Ilpo K. Hanski

HOME-RANGE SIZE, MOVEMENTS, AND NEST-SITE USE IN THE SIBERIAN FLYING SQUIRREL, PTEROMYS VOLANS

Abstract The Siberian flying squirrel (Pteromys volans) is a herbivorous, nocturnal, and arboreal rodent living in boreal coniferous forests. Home-range sizes, movements, and nest-use behavior of Pteromys were studied by radiotelemetry in southern Finland in 1996–1998. Thirty-seven animals were tracked. Average home-range size measured by 100% minimum convex polygons was 59.9 ha for males and 8.3 ha for females. Both sexes concentrated their activities in core areas that represented 9% and 11% of the home-range areas in males and females, respectively. Home ranges of males and females were several times larger than predicted according to body mass. Similarly, home ranges of Pteromys were much larger than in other gliding herbivores. Males especially showed great mobility; the average distance moved from the nest at night was 292 m, and the longest distances recorded were >2 km. A plausible explanation for the large home ranges and great mobility of Pteromys is its gliding ability; both sexes can reach distant parts of the home range for foraging, and males also can reach distant parts for receptive females. Pteromys had several nests, both cavities and dreys (nests in branches of trees), which they changed frequently.

Home Ranges and Habitat Use in the Declining Flying Squirrel Pteromys volans in Managed Forests

The flying squirrel Pteromys volans is an arboreal rodent and inhabitant of Palearctic boreal forests. In Finland, the flying squirrel has been classified as a declining species which needs to be monitored. I studied home ranges, habitat use and nocturnal activity of eight adult flying squirrels by radio tracking in fragmented coniferous forests in Finland during June - December, 1996. Average home-range size of the flying squirrel measured by the 100% MCP was 6.5 ha. In summer, the average size of the 95% cluster area was 2.3 ha and the 80% core area 0.5 ha. The core areas represented only 7.8% of the 100% MCP area and were composed of 2–6 separate patches in the home ranges of individual squirrels. Radio-tagged squirrels used several nests, both old woodpecker cavities and dreys for nesting and diurnal roosting. The combined density of all deciduous tree species was significantly greater in the 80% core areas than within the 100% MPC in the summer data set. In the polychotomous logistic regression model the great canopy cover, high densities of alders Alnus incana and A. glutinosa and aspen Populus tremula significantly explained the ranked utilisation classes (utilisation rank from highly used areas to least used areas: 80% core - 95% cluster - 100% MCP). The three most abundant deciduous trees species (birches Betula pendula and B. pubescens, aspen, alder) constituted 87% of trees used by squirrels in summer. Flying squirrels were found in aspens more often than expected according to their availability. The results show a clear preference for deciduous trees and a preference for the parts of home ranges with higher densities of alders and aspen. The flying squirrel seems to be capable of using several cover types, including young forest stands, as foraging and moving areas and are able to move across semi-open clear-cut areas.

Siberian flying squirrel responses to high- and low-contrast forest edges

We examined responses of Siberian flying squirrels ( Pteromys volans ) to edges between nesting habitat (mature spruce forests), movement habitat (other forests, pine bogs), and open areas within their home ranges in southern Finland in 1996-2000. Radio-tracked squirrels (n=146) were generally associated to edges when they were ac tive at night. Compared to distances expected from the habitat pattern of their home range, squirrels occurred closer to high-contrast edges (of open areas) and low-contrast edges (nesting or movement forest types). Asso ciation with edges of open areas was more pronounced when squirrels were in movement habitat than in nesting habitat, possibly because of stronger channeling of movements in the former habitat. When in nesting habitat, squirrels responded more strongly to field edges than to recent clearcut edges, probably as a result of the pres ence of more deciduous trees on field edges, unlike clearcut edges. Responses to open areas were independent of spatial scale. However, responses to movement habitat from nesting habitat, and vice versa, were more pronounced over hundreds than tens of meters. Nesting cavities and dreys were generally located at random with respect to edges. We conclude that squirrel responses to edges of landscape attributes are diverse and depend both on spatial scale and edge contrast.

Condition-dependent, phenotype-dependent and genetic-dependent factors in the natal dispersal of a solitary rodent

1. Dispersal can be condition- and phenotype-dependent and related to individual genetic differences. Few studies have addressed the relative importance of these factors on dispersal. We studied the factors behind philopatry and dispersal in juvenile Siberian flying squirrels, Pteromys volans L. 2. The dispersal distance and the distances explored before abandoning the natal nest were not related to any of the condition-dependent factors studied such as the area of high-quality habitat or the number of conspecifics near the natal area. In addition, the body mass (a phenotypic trait) of individuals was not related to philopatry and dispersal in flying squirrels. 3. Genetic variability, measured by microsatellite heterozygosity, was positively correlated with dispersal. The correlation was mainly driven by one locus related to the distances explored before abandoning the natal nest. 4. We conclude that condition- and phenotype-dependent factors did not have detectable effects on philopatry and dispersal, but individual heterozygosity was related to dispersal in flying squirrels. Our results suggest that genetic variability is important behind the dispersal of the species.

Gene flow and natal dispersal in the Siberian flying squirrel based on direct and indirect data

Dispersal is a key determinant of the evolution and ecology of species. For a comprehensive picture of dispersal, a combination of both field observations and indirect genetic measures are required, as both of these have strengths that may mitigate the other’s limitations. Here, we used microsatellite markers and radio-telemetry data to study dispersal and gene flow in Siberian flying squirrels. Genetic data confirmed our empirical results that dispersal is female biased in the flying squirrel. Female bias in dispersal is exceptional among mammals and in flying squirrels is probably explained by competition for food resources and nesting cavities among mothers and daughters. The individual-level genetic pattern was influenced by isolation by distance. Using this information fairly comparable dispersal distances were derived using indirect data as observed directly with radio telemetry. Thus, our results support the recent conclusion that individual-level genetic data can be useful in inferring dispersal distances for species for which direct data are lacking.

