Winter nest trees of sympatric northern and southern flying squirrels: a test of reinforcement in a hybrid zone

Shifting range boundaries can lead to secondary contact of closely related species, which might in turn lead to hybridization when the evolution of reproductive isolation is incomplete. We examined winter nest use of northern (Glaucomys sabrinus Shaw, 1801) and southern flying squirrels (G. volans Linnaeus, 1758) in an area of recent secondary contact and known hybridization in Ontario, Canada to test for evidence of reinforcement due to different and diverging nesting behaviours. We radio-collared 26 flying squirrels (12 G. sabrinus and 14 G. volans) between two survey periods (winters of 2008-9 and 2019-20) and identified all nest trees used by individuals throughout each winter. For each nest tree we identified the nest type and collected tree classification information to compare differences in nest use between species. We also present a novel application of habitat suitability modelling to test for evidence of divergence in nest use through time, which would suggest reinforcement. We found southern flying squirrels used a higher proportion of cavities in large, hardwood trees, whereas northern flying squirrels used more external nests and softwood trees. Conditional probabilities provided some evidence for increased differentiation in nest use by flying squirrels through time. Overall, we found relatively little overlap in winter nest use between flying squirrel species, despite evidence for hybridization at this site, and some weak evidence for increased divergence between species in nest use over 11 years

A novel approach for tracing mycophagous small mammals and documenting their fungal diets

We developed a method combining passive baiting (animals that are not trapped) with DNA meta-barcoding of the feces acquired, to study fungi in the diet of small mammals. Mammal and fungal species were identified using genomic DNA of 596 fecal samples collected in five regions of the eastern Canadian boreal forest. For identification of the small mammal species, the cytochrome b region was used. A total of eight species of small mammals displayed hypogeous fungi consumption, with northern flying squirrels (Glaucomys sabrinus) and red-backed voles (Myodes gapperi) as the top consumers. For identification of their fungal diets, the ribosomal internal transcribed spacer (ITS) region was used. We recovered 722 taxa of Ascomycota, 429 Basidiomycota, 81 Zygomycota, 4 Chytridiomycota, 1 Glomeromycota, and 44 unidentified fungal taxa. Of these, 28 were hypogeous sequestrate fungi (underground fructification), which presumably are dug out by small mammals for consumption. Otherwise, for the remaining fungi [epigeous (above ground fructification) or microscopic fungal species], it is unclear which ones are selected by the animal as a dietary source or result from incidental contamination. Our paper presents a promising approach for tracing mycophagy in small mammals, and our results suggest that fungal diversity is important for the diet of some small mammals.

Corticosteroid-binding globulin levels in North American sciurids: implications for the flying squirrel stress axis

Corticosteroid-binding globulin (CBG) helps to regulate tissue bioavailability of circulating glucocorticoids (GCs), and in most vertebrates, ≥80%–90% of GCs bind to this protein. New World flying squirrels have higher plasma total cortisol levels (the primary corticosteroid in sciurids) than most vertebrates. Recent research suggests that flying squirrels have either low amounts of CBG or CBG molecules that have a low binding affinity for cortisol, as this taxon appears to exhibit very low proportions of cortisol bound to CBG. To test whether CBG levels have been adjusted over evolutionary time, we assessed the levels of this protein in the plasma of northern (Glaucomys sabrinus (Shaw, 1801)) and southern (Glaucomys volans (Linnaeus, 1758)) flying squirrels using immunoblotting, and compared the relative levels among three phylogenetically related species of sciurids. We also compared the pattern of CBG levels with cortisol levels for the same individuals. Flying squirrels had higher cortisol levels than the other species, but similar levels of CBG to their closest relatives (tree squirrels). We conclude that CBG levels in flying squirrels have not been adjusted over evolutionary time, and thus, the uncoupling of CBG levels from cortisol concentrations may represent an evolutionary modification in the lineage leading to New World flying squirrels.

Home range, den selection and habitat use of Carolina northern flying squirrels (Glaucomys sabrinus coloratus)

Context Understanding habitat selection is important for determining conservation and management strategies for endangered species. The Carolina northern flying squirrel (CNFS; Glaucomys sabrinus coloratus) is an endangered subspecies found in the high-elevation montane forests of the southern Appalachians, USA. The primary use of nest boxes to monitor CNFS has provided biased information on habitat use for this subspecies, as nest boxes are typically placed in suitable denning habitat. Aims We conducted a radio-telemetry study on CNFS to determine home range, den site selection and habitat use at multiple spatial scales. Methods We radio-collared 21 CNFS in 2012 and 2014–15. We tracked squirrels to diurnal den sites and during night-time activity. Key results The MCP (minimum convex polygon) home range at 95% for males was 5.2±1.2ha and for females was 4.0±0.7. The BRB (biased random bridge) home range at 95% for males was 10.8±3.8ha and for females was 8.3±2.1. Den site (n=81) selection occurred more frequently in montane conifer dominate forests (81.4%) vs northern hardwood forests or conifer–northern hardwood forests (9.9% and 8.7%, respectively). We assessed habitat selection using Euclidean distance-based analysis at the 2nd order and 3rd order scale. We found that squirrels were non-randomly selecting for habitat at both 2nd and 3rd order scales. Conclusions At both spatial scales, CNFS preferentially selected for montane conifer forests more than expected based on availability on the landscape. Squirrels selected neither for nor against northern hardwood forests, regardless of availability on the landscape. Additionally, CNFS denned in montane conifer forests more than other habitat types. Implications Our results highlight the importance of montane conifer to CNFS in the southern Appalachians. Management and restoration activities that increase the quality, connectivity and extent of this naturally rare forest type may be important for long-term conservation of this subspecies, especially with the impending threat of anthropogenic climate change.