Limb preference in the gallop of dogs and the half-bound of pikas on flat ground

51 R. HACKERT, L.D. MAES, M. HERBIN, P.A. LIBOUREL, A. ABOURACHID 52 UMR 7179, Dpt EGB, Museum National d’Histoire Naturelle, Paris, France 53 57 rue Cuvier CP 55 75231 Cedex 54

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56 Abstract - During fast locomotion — gallop, half bound — of quadruped mammals, the ground contact of 57 the limbs in each pair do not alternate symmetrically. Animals using such asymmetrical gait thus choose 58 whether the left or the right limb will contact the ground first, and this gives rise to limb preference. Here, 59 we report that dogs (Mammalia, Carnivora) and pikas (Mammalia, Lagomorpha) prefer one forelimb as 60 trailing limb and use it as such almost twice as often as the other. We also show that this choice depends on 61 the individual and is not a characteristic of the species, and that the strength of the preference was not 62 dependent on the animal’s running speed. 63

64 Keyword: handedness, quadruped, locomotion, laterality, asymmetrical gaits, Carnivora, Lagomorpha, 65 stability. 66

67 INTRODUCTION 68 The expression of brain lateralization has been observed in behaviours as different as feeding, manipulation, 69 and communication in several families of vertebrates. Limb preference is a subset of this expression and has 70 been documented in birds and in most families of tetrapods including toads, anurans and mammals (for review 71 in Vallortigara et al., 1999). In some species, sex was identified as a determinant of the direction of the 72 preference at population level (in dogs: Wells, 2003; Quaranta et al., 2004; Poyser et al., 2006; in cats: Tan et 73 al., 1990) but not of the strength of preference. The strength of preference is thought to be task dependent and 74 increases as tasks require more cognitive function (Fagot and Vauclair, 1991) and coordination (Hopkins, 75 1995).
76 Curiously, limb preference during locomotion has not been extensively explored, although locomotion is the 77 primary function of limbs. Malashichev (2006) observed five species of anuran and found a limb preference in 78 those that exhibit an “alternated limb locomotion or other unilateral limb activity”. He suggested that the 79 degree of lateralization in the motor response depends on the mode of locomotion used by a species. In the 80 asymmetrical gaits of mammals– i.e. gallop, bound, and half-bound — the fore and hind pairs of legs make 81 contact with the ground alternately. During gallops or half-bounds, the motions of the two limbs of a pair are 82 not symmetrically alternated, such that one limb of each pair touches the ground first and is called the trailing 83 limb, the other one is called leading limb. The trailing and leading limb of each pair cannot be systematically 84 assigned to the left or right limb because during locomotion animals switch trailing and leading limbs from 85 time to time. In their study of the gallop of four race horses, Deuel and Lawrence (1987) compared the 86 kinematics of trailing and leading limbs. All the horses preferred the right limb as trailing limb. Moreover, the 87 duration of the stance phase of the trailing forelimb differed significantly according to whether it was the right 88

2 or the left limb. Walter and Carrier (2007) observed that four of the six dogs in their study preferred one limb 89 as the trailing limb during gallop. Finally, Hook and Rogers (2002) found that marmosets land preferentially 90 with the right limb and that the choice of this limb is correlated with the leading limb used during leaping and 91 walking. 92 There have been few quantitative studies of the direction or the strength of limb preference during animal 93 locomotion. We therefore investigated this issue and changes in laterality with speed in dogs and pikas, two 94 mammals with different morphologies, behaviours and gaits. Dogs are medium-sized digitigrades that trot over 95 a large range of speeds before switching to galloping at high speeds. In contrast, pikas are small plantigrades 96 that switch to half-bound locomotion for all speeds faster than walking (Gambaryan, 1974; Fischer and 97 Lehmann, 1998). 98 ANIMALS AND METHODS 99 We studied five dogs (Canis familiaris, Mammalia: Carnivora) and four pikas (Ochotona rufescens, 100 Mammalia: Lagomorpha). The dogs were male Belgian shepherd malinois of similar size (withers 101 height=0.61±0.05 m, BW=28.3±2.0 kg); they belonged to the French army and were thus healthy, obedient, 102 and used to energetic exercise. Pikas (withers height=0.05 m, body length=0.16 m, BW=150-200 g) were 103 reared at the IPBS (CNRS, Toulouse) by A. Puget and were kept at the Motion Lab, University of Iena, 104 Germany; they were reared and used in conformity with German animal welfare regulations. 105 Methods 106 Dogs were studied while running outdoors along a 1 m

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