Bear Anatomy

[brotherbear]

http://jeb.biologists.org/content/218/19/3102

RESEARCH ARTICLE
Grizzly bear (Ursus arctos horribilis) locomotion: gaits and ground reaction forces
Catherine L. Shine, Skylar Penberthy, Charles T. Robbins, O. Lynne Nelson, Craig P. McGowan
Journal of Experimental Biology 2015 218: 3102-3109; doi: 10.1242/jeb.121806

Locomotion of plantigrade generalists has been relatively little studied compared with more specialised postures even though plantigrady is ancestral among quadrupeds. Bears (Ursidae) are a representative family for plantigrade carnivorans, they have the majority of the morphological characteristics identified for plantigrade species, and they have the full range of generalist behaviours. This study compared the locomotion of adult grizzly bears (Ursus arctos horribilis Linnaeus 1758), including stride parameters, gaits and analysis of three-dimensional ground reaction forces, with that of previously studied quadrupeds. At slow to moderate speeds, grizzly bears use walks, running walks and canters. Vertical ground reaction forces demonstrated the typical M-shaped curve for walks; however, this was significantly more pronounced in the hindlimb. The rate of force development was also significantly higher for the hindlimbs than for the forelimbs at all speeds. Mediolateral forces were significantly higher than would be expected for a large erect mammal, almost to the extent of a sprawling crocodilian. There may be morphological or energetic explanations for the use of the running walk rather than the trot. The high medial forces (produced from a lateral push by the animal) could be caused by frontal plane movement of the carpus and elbow by bears. Overall, while grizzly bears share some similarities with large cursorial species, their locomotor kinetics have unique characteristics. Additional studies are needed to determine whether these characters are a feature of all bears or plantigrade species.

Within terrestrial animals a continuum of foot postures exists, from plantigrade species with their entire foot on the ground, to unguligrade animals that stand on the tips of their toes (Ginsburg, 1961; Carrano, 1997). The plantigrade posture is ancestral for mammals and it is generally agreed that digitigrade and unguligrade postures evolved as adaptations for speed and endurance. Because of this, numerous studies have examined the gait mechanics of digitigrade and unguligrade species (Budsberg et al., 1987; Hutchinson et al., 2006; Robilliard et al., 2007; Hudson et al., 2012). However, relatively few studies have examined the links between the plantigrade posture and locomotor mechanics. Plantigrade species are considered locomotor generalists, and because of the lack of cursorial specialisations, their limb movements are less restricted to the sagittal plane (Liem et al., 2001). Within mammals, plantigrade species include raccoons, badgers, weasels, as well as all rodents and primates. All of these animals are small compared with most digitigrade and especially unguligrade species; however, bears also retain the plantigrade stance. The goal of this study was to determine whether the locomotor mechanics of a stereotypical plantigrade quadruped, the grizzly bear (Ginsburg, 1961), differ from those of more extensively studied cursorial quadrupeds. 
 
The selection of gaits used by plantigrade and cursorial species could represent some of the locomotor differences observed between these postures. Analysis of gaits, through footfall patterns, has been applied broadly to a wide range of terrestrial species (e.g. Gray, 1968; Hildebrand, 1976, 1977). Within quadrupedal animals, a lateral walk, in which the placement of the hindfoot is followed by the placement of the ipsilateral forefoot, is the gait used at slow speeds by the majority of species, including bears (Hildebrand, 1976). But, there is variation in terms of intermediate and faster gaits. The most common intermediate gait is the trot, defined by diagonal couplets, as this is seen in digitigrade (e.g. dogs and cats) and unguligrade (e.g. horses) animals, although these animals will also use a pace (ipsilateral couplets; Alexander, 1984). Interestingly, plantigrade carnivorans have not been shown to trot, but there have been a few observations of a pace (McClearn, 1992). Faster gaits include canters and gallops. Canters can be considered a slow gallop; however, they are characterised as being a three beat gait with one diagonal couplet (Hildebrand, 1976). Rotary gallops, as described above for the lateral walk, and transverse gallops, with the leading hindfoot placement being followed by the contralateral forefoot, can both be observed in the same species (Vilensky and Larson, 1989; Walter and Carrier, 2007), although there may be energetic differences between them (Bertram and Gutmann, 2009). Gallops are the fastest gait used by quadrupedal animals and studies have demonstrated that galloping occurs in species representing all three foot postures – unguligrade, digitigrade and bears within plantigrade species (Hildebrand, 1989; Renous et al., 1998; Robilliard et al., 2007; Walter and Carrier, 2007).



