of germinating acorns. We conclude that there is a strong innate basis for some of the hoarding decisions made by tree squirrels, strengthening an emerging argument that tree squirrels show specific behavioural adaptations to the oaks.

Scaling of the mandible in squirrels.

We compared the shape of the mandible among New World tree squirrels and selected outgroup taxa using linear measurements and areas defined by the median axis and conventional anatomical landmarks. We modified the median axis technique to define novel measurements, which proved complementary to those obtained from conventional landmarks. Allometric analyses showed that the scaling of the mandible among the New World tree squirrels is generally isometric (as has been observed in other groups of mammals), but diverges from isometry in a tendency in smaller animals for the masseteric ridge to be displaced anteriorly, the condylar process and posterior portion of the ascending ramus to be relatively elongated, and the coronoid process to be shortened. Allometric analyses also revealed the ways and extent that outgrowth taxa deviated from the scaling pattern observed for the New World tree squirrels. A flying squirrel (subfamily Pteromyinae), a moderate-sized callosciurine squirrel, and three species of pygmy tree squirrels from Asia and Africa show mandibular proportions very similar to those predicted for New World tree squirrels of corresponding size. Ground squirrels (tribe Marmotini) and successively more distant relatives such as Aplodontia, two myomorph rodents, and a rabbit show greater differences from the New World tree squirrels in their mandibular proportions. Combining the use of median-axis and conventional measurements makes it possible to examine changing relationships between locations of anatomically homologous landmarks and the geometry of the form.

Reproductive Biology of a Tropical, Non-Hibernating Ground Squirrel

To examine the effects of climate and hibernation on reproductive patterns, I compared tropical and temperate species of ground and tree squirrels. I studied the reproductive biology of a tropical species, the Cape ground squirrel ( Xerus inauris ), by observing marked individuals in the Kalahari-bushveld region in Namibia. Breeding occurred throughout the year, although there was some seasonality. Interestrous intervals were 76.5–146 days, gestation averaged 47.7 days, lactation averaged 52 days, there were one to three litters per year, with litters of one or two young. Subadult males reached reproductive maturity at 8 months, while females had their first estrus at 10 months. Comparisons of these reproductive traits with other species in the family Sciuridae suggest that reproduction in the Cape ground squirrel is influenced by a long active season in an unpredictable habitat. Of the potential factors affecting reproduction in squirrels, I conclude that climate is the most critical, and nutrition, predation, and allometry are less important.