High incidence of cervical ribs indicates vulnerable condition in Late Pleistocene woolly rhinoceroses

Mammals as a rule have seven cervical vertebrae, a number that remains remarkably constant. Changes of this number are associated with major congenital abnormalities (pleiotropic effects) that are, at least in humans, strongly selected against. Recently, it was found that Late Pleistocene mammoths (Mammuthus primigenius) from the North Sea have an unusually high incidence of abnormal cervical vertebral numbers, approximately ten times higher than that of extant elephants. Abnormal numbers were due to the presence of large cervical ribs on the seventh vertebra, indicating a homeotic change from a cervical rib-less vertebra into a thoracic rib-bearing vertebra. The high incidence of cervical ribs indicates a vulnerable condition and is thought to be due to inbreeding and adverse conditions that may have impacted early pregnancies in declining populations. In this study we investigated the incidence of cervical ribs in another extinct Late Pleistocene megaherbivore from the North Sea and the Netherlands, the woolly rhinoceros (Coelodonta antiquitatis). We show that the incidence of abnormal cervical vertebral numbers in the woolly rhinoceros is unusually high for mammals (15,6%, n = 32) and much higher than in extant Rhinoceratidae (0%, n = 56). This indicates that woolly rhinoceros lived under vulnerable conditions, just like woolly mammoths. The vulnerable condition may well have contributed to their eventual extinction

The paradox of anthropogenic enrichment: homogenization and downsizing of insular mammals

Oceanic islands are global hotspots of biodiversity – many of them harboring marvels of evolution in isolation. Unfortunately, insular biotas are also highly susceptible to extinction, especially following colonization by humans. Here, we assess the influence of humanity on the diversity and biological distinctiveness of mammals inhabiting 37 oceanic islands. We compiled lists of mammals inhabitingthese islands prior to and then following colonization by hominids (including Homo erectus and H. sapiens). We then quantified the dynamics in diversity (as measured by species richness) and distinctiveness (measured as beta-diversity) among islands. We compared mammalian assemblages on islands prior to humanity, following colonization by early hominids and then following colonization by H. sapiens (in the latter case, separating assemblage dynamics resulting from extinctions of native species from the effects of species introductions). As expected, early hominids hardly influenced mammalian diversity or distinctiveness. In contrast, colonization by H. sapiens was initially followed by numerous extinctions and substantial declines in species richness, which then however rebounded to exceed pre-humanity levels. These post-humanity increases in species richness were paradoxically accompanied by substantial declines in distinctiveness among islands. This paradox of anthropogenic enrichment is readily resolved by observing that species introductions to the islands (the sources of the post-humanity surges in species richness) were comprised of a highly redundant set of small species (primarily rats and house mice), resulting in the homogenization and downsizing of island life

Dwarfism and gigantism drive human-mediated extinctions on islands

Islands have long been recognized as distinctive evolutionary arenas leading to morphologically divergent species, such as dwarfs and giants. We assessed how body size evolution in island mammals may have exacerbated their vulnerability, as well as how human arrival has contributed to their past and ongoing extinctions, by integrating data on 1231 extant and 350 extinct species from islands and paleo islands worldwide spanning the past 23 million years. We found that the likelihood of extinction and of endangerment are highest in the most extreme island dwarfs and giants. Extinction risk of insular mammals was compounded by the arrival of modern humans, which accelerated extinction rates more than 10-fold, resulting in an almost complete demise of these iconic marvels of island evolution. Includes supplementary materia