Within carnivorans, bears are the most plantigrade along the posture continuum (Ginsburg, 1961). The specific morphological features defining plantigrady include: well-developed digits on both forefeet and hindfeet; different sizes of the metapodials, e.g. metapodials 3 and 4 are rarely the same length in plantigrade species; and a substantial angle produced between the ulna and the humerus during elbow extension (20 deg in bears; Ginsburg, 1961). Ursidae is considered a generalist family; yet, the individual species exhibit substantial differences in diet, habitat and ecology. Grizzly bears have the broadest range of behaviours in Ursidae and are able to climb (particularly as juveniles), swim and have been reported to run as fast as 13.3 m s−1 (Garland and Janis, 1993; Brown, 2009). There has been very limited research into the locomotion and biomechanics of Ursidae (Gambaryan, 1974; Inuzuka, 1996; Renous et al., 1998); however, it is likely that differences in limb morphology and locomotor behaviour may exist within Ursidae (Irschick and Garland, 2001), as well as between bears and other quadrupeds.


Previous studies have shown that locomotion by cursorial animals over a large size range can be described as dynamically similar across all speeds (Farley et al., 1993; Alexander, 2005). Locomotion is considered to be dynamically similar if, at a given dimensionless speed (Froude number), parameters can be made identical by multiplying forces, linear dimensions and time intervals by constant factors (Alexander and Jayes, 1983). In their seminal study, Alexander and Jayes (1983) characterised cursorial animals as those that stand with the humerus and femur closer to vertical than horizontal, which excludes other morphological characteristics that are considered cursorial in other studies (described above). 
 
Relative to cursorial species, bears appear to have substantial movement in the frontal plane during locomotion. For example, bears have an unusual carpal movement, which manifests as a medial rotation during swing (Davis, 1949; Gray, 1968; Inuzuka, 1996). Further, grizzly bears have a medially directed forefoot position during stance, relative to the direction of travel. This differs from most cursorial species, which limit movement to the frontal plane to enhance efficiency and restrict forces to the direction of travel (Liem et al., 2001). Because of this, the mediolateral forces generated by cursorial animals are comparatively small and frequently ignored in the analysis of locomotion (Budsberg et al., 1987). However, some primates walking bipedally and animals with sprawling gaits have been shown to produce mediolateral ground reaction forces equal to or greater than the magnitude of their anterior–posterior forces (Willey et al., 2004). Currently, it is unclear to what extent the forces generated by bears during locomotion are similar to or differ from those of well-studied groups of terrestrial mammals, particularly considering the angle of the forefoot during stance.



In addition to terrestrial locomotion, the forelimbs may be involved in a wide range of other activities, especially in non-predatory carnivorans that may forage for food or exhibit escape behaviours such as climbing. The requirement of predators to chase down vertebrate prey overcomes the need for dexterity upon capture; therefore, forelimb dexterity in carnivores is negatively correlated with vertebrate predation. Bears and other plantigrade carnivores (i.e. generally omnivorous species) have higher dexterity scores than digitigrade carnivorans (Iwaniuk et al., 2000). Contributing to this dexterity is the morphology of the forelimbs, such that the ulna and radius are separate in plantigrade animals, resulting in the ability to supinate and pronate (rotate the forearm to point the palm up or down). In cursorial animals, the ulna and radius are fused to increase stability and therefore speed (Liem et al., 2001). Additionally, pentadactyly is only retained in plantigrade species as loss of digits is characteristic of digitigrade and unguligrade postures; this is associated with the reduction of distal limb mass that, along with elongation of the distal limbs, increases speed in cursorial animals (Garland and Janis, 1993). The difference in forelimb bone anatomy, as well as the differences in ecology, between cursorial and plantigrade species of the Carnivora is likely to have resulted in differences in locomotion.



The overall goal of this study was to determine whether locomotion by grizzly bears differs from that of other large quadrupedal animals, which tend to be digitigrade or unguligrade. We hypothesised that the gaits used by grizzly bears would be similar to those used by smaller plantigrade animals, as opposed to similarly sized cursorial animals, because of the differences in morphology of the distal limb. Further, we predicted that the mediolateral ground reaction forces would be higher in forelimbs of bears, compared with other species, as a result of their medially directed stance. These hypotheses were addressed by examining the footfall patterns and stride parameters of grizzly bears to identify gaits, and characterising the magnitude, time-varying shape and relative distribution of three-dimensional ground reaction forces generated by the forelimbs and hindlimbs over a range of speeds. 

 
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