Brain Changes during Phyletic Dwarfing in Elephants and Hippos

Abstract

Of all known insular mammals, hippos and elephants present the extremes of body size decrease, reducing to 4 and a mere 2% of their ancestral mainland size, respectively. Despite the numerous studies on these taxa, what happens to their relative brain size during phyletic dwarfing is not well known, and results are sometimes conflicting. For example, relative brain size increase has been noted in the Sicilian dwarf elephant, Palaeoloxodon falconeri, whereas relative brain size decrease has been postulated for Malagasy dwarf hippos. Here, I perform an analysis of brain, skull, and body size of 3 insular elephants (Palaeoloxodon “mnaidriensis,” P. tiliensis, and P. falconeri) and 3 insular hippos (Hippopotamus madagascariensis, H. lemerlei, and H. minor) to address this issue and to test whether relative brain size in phyletic dwarf species can be predicted. The results presented here show that the encephalization of all insular elephants and hippos is higher than that of their continental relatives. P. falconeri in particular has an enormous encephalization increase, which has so far not been reported in any other insular mammal. Insular brain size cannot be reliably predicted using either static allometric or ontogenetic scaling models. The results of this study indicate that insular dwarf species follow brain-body allometric relationships different from the expected patterns seen for their mainland relatives.

Size matters: micro-evolution in Polynesian rats highlights body size changes as initial stage in evolution

“…The separation in PC1 (dorsal view, lateral view) of specimens from Remote Oceania and New Zealand from the rest of the geographic distribution is in accordance with body mass distributions, where specimens from the former regions are on average larger (Motokawa, Lin & Lu, 2004;Van der Geer, Lomolino & Lyras, 2018;Van der Geer, 2018). Also, the proportional scaling of the foramen magnum with skull size is according to expectations of the existence of a correlation between body mass and skull size (Lyras, 2018), because the diameter of the foramen magnum scales with body mass in rodents (Bertrand, Schillaci & Silcox, 2016). Apart from confirming to CREA, another allometric aspect observed in the Polynesian rats is that larger skulls are more tubular in shape than the smaller skulls, which are more balloon-shaped with a rounder and wider braincase relative to those of large skulls.…”

Epidermal Keratinocyte Sensing and Processing of Environmental Information Together with the Brain’s Simulation and Prediction Abilities Helped to Enable <i>Homo sapiens’</i> Evolutionary Success

“…These three examples suggest that the combina-M. Denda, S. Nakanishi DOI: 10.4236/aa.2020.101002 24 Advances in Anthropology tion of an exposed, environmentally sensitive skin and a large brain might have played a key role during animal evolution. Several terrestrial mammals are largely hairless, including the hippopotamus, elephant, rhinoceros and naked mole rat, and the hippopotamus and rhinoceros do not have large brains (Lyras, 2018;Bhagwandin et al, 2017). We propose that because these two animals have thick skin, it might have been difficult to construct tightly-organized, arborizing peripheral nerve networks in their skin.…”

“…We propose that because these two animals have thick skin, it might have been difficult to construct tightly-organized, arborizing peripheral nerve networks in their skin. Elephants have relatively large brains (Lyras, 2018), but their skin could also be too thick to construct peripheral nerve networks. We speculate instead that the elephant has a flexible arm, its long nose.…”

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

“…Its magnitude and direction result from a combination of selective biotic and abiotic factors (Durst & Roth, 2015; Heaney, 1978; Lomolino, Sax, Palombo, & Van der Geer, 2012; Meiri, Cooper, & Purvis, 2008; Palombo, 2007; Rozzi & Lomolino, 2017; Van der Geer, Lyras, Lomolino, Palombo, & Sax, 2013; Van der Geer et al., 2016). However, in addition to shifts in body size towards that of intermediate‐sized taxa, mammals on islands often undergo morphological changes in their skull, brain, teeth and appendicular skeleton (Angelone et al., 2018; Bover & Alcover, 1999a; Bover & Fornós, 2005; De Vos, 2000; Diniz‐Filho & Raia, 2017; Jordana, Marín‐Moratalla, DeMiguel, Kaiser, & Köhler, 2012; Köhler & Moyà‐Solà, 2004; Larramendi & Palombo, 2015; Lyras, 2018; Palombo, Rozzi, & Bover, 2013; Quintana, Köhler, & Moyà‐Solà, 2011; Rozzi, 2017; Rozzi, Winkler, De Vos, Schulz, & Palombo, 2013; Sondaar, 1977; Van der Geer, Lyras, Mitteroecker, & MacPhee, 2018; Weston & Lister, 2009; Winkler et al., 2013). 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).…”