Evolution of body size in anteaters and sloths (Xenarthra, Pilosa): phylogeny, metabolism, diet and substrate preferences

Abstract

Pilosa include anteaters (Vermilingua) and sloths (Folivora). Modern tree sloths are represented by two genera, Bradypus and Choloepus (both around 4-6 kg), whereas the fossil record is very diverse, with approximately 90 genera ranging in age from the Oligocene to the early Holocene. Fossil sloths include four main clades, Megalonychidae, Megatheriidae, Nothrotheriidae, and Mylodontidae, ranging in size from tens of kilograms to several tons. Modern Vermilingua are represented by three genera, Cyclopes, Tamandua and Myrmecophaga, with a size range from 0.25 kg to about 30 kg, and their fossil record is scarce and fragmentary. The dependence of the body size on phylogenetic pattern of Pilosa is analysed here, according to current cladistic hypotheses. Orthonormal decomposition analysis and Abouheif C-mean were performed. Statistics were significantly different from the null-hypothesis, supporting the hypothesis that body size variation correlates with the phylogenetic pattern. Most of the correlation is concentrated within Vermilingua, and less within Mylodontidae, Megatheriidae, Nothrotheriidae and Megalonychidae. Influence of basal metabolic rate (BMR), dietary habits and substrate preference is discussed. In anteaters, specialised insectivory is proposed as the primary constraint on body size evolution. In the case of sloths, mylodontids, megatheriids and nothrotheriids show increasing body size through time; whereas megalonychids retain a wider diversity of sizes. Interplay between BMR and dietary habits appears to be the main factor in shaping evolution of sloth body size.

The first fossil skull of an anteater (Vermilingua, Myrmecophagidae) from northern South America, a taxonomic reassessment ofNeotamanduaand a discussion of the myrmecophagid diversification

“…7), from low to medium-high latitudes. This is in line with the evolution of larger body sizes since when this attribute increases, the foraging area also increases and, with it, the distributional range, according the general foraging strategy of the extant myrmecophagids (Naples 1999; Toledo et al 2017; Gaudin et al 2018). The co-occurrence pattern of N. borealis and Gen. et sp.…”

“…However, several inferences and hypotheses about the evolutionary history of anteaters and, particularly, the myrmecophagids, can be outlined from the current evidence, including that presented in this work. Following to Pascual & Ortiz-Jaureguizar (1990), McDonald et al (2008) and Toledo et al (2017), the next discussion is based on multiple palaeobiological, ecological and biogeographical aspects as major constraints and/or consequences of the myrmecophagid evolution.…”

“…Whether the ancestral condition of substrate use in Myrmecophagidae is arboreal, as held by Gaudin & Branham (1998), the preference for open biomes (e.g. savannah) and terrestriality in Myrmecophaga (and possibly in Neotamandua ) is a derived condition (McDonald et al 2008; Toledo et al 2017). The semiarboreal habits of Tamandua are explained from niche conservatism or, alternatively, from convergence with Protamandua if the ancestor of Tamandua was hypothetically terrestrial.…”

“…8), linked to latitudinal temperature gradient reduction and expansion of the tropical (warm) forest belt towards higher latitudes in the continents (including South America; see Anderson 2009; Herold et al 2011; Morley 2011; Palazzesi et al 2014), has influenced on the evolutionary differentiation of the myrmecophagids, maybe predominantly in situ as in the climatically-induced evolution of other small Cenozoic mammals (Fortelius et al 2014), such as Protamandua . This differentiation would have been triggered by increase in suitable area in terms of preferred biomes (warm forests in this case) and, especially, temporarily sustained availability of social insects for their feeding (McDonald et al 2008; Kay et al 2012; Toledo et al 2017). Indeed, extant termites and ants (Termitidae and Formicidae, respectively) concentrate the vast majority of their biomass (and species richness) in the tropics and warm subtropical regions (Hölldobler & Wilson 1990; Tobin 1995; Davidson & Patrell-Kim 1996; Eggleton et al 1996; Davidson et al 2003; Ellwood & Foster 2004; Keller & Gordon 2009).…”

“…On the basis of the foregoing and by generalization of morphological and ecological features of the living vermilinguans, e.g. less dependence on trees related to greater taxonomic and/or ecological diversity of consumed insects (Hirschfeld 1976; Montgomery 1985a; Rodrigues et al 2008; Toledo et al 2017; Table 5), it is hypothesized that, as early as the late Middle Miocene, with the triggering of a global cooling (Fig. 8), Neotamandua was involved in a niche evolution process within Myrmecophagidae which implied a significative increase in dietary diversity as myrmecophagous and expansion of substrate use and biome selection.…”