John L. Koprowski

The response of tree squirrels to fragmentation: a review and synthesis

Habitat fragmentation is often considered a major threat to biodiversity; however, our understanding of how fragmentation impacts populations is poor. Identifying appropriate models for such studies is difficult. Tree squirrels are dependent on mature forests for food, cover and nests; these are habitats that are being fragmented rapidly and that are easily defined by humans. Squirrels represent excellent models for study of fragmentation. The literature on tree squirrels was reviewed to glean data on density and home-range size in forest fragments. Sufficient data were available on four species (Sciurus carolinensis, S. niger, S. vulgaris, Tamiasciurus hudsonicus). Density was negatively related to fragment size for S. carolinensis and S. niger and marginally so for T. hudsonicus. Sciurus vulgaris did not exhibit this relationship. Home-range size was analysed for three species of Sciurus and was positively related to forest fragment size for S. carolinensis and S. niger. Again, only S. vulgaris did not to show this relationship. Sciurus vulgaris is rarely found in small forest fragments and is believed to be especially sensitive to fragmentation; other tree squirrels appear to be sensitive to fragmentation in more subtle ways. Home range compaction provides a mechanism by which densities may increase in small fragments. The demographic consequences resultant from the high densities of squirrels found in small woodlots are not known but may explain the forest damage, avian nest predation and reduced diversity often cited to occur in woodland fragments.

Natal philopatry, communal nesting, and kinship in fox squirrels and gray squirrels

Tree squirrels (Sciuridae: Sciurini) are considered to be asocial; however, communal nesting is reported for most species. I examined the sociality of uniquely marked fox squirrels ( Sciurus niger ) and gray squirrels ( S. carolinensis ) to investigate interspecific patterns of natal philopatry, communal nesting, and kinship. Natal philopatry was female-biased in gray squirrels, resulting in the formation of kin clusters, but no fox squirrels remained in their natal areas. Adult male and female squirrels interacted less than expected, suggesting that the social systems of the sexes are separate. Communal nesting occurred in all seasons, but was most frequent in winter. Unrelated adult males commonly nested together in both species. However, adult female fox squirrels rarely nested in groups, although communal nests were common among female gray squirrels. Amicable behaviors were directed only toward closely related females ( r = 0.5) within kin groups of gray squirrels; aggression was directed primarily at unrelated females. The female-female bond was important in the formation of groups in gray squirrels.

Removal of copulatory plugs by female tree squirrels

Copulatory plugs form in the vagina of females from the semen of male fox squirrels ( Sciurus niger ) and eastern gray squirrels ( S. carolinensis ). Plugs are opaque white with waxy to rubbery consistency resembling those of other rodents. Although copulatory plugs frequently are hypothesized to prevent the successful copulation of subsequent males, female tree squirrels often remove the plug within 30 s of copulating and either discard or consume it. The removal by the female of a copulatory plug suggests that conflict may exist between the reproductive strategies of the sexes.

Response of Fox Squirrels and Gray Squirrels to a Late Spring-Early Summer Food Shortage

The response of adult and juvenile fox squirrels ( Sciurus niger ) and gray squirrels ( S. carolinensis ) to a shortage in their two major May-June foods was monitored and compared to a year of typical food abundance. Squirrels foraged more frequently, but less efficiently, during the year of fruit failure than during the typical year. Juvenile survival of both species was reduced during the year of fruit failure, but adult survival remained high. Juvenile losses occurred during late June when weights of adults and juveniles were lower than in the year of typical fruit abundance. By influencing juvenile survival and body condition of adults, late spring-early summer can be a critical period for populations of tree squirrels.

TREE SQUIRREL INTRODUCTION: A THEORETICAL APPROACH WITH POPULATION VIABILITY ANALYSIS

Abstract 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 between-patch 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.

DIRECT EFFECTS OF FIRE ON ENDANGERED MOUNT GRAHAM RED SQUIRRELS

Abstract Direct mortality of forest wildlife due to fire is rarely documented. In June and July 2004, the Nuttall Complex Fire burned 11,898 ha in the Pinaleño Mountains, southeastern Arizona. Portions of these mountains serve as the only habitat of endangered Mount Graham red squirrels (Tamiasciurus hudsonicus grahamensis). Survival of radio-collared red squirrels over a period that included the fire was lower in areas that were burned than on unburned areas; however, the majority of squirrels on our study areas survived the fire. Distance of nests from middens did not differ between pre-fire and post-fire periods for animals in burned or unburned areas, indicating that squirrels did not abandon middens immediately after surviving the effects of fire.

SELECTION OF DREY SITES BY ABERT'S SQUIRRELS IN AN INTRODUCED POPULATION

Abstract Aberts squirrels (Sciurus aberti) are reported to depend on ponderosa pine (Pinus ponderosa) for food and nest sites. Introduced Aberts squirrels in the Pinaleño Mountains of Arizona, however, occupy mixed-conifer forests that contain almost no ponderosa pine (about 2%). We examined selection of drey sites in this introduced population. Dreys (i.e., spherical nests) were built adjacent to the trunk at 75% of the tree height. Dreys were found in 5 different conifer species and <2% were in ponderosa pine. Drey trees were larger and had more access routes than did random trees. Drey sites were steeper, had more large trees, Douglas-fir (Pseudotsuga menziesii), and southwestern white pine (Pinus strobiformis), and less corkbark fir (Abies lasiocarpa var. arizonica) than random sites. The structural characteristics of drey trees in the Pinaleños population also were very similar to drey trees used by natural populations of Aberts squirrels in ponderosa pine forests. Our results suggest that the dependence of Aberts squirrels on ponderosa pine is not as strong as previously reported. Structural features such as tree size and access routes appear to be more important to selection of drey sites than tree species.

TIME BUDGETS, ACTIVITY PERIODS, AND BEHAVIOR OF MEXICAN FOX SQUIRRELS

Abstract Resources that limit reproductive success differ between the sexes in many vertebrates. We used focal animal observations to determine sex-specific activity periods, time budgets, and movements of endemic Mexican fox squirrels (Sciurus nayaritensis chiricahuae) in montane forests of southeastern Arizona during June, August, and December 1994 and January 1995. Activity periods during summer were bimodal with peaks in morning and afternoon separated by a midday lull in activity; winter activity was unimodal. No sex differences were detected in activity periods, although males retired later than females in winter. Resting, feeding, and locomotion were the most common behaviors and accounted for >75% of activities of each sex. Activity by females did not differ between seasons. Males spent more time in locomotion in summer and more time feeding in winter than in other seasons. Differences in time budgets of the sexes suggest that males and females apportion daily activity to maximize access to resources that limit fitness of each sex.

USE OF MIXED-CONIFER AND SPRUCE-FIR FORESTS BY AN INTRODUCED POPULATION OF ABERT'S SQUIRRELS (SCIURUS ABERTI)

Abstract Aberts squirrels (Sciurus aberti) are thought to depend on ponderosa pine (Pinus ponderosa) for food sources, cover, and nest sites. Records of Aberts squirrels using other food sources, forest types, and nest trees are rare. In the 1940s, Aberts squirrels were introduced to ponderosa pine forests on Mount Graham in the Pinaleño Mountains of southeastern Arizona. Since 1989, while studying Mount Graham red squirrels (Tamiasciurus hudsonicus grahamensis), we recorded Aberts squirrel sightings. In over 41,000 field-hours of studying Mount Graham red squirrels, we documented 498 Aberts squirrel sightings in both mixed-conifer and spruce-fir forests. Behaviors observed included feeding, collecting nest material, nest building, intraspecific chases, and interactions with the endangered red squirrels in this nontraditional habitat.

ANNUAL CYCLES IN BODY MASS AND REPRODUCTION OF ENDANGERED MT. GRAHAM RED SQUIRRELS

Abstract Annual cycles in life-history traits often are interpreted as adaptations. The Mt. Graham red squirrel (Tamiasciurus hudsonicus grahamensis) is an endangered species isolated in the Pinaleno Mountains of Arizona (United States) at the southernmost range terminus where the ecology is poorly known. I monitored annual cycles of reproduction and body mass in male and female Mt. Graham red squirrels. Males with scrotal testes were present during all months except October, with a peak in testis size in April. Lactating females were found from April to September. Male body mass was highest in winter and lowest in summer; females did not fluctuate seasonally in mass. Male body mass exceeded that of females during winter and was positively related to size of testes late in the breeding season. Annual cycles in reproduction and body mass of Mt. Graham red squirrels are similar to those of other red squirrels in similar life zones.