Arboreality in acacia rats (Thallomys paedulcus; Rodentia, Muridae): gaits and gait metrics

Abstract

The acacia rat Thallomys paedulcus is a small arboreal rodent, extensively dependent on Acacia sp. trees. In order to understand the arboreal locomotor adaptations of the species, we examined their gaits in arboreal locomotion (i.e. diagonality, duty factor, duty factor index, velocity, and stride length and frequency). For these purposes, we filmed 12 captive specimens on simulated arboreal substrates of variable sizes (2 mm, 5 mm, 10 mm, 25 mm) and inclinations (0° and 45°). Acacia rats employed slow, symmetrical gaits with lower diagonality on the smaller substrates, which were progressively substituted by faster, asymmetrical half‐bounding gaits on larger substrates. In general, inclination had no impact on gait metrics, except that ascents were slower than horizontal locomotion. Velocity increase was regulated primarily due to an increase in stride frequency, a pattern encountered in many small mammals, although stride length contributed significantly as well. These locomotor adaptations serve as a behavioural mechanism to cope with the challenges of the arboreal milieu. They appear to provide stability and enable safe negotiation of arboreal substrates, ultimately leading to the successful exploitation of Acacia trees in their natural habitat.

Comparison of spatiotemporal gait characteristics between vertical climbing and horizontal walking in primates

“…Accordingly, almost all of the species in our study primarily regulate speed by increasing stride frequency rather than stride length (although both factors are important contributors in almost all species). Similar findings have been reported in geckos (Zaaf et al, 2001), cotton-top tamarins (Nyakatura et al, 2008), bonobos (Schoonaert et al, 2016), acacia rats (Karantanis et al, 2017c) and feathertail gliders (Karantanis et al, 2015). The regulation of speed by stride frequency versus stride length is thought to reduce body oscillations.…”

“…On thin, arboreal substrates, body oscillations may cause substrate oscillations. Therefore, reducing body oscillations during arboreal locomotion (horizontally or vertically oriented) may increase arboreal stability (Delciellos and Vieira, 2007;Karantanis et al, 2015Karantanis et al, , 2017cStrang and Steudel, 1990). With this in mind, it is possible that the need to maximize stride length during vertical climbing may be less important than previously thought.…”

The gaits of marsupials and the evolution of diagonal‐sequence walking in primates

“…Other investigators report similar upshifted horse‐line patterns—DS at low duty factors (high speeds), LS at high duty factors (lower speeds)—for symmetrical walking gaits in a wide variety of arboreal mammals: Petaurus breviceps (Shapiro & Young, , ), the didelphids Didelphis virginiana (White, ), and Caluromys philander (Cartmill, Lemelin, & Schmitt, ; Lemelin et al, ), the basal australidelphian marsupial Dromiciops australis (Pridmore, ), the tamarin Saguinus oedipus (Nyakatura, Fischer, & Schmidt, ), the sloths Bradypus variegatus , Choloepus didactylus , and C . hoffmani (Mendel, ; Nyakatura et al, 2010), the kinkajou Potos flavus (Lemelin & Cartmill, ), the acacia rat Thallomys paedulcus (Karantanis, Rychlik, Herrel, & Youlatos, ), and the climbing mice Apodemus agrarius and Myodes glareolus (Karantanis, Rychlik, Herrel, & Youlatos, ).…”

Developments in development: What have we learned from primate locomotor ontogeny?

“…Therefore, functional and adaptive explanations for DSDC gait have focused on biomechanical stability, particularly in a precarious arboreal setting. An arboreal explanation for DSDC gait usage is complicated by the fact that some arboreal mammals that regularly navigate (or are capable of navigating) small, flexible branches do not prefer this gait (Karantanis, Rychlik, Herrel, & Youlatos, , , ; Schmidt & Fischer, ; Shapiro & Young, ; Shapiro, Young, & VandeBerg, ). Regardless, ontogeny has provided a means to test some of the more proximate biomechanical hypotheses for why some mammals, and primates in particular, prefer DSDC gaits.…”

“…A third benefit that has been identified for DSDC gait (tested in a non‐primate arboreal mammal, the kinkajou) is that it increases not only the duration of support by diagonal limb pairs, but the linear distance between them, further enhancing stability (Lemelin & Cartmill, ). It is important to note that although the spatiotemporal features of diagonal sequence, diagonal couplets gait are often presumed to have evolved in response to relatively small‐diameter substrates, they are not strictly necessary for navigating small‐diameter supports (Karantanis et al, , , ; Schmidt & Fischer, ; Shapiro & Young, ; Shapiro et al, ), and changes in substrate orientation or compliance tend to have a more significant effect on gait parameters than substrate diameter per se (Shapiro & Young, ; Shapiro et al, ; Young, Stricklen, & Chadwell, ).…”