Causal explanations for the evolution of ‘low gear’ locomotion in insular ruminants

Roberto Rozzi,Jeff M Martin,Sara Varela,Pere Bover

doi:10.1111/jbi.13942

Abstract

AbstractAimMammals on islands often undergo remarkable evolutionary changes. The acquisition of ‘low gear’ locomotion, namely short and robust limb elements, has been typically associated with the island syndrome in large mammals and, especially, ruminants. Here we provide an investigative framework to examine biotic and abiotic selective factors hypothesized to influence evolution of this peculiar type of gait.LocationIslands worldwide.TaxonBovidae.MethodsWe calculated response variables associated with ‘low gear’ locomotion in 21 extinct and extant insular bovids. We assembled data on the physiography of 11 islands and on life history and ecological traits of the focal taxa. We estimated 10 predictors (island area and four topographic indices, body mass, body size divergence, number of predators and competitors, large mammal richness) and used multiple regressions, regression trees, and random forests to assess their contextual importance.ResultsThe acquisition of ‘low gear’ locomotion generally happens on islands with a small number of competitors. However, the roughness of the island terrain appears to be also important, without being a main driver. Finally, although the most extreme cases of ‘low gear’ locomotion occurred on islands with no mammalian predators, our models show a non‐significant relationship with this factor.Main conclusionsThe evolution of ‘low gear’ locomotion in insular ruminants does not simply result from phyletic dwarfing and predatory release. Variation in morphological responses within Bovidae to ecological and topographic traits suggests, instead, a complex interplay of biotic and abiotic factors. Current understanding on the main drivers of species evolutionary pathways and biogeographic patterns are disproportionally based on few taxa, mainly vertebrates, and in some extreme cases (like this one) even on few species. Here we show how adding more data, even within the same taxonomic group, can challenge historically accepted macroevolutionary and macroecological concepts.

Highlights

Mammals are a prime target to investigate the suite of responses to the geographic isolation and ecological simplicity of island environments known as the island syndrome (Adler & Levins, 1994; Heaney, 1978; Lomolino, 1985; Lomolino et al, 2013; Wallace, 1880)
We observed a negative relationship between robustness indices (RI) Mc and richness of large mammals, but we found no evidence for an interaction between the latter and subfamily (F1,15 = 0.4397, p = 0.52, Figure 4a)
Given that predator diversity is the main factor influencing body size evolution of insular bovids (Rozzi, 2018), different traits associated with the island syndrome may evolve in response to varied drivers, even within the same taxonomic group

Summary

| INTRODUCTION

Mammals are a prime target to investigate the suite of responses to the geographic isolation and ecological simplicity of island environments known as the island syndrome (Adler & Levins, 1994; Heaney, 1978; Lomolino, 1985; Lomolino et al, 2013; Wallace, 1880). Extinct and extant insular ruminants, and to a lesser degree insular elephants and hippopotamuses, often evolved a peculiar structure of the limbs by shortening limb elements—most markedly the metapodials, increasing their robustness, and occasionally developing bone fusions (Bover & Fornós, 2005; Bover, Quintana, & Alcover, 2010; Leinders & Sondaar, 1974; Rozzi & Palombo, 2014; Sondaar, 1977; Van der Geer, 2014a; Van der Geer, Lyras, De Vos, & Dermitzakis, 2011) This has been explained as an adaptation for what P.Y. Sondaar (Sondaar, 1977) described as ‘low gear’ locomotion—a frequent phenomenon believed to be advantageous, in the absence of predators, for low-speed walking in mountainous environments. Species that inhabited or are still living on islands characterized by homogenously mountainous landscapes (Figure 1)

| MATERIALS AND METHODS

| DISCUSSION

Findings

| CONCLUSIONS