The anatomy of a range expansion: changes in cranial morphology and rates of energy extraction for North American red squirrels from different latitudes

Abstract

2003. The anatomy of a range expansion: changes in cranial morphology and rates of energy extraction for North American red squirrels from different latitudes. -Oikos 102: 33 -44.Species with expanding ranges provide unique opportunities to examine environmentally induced adaptations in ecological traits and anatomical characteristics. Since the late 1800s, the North American red squirrel (Tamiasciurus hudsonicus) has expanded its range into the central hardwoods of the United States in conjunction with increasing agricultural fragmentation. We examined whether red squirrels from the central hardwoods (west-central Indiana, USA) displayed differences in foraging behaviors and morphology relative to red squirrels from conifer-dominated environments (upper peninsula of Michigan, USA), a biome in which red squirrels evolved. Specifically, we measured rates of energy extraction, variation in cranial morphology, and diet preference between red squirrels from both regions. In addition, we compared foraging behaviors of red squirrels from the central hardwoods to those of a competitor that coevolved with nut-producing trees, the gray squirrel (Sciurus carolinensis). Red squirrels from Indiana and Michigan differed significantly in the efficiency with which they used food items, with individuals from each region extracting calories at a more rapid rate for items that were common in their region. The enhanced efficiency of southern red squirrels feeding on black walnuts (Juglans nigra) was correlated with geographic differences in cranial morphology; skulls of southern squirrels were larger, with longer jaws and higher metrics associated with greater mandibular force than northern squirrels. Contrary to our expectations, red squirrels from Indiana and Michigan did not differ qualitatively in preferences for food items, suggesting that diet choice may be governed by perishability of food items rather than by rates of energy extraction. Gray squirrels were more efficient than Indiana red squirrels in using all food items, and differed only slightly from red squirrels with regard to preference for food items. Measures of efficiency of resource use, after accounting for species-specific metabolic requirements, suggest that red squirrels are unlikely to compensate ecologically for declining gray squirrel populations in fragmented portions of the central hardwoods, with potentially adverse effects for forest regeneration and succession. Our results demonstrate that invading species can display significant flexibility in adapting to new environments, but they may not be flexible enough to exploit resources in a manner comparable to native species.

Relating Red Squirrel Body Size to Different Conifer Cone Morphologies within the Same Geographic Location

“…Likewise, Boutin and Larsen (1993) report heavier squirrels in spruce habitat relative to pine habitat (Pinus banksiana), a difference of approximately 35-40 g for both males and females, but larger squirrels from spruce habitat were sampled further north (Yukon) than squirrels from pine habitat (Alberta), suggesting a latitudinal effect. In contrast, Smith (1970) found heavier squirrels in pine habitat (T. hudsonicus X ¼ 243.9 g) versus those caught within other conifer habitats (T. hudsonicus X ¼ 229.0 g; T. douglasii X ¼ 209.9 g), and Lindsay (1986) found a similar multivariate result for T. douglasii; larger squirrels associated with larger, tougher cones (also see Goheen et al [2003] for similar results in nutproducing forests). Published literature presents mixed results regarding squirrel size and cone serotiny.…”

Finite element analysis of performance in the skulls of marmosets and tamarins

“…2008). This very proximate measure of performance speaks to the direct interaction between an organism and its environment, and field studies have demonstrated the rapid evolution of bite force in populations that encounter selection for a more resistant diet (Goheen et al. 2003; Herrel et al.…”

“…In studies of feeding, bite force is often used as a performance variable because it reflects the potential breadth of an animal's diet based on food hardness (Anderson et al 2008). This very proximate measure of performance speaks to the direct interaction between an organism and its environment, and field studies have demonstrated the rapid evolution of bite force in populations that encounter selection for a more resistant diet (Goheen et al 2003;Herrel et al 2008). The question is, how do FEA-based measures of mechanical performance relate to aspects of organismal performance that are seen by natural selection?…”

Degradation of Boreal Forests by Nonnative Herbivores in Newfoundland's National Parks: Recommendations for Ecosystem Restoration

“…Squirrels (Sciurus spp.) are recognized as important predators of pollen and seed cones (Hurly et al 1987) and have the capacity to reduce seed availability and to influence forest regeneration (Gill et al 1995;Goheen et al 2002). In TNNP, Boa-Antwi (2009) documented a 66% reduction in balsam fir seed from the combined effects of cone predation by red squirrels and cone/seed eating insects during a season of high cone production.…”