Morphology, locomotor behaviour and microhabitat use in North American squirrels

Abstract

The rodent family Sciuridae (squirrels) presents an ideal system for examining the morphological, behavioural and ecological correlates of locomotor novelty. Ancestrally, sciurids were arboreal, a condition retained by modern tree squirrels. Two major transitions from the arboreal condition are hypothesized to have occurred in the sciurid clade: (1) flying squirrels evolved gliding from arboreal leaping (parachuting) and (2) chipmunks and ground squirrels became semiarboreal/terrestrial. This study examines morphology, locomotor behaviour and microhabitat use under controlled laboratory conditions in three North American sciurids: eastern chipmunk Tamias striatus, red squirrel Tamiasciurus hudsonicus and southern flying squirrel Glaucomys volans. Multivariate space was defined using a series of morphological measurements and by continuously sampling individuals moving under identical conditions in a naturalistic enclosure, controlling for the proximate effects of microhabitat structure. Morphospace was characterized by a contrast between proximal and distal limb elements -flying squirrels exhibited elongated forelimbs and shanks and shortened forefeet and hindfeet, while chipmunks exhibited the opposite pattern. Ethospace was characterized by a contrast between aerial locomotion in flying squirrels and groundbased locomotion in chipmunks. In both instances, red squirrels occupied intermediate positions, as predicted on the basis of retention of ancestral features. Despite significant differences among species at morphological and behavioural levels, ecospace was characterized by overlap between flying squirrels and red squirrels in the use of high supports, and between flying squirrels and chipmunks in the use of large-diameter supports. The lack of concordance at the ecological level underscores the difficulty in making predictions based solely upon organismal design.

The kinematic consequences of locomotion on sloped arboreal substrates in a generalized (Rattus norvegicus) and a specialized (Sciurus vulgaris) rodent

“…Studies dealing with the effects of sloped substrates on locomotion in mammals have focused on locomotor behavior in general (Zwahlen, 1975;Youlatos, 1999;Essner, 2007) and specifically on inclined terrestrial substrates [treadmill (Prost and Sussman, 1969;Vilensky et al, 1994); trackway (Carlson-Kuhta et al, 1998;Lammers et al, 2006;Kivell et al, 2010)], kinetics (Lammers, 2007), the cost of transport (e.g. Wickler et al, 2000;Hanna et al, 2008), metabolism (Wunder and Morrison, 1974;Armstrong et al, 1983), rates of oxygen consumption (Snyder and Carello, 2008) and muscle activity (Carlson-Kuhta et al, 1998;Gillis and Biewener, 2001;Nakano, 2002).…”

Arboreal locomotor and postural behaviour of European red squirrels (Sciurus vulgaris L.) in northern Greece

“…The extended lumbar region, the moderately elongated limbs and the arrangement of the main propulsive muscles in these squirrels appear to facilitate the required regular fore and aft movements of the limbs during such bounding and walking (Peterka 1936;Bryant 1945;Thorington and Heaney 1981;Emry and Thorington 1982;Thorington and Thorington 1989;Thorington et al 1997). Based on these morphological and behavioural observations, relatively high rates of quadrupedalism can be expected in larger squirrels, although shifts between morphological and behavioural attributes may not coincide (Essner 2007). Unfortunately, we lack any information on the positional behavior of larger, purely arboreal squirrels.…”

“…In contrast, larger species ([300 g) engage more frequently in quadrupedalism and leaping, mainly using smaller horizontal supports (Garber and Sussman 1984;Youlatos 1999;Stafford et al 2003;Essner 2007). However, eco-behavioural divergence does not always follow morphological variability, implying that interactions among levels are complex (Youlatos 1999;Essner 2007).…”

“…Detailed data on the positional behaviour of arboreal squirrels are relatively limited but they do suggest considerable diversity: small-bodied species (\200 g) tend to engage in extensive clawed locomotion and frequently use large vertical supports in the lower parts of the forest (Youlatos 1999;Essner 2007;Youlatos et al 2008). In contrast, larger species ([300 g) engage more frequently in quadrupedalism and leaping, mainly using smaller horizontal supports (Garber and Sussman 1984;Youlatos 1999;Stafford et al 2003;Essner 2007).…”

“…In this context, mammalian groups, such as arboreal squirrels, with a relatively recent evolutionary history (early Oligocene: Emry and Thorington 1982), but with remarkable diversity in size, morphology, ecology and behaviour (Cartmill 1974;Emry andThorington 1982, 1984;Roth 1996;Steppan et al 2004;Essner 2007;Hayssen 2008), would be good models for the study of similar complexes.…”