Size differentiation in Finnish house sparrows follows Bergmann's rule with evidence of local adaptation

Bergmanns rule predicts that individuals are larger in more poleward populations and that this size gradient has an adaptive basis. Hence, phenotypic divergence in size traits between populations (PST) is expected to exceed the level of divergence by drift alone (FST). We measured 16 skeletal traits, body mass and wing length in 409 male and 296 female house sparrows Passer domesticus sampled in 12 populations throughout Finland, where the species has its northernmost European distributional margin. Morphometric differentiation across populations (PST) was compared with differentiation in 13 microsatellites (FST). We find that twelve traits phenotypically diverged more than FST in both sexes, and an additional two traits diverged in males. The phenotypic divergence exceeded FST in several traits to such a degree that findings were robust also to strong between‐population environmental effects. Divergence was particularly strong in dimensions of the bill, making it a strong candidate for the study of adaptive molecular genetic divergence. Divergent traits increased in size in more northern populations. We conclude that house sparrows show evidence of an adaptive latitudinal size gradient consistent with Bergmanns rule on the modest spatial scale of ca. 600 km.

Communal nesting is explained by subsequent mating rather than kinship or thermoregulation in the Siberian flying squirrel

Research on group living in animals is concentrated on highly social species, but studying less social species may hint at the factors possibly leading to the evolution of increased sociality. Thermoregulation is often thought to explain communal nesting in solitarily breeding mammals but also other factors may be involved. For example, it is observed that even solitary species may have cryptic kin cooperation. We studied factors affecting communal nesting in the Siberian flying squirrel. Flying squirrels breed solitarily but, similar to most other rodents, adults may sometimes huddle in groups. Communal nesting in flying squirrels was most frequent during winter and autumn, but also occurred during other seasons. This pattern was explained by the breeding season, which took place in the spring–summer, when communal nesting was less common. Neither monthly temperature, after accounting for breeding season, nor daily temperatures in winter explained communal nesting. Group size was small, two to three individuals. In most cases the group was a pair consisting of unrelated male and female, possibly indicating that group formation was related to mating behavior. This study contributes to the understanding of mammalian group formation in two major ways. First, our study contributes to the understanding of the role of relatedness in rodent group formation, demonstrating a case where close relatedness is not related to group formation. Second, our study indicates that in a solitarily breeding, rodent huddling may be more driven by other factors than temperature.

Mating system and reproductive success in the Siberian flying squirrel

Abstract Mating systems and factors affecting reproductive success are much studied especially for mammals that are large bodied, have marked sexual size dimorphisms, and have a female-defense mating system. For species that deviate from these patterns, we need more information on mating systems and reproductive success. Here, we study mating system and factors related to reproductive success in a solitary rodent, the Siberian flying squirrel (Pteromys volans). In contrast to most other mammals, males are not larger than females in flying squirrels. Similarly to most mammalian species, we observed multimale paternity within litters and reproductive success of males being positively related to body mass. Variation in reproductive success was clearly higher for males than for females, although remained lower than observed in species with highly male-biased sexual size dimorphism. Female flying squirrels lived in nonoverlapping home ranges and reproductive success was positively related to body mass, in line with earlier predictions for large female size in mammals.

Predictors of long-distance dispersal in the Siberian flying squirrel

During dispersal the distances moved differ between individuals. The evolutionary causes of dispersal rate are much studied, for example, it is observed that dispersal is often a condition- and phenotype-dependent strategy. However, more empirical information is needed on factors affecting the dispersal distance. We study factors behind dispersal distance in the juvenile Siberian flying squirrel. The longer dispersing individuals abandoned natal site earlier in the season and were larger, perhaps being born earlier, than shorter dispersing individuals. These patterns did not hold between same-sex siblings, indicating that the early long-distance dispersal was more a between than a within-litter related phenomenon. Our results indicate differences between litters that are related to dispersal strategies of individuals. In flying squirrels, long-distance dispersal is not merely a secondary effect of short-distance dispersal. Instead, the distribution of dispersal distance is affected by factors enhancing long-distance dispersal.

Movements of dispersing flying squirrels in relation to siblings and parents

Knowledge of kin interactions can be informative in explaining the processes underlying dispersal. By dispersing, relatives can avoid kin competition for resources or mates and prevent inbreeding. We investigated sibling movements in relation to each other and parents before and after dispersal in radio-collared juvenile flying squirrels (Pteromys volans L.). Before dispersal, most siblings moved in different areas and were not in contact while exploring the surroundings of natal area. After dispersal, all siblings settled far away from each other, as they dispersed different distances and to random directions compared to each other. No clear effect of litter size or presence of same-sex siblings was observed on dispersal. Time spent in the same nest with the mother did not differ between dispersing sons and daughters. Mother did not force dispersers to leave the natal nest; instead, the mother often moved away from the nest before offspring. Father and offspring were not observed to be in contact before dispersal. Dispersal direction was unaffected by the location of fathers’ home range. We conclude that after dispersal interaction between relatives is limited in flying squirrels, but we did not find any indication that interaction between relatives before dispersal is determining dispersal decision. Siblings’ settlement far away from each other was already indicated by the movement within the natal home range, but, interestingly, the latter could not be used to predict dispersal patterns of sexes.