Abstract
While running, small animals frequently encounter large terrain variations relative to their body size, therefore, terrain variations impose important functional demands on small animals. Nonetheless, we have previously observed in lizards that running specialists can maintain a surprisingly good running performance on very uneven terrains. The relatively large terrain variations are offset by their capacity for leg adjustability that ensures a ‘smooth ride’ of the centre of mass (CoM). The question as to how the effect of an uneven terrain on running performance and locomotor costs differs between species exhibiting diverse body build and locomotor specializations remains. We hypothesise that specialized runners with long hind limbs can cross uneven terrain more efficiently than specialized climbers with a dorso-ventrally flattened body and equally short fore and hind limbs. This study reports 3D kinematics using high-speed videos (325 Hz) to investigate leg adjustability and CoM movements in two lacertid lizards (Acanthodactylus boskianus, running specialist; Podarcis muralis, climbing specialist). We investigated these parameters while the animals were running on a level surface and over a custom-made uneven terrain. We analysed the CoM dynamics, we evaluated the fluctuations of the positive and negative mechanical energy, and we estimated the overall cost of transport. Firstly, the results reveal that the climbers ran at lower speeds on flat level terrain but had the same cost of transport as the runners. Secondly, contrary to the running specialists, the speed was lower and the energy expenditure higher in the climbing specialists while running on uneven terrain. While leg movements adjust to the substrates’ variations and enhance the stability of the CoM in the running specialist, this is not the case in the climbing specialist. Although their legs are kept more extended, the amplitude of movement does not change, resulting in an increase of the movement of the CoM and a decrease in locomotor efficiency. These results are discussed in light of the respective (micro-)habitat of these species and suggest that energy economy can also be an important factor for small vertebrates.
Highlights
While running, small animals frequently encounter large terrain variations relative to their body size, terrain variations impose important functional demands on small animals
In the present study we compare the centre of mass dynamics, limb behaviour and locomotor costs of A. boskianus with results for P. muralis when tested on the same terrain
The mass of the hind limbs is more than 2 times larger than the mass of the forelimbs in A. boskianus (5.5 g vs 2.2 g), while fore- and hind limbs masses are almost equal in P. muralis (1.8 g vs 2.3 g)
Summary
Small animals frequently encounter large terrain variations relative to their body size, terrain variations impose important functional demands on small animals. The climbing specialist commonly lives in rocky and uneven environments, our experimental terrain should strongly perturbate the running performance of this species because its habitat commonly offers many hiding places that do not require running any great distance Their anatomy, i.e. a flattened body with short limbs (see above), allows it to maintain close and firm contact with the substrate, P. muralis are expected to follow the uneven substrate topology closely, leading to perturbations in their running mechanics. In this context, we hypothesise that, on the flat terrain, A. boskianus will show better locomotor performance and lower costs to those of P. muralis[17,30,31]. The respective limb and CoM dynamics should result from the differences in limb length and design[4,32] and from the respective ecologically-relevant escaping strategies of these species, i.e. running a great distance in A. boskianus and hiding as fast as possible in P. muralis
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