Uniformity in variety: Antler morphology and evolution in a predator-free environment

Abstract

The Late Pleistocene mammal fauna of Crete was impoverished, as typical for oceanic islands, and consisted only of deer, dwarf elephants, an otter, a shrew and giant mice. Dwarf deer (Candiacervus spp.) were the dominant endemic herbivorous species. Here, I describe the adult antler morphology of this deer. Antler variety appears to be remarkably large, yet a few concise morphological groups without intermediate forms can be recognized, likely representing separate species. Antler variety is not a product of random variation induced by ecological release in a predator-free environment. Three new species are described here (Candiacervus spp. nov.), differing in antler and skull morphology, and the diagnosis of existing species (C. ropalophorus, C. rethymnensis) is emended based on new material. Antler variation can be explained by two evolutionary trends: showiness versus a classic fighting type. Divergence is driven and accelerated by intra-specific competition among males. The classic type is best explained as a result of allometric down-scaling during dwarfism. The display type is best explained as a result of restructuring of antler bauplan (simplification and extreme elongation of the main beam). Under predator-free scenarios, deer have the potential to evolve antler morphologies and behavioural changes unknown on the mainland.

New skull material of Pleistocene dwarf deer from Crete (Greece)

“…The concurrence of this finding in these two insular cervids suggests that this may be yet another adaptation to the insular environment. The probable changes in the sense of smell (see above) and vision in Candiacervus would be well in line with the notion that this cervid was living in a predator-free environment (Palombo et al 2008;van der Geer 2018;van der Geer et al 2014).…”

“…This is fully consistent with the observation of Palombo et al (2008), that brain size in Candi acervus is little affected by dwarfing: as the side lengths of its braincase approach equality, its overall volume gets optimized with respect to its surface size to house a brain of "minimal size needed according to environmental conditions" (Palombo et al 2008: p. 178). The relatively short braincase, together with the rostrally displaced occipital condyles also result in a mechanical optimization to carry the long antlers present in Candiacervus (Strasser et al 2018;van der Geer 2018).…”

“…Remains of Candiacervus have been found at several sites, mainly in karst caves along the Cretan coastline (Lax 1996;Iliopoulos et al 2010). Most of the studied material of Candiacervus comes from the Simonelli Cave (Accordi 1972;Malatesta 1980;Caloi & Palombo 1995;Palombo et al 2008), the Bate Cave (Raia & Meiri 2006;Kolb et al 2015), the Liko Cave, and the Gerani caves (de Vos 1984;Raia & Meiri 2006;van der Geer et al 2006avan der Geer et al , b, c, 2014Vislobokova 2013;Kolb et al 2015;van der Geer 2018). Post-cranial and cranial remains, including antlers, as well as dental remains have been obtained from these sites (see references given above and also Simonelli 1908;Kuss 1965Kuss , 1975de Vos 1979;Capasso Barbato & Petronio 1986), and were dated to the late Pleistocene.…”

Brain Changes during Phyletic Dwarfing in Elephants and Hippos

“…The relative brain size of C. ropalophorus is evaluated using 2 living Cervinae: the fallow deer (D. dama) and the red deer (Cervus elaphus). Fallow deer is the living relative of Candiacervus [van der Geer, 2018]. Candiacervus is a late Pleistocene species, and hence it is feasible to compare it with living taxa.…”

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

“…sp. (size II), C. cretensis ; Van der Geer, 2014a, 2018; Van der Geer et al., 2011). In a few cases ( Myotragus balearicus , C. ropalophorus , and the smallest Hoplitomeryx ) short and robust distal limb bones are already present at birth, but they become even more stout during postnatal ontogeny (Bover & Alcover, 1999b; Van der Geer, 2014a; Van der Geer et al., 2011).…”