Elastic energy storage across speeds during steady-state hopping of desert kangaroo rats (Dipodomys deserti)

Abstract

Small bipedal hoppers, including kangaroo rats, are thought to not benefit from substantial elastic energy storage and return during hopping. However, recent species-specific material properties research suggests that, despite relative thickness, the ankle extensor tendons of these small hoppers are considerably more compliant than had been assumed. With faster locomotor speeds demanding higher forces, a lower tendon stiffness suggests greater tendon deformation and thus a greater potential for elastic energy storage and return with increasing speed. Using the elastic modulus values specific to kangaroo rat tendons, we sought to determine how much elastic energy is stored and returned during hopping across a range of speeds. In vivo techniques were used to record tendon force in the ankle extensors during steady-speed hopping. Our data support the hypothesis that the ankle extensor tendons of kangaroo rats store and return elastic energy in relation to hopping speed, storing more at faster speeds. Despite storing comparatively less elastic energy than larger hoppers, this relationship between speed and energy storage offer novel evidence of a functionally similar energy storage mechanism, operating irrespective of body size or tendon thickness, across the distal muscle-tendon units of both small and large bipedal hoppers.

Unlocking the secrets of kangaroo locomotor energetics: Postural adaptations underpin increased tendon stress in hopping kangaroos

“…The only mammals with comparable EMA values to kangaroos are rodents <1 kg (Fig. 7b) (Biewener 1990), such as kangaroo rats which have relatively thicker tendons that may be less suited for recovering elastic strain energy (Biewener and Blickhan 1988) (but see: Christensen et al (2022)). Fig.…”

“…In tammar wallabies, ankle tendon stress increased with hopping speed, leading to a greater rise in elastic strain energy return than muscle work, which increases the proportion of work done by tendon recoil while muscle work remains near constant (Baudinette and Biewener 1998, Biewener et al 1998). Size-related differences in ankle extensor tendon morphology (Bennett and Taylor 1995, McGowan et al 2008), and the resultant low strain energy return, may explain why small (<3 kg) hopping macropods and rodents appear not to be afforded the energetic benefits observed in larger macropods (Thompson et al 1980, Biewener et al 1981, Biewener et al 1998) (but see Christensen et al (2022)). However tendon morphology alone is insufficient to explain why large macropods can increase speed without cost, while large quadrupeds with similar tendon morphology cannot (Dawson and Webster 2010).…”

Alternative highland adaptations: organ masses and fur insulation in Afroalpine rodents

“…3). Typical for cursorial runners, such as vole‐like animals, this type of locomotion is postulated to be energetically costly when compared to saltatory locomotion (Christensen et al., 2022; McGowan & Collins, 2018). Therefore, cursorial species might need to invest more energy in endurance running, for example, to evade predators, in contrast to saltatory species, that can jump to avoid becoming prey (Moore et al., 2017).…”