We consider sex differences in human facial morphology in the context of developmental change. We show that at puberty, the height of the upper face, between the lip and the brow, develops differently in males and females, and that these differences are not explicable in terms of sex differences in body size. We find the same dimorphism in the faces of human ancestors. We propose that the relative shortening in men and lengthening in women of the anterior upper face at puberty is the mechanistic consequence of extreme maxillary rotation during ontogeny. A link between this developmental model and sexual dimorphism is made for the first time, and provides a new set of morphological criteria to sex human crania. This finding has important implications for the role of sexual selection in the evolution of anthropoid faces and for theories of human facial attractiveness.

The earliest known hippopotamids, attributed to the subfamily Kenyapotaminae, are known essentially from dental remains of two species. The first was found in the middle Miocene of Kenya, the second at the beginning of the upper Miocene in eastern and northern Africa. The exact affinities of the Kenyapotaminae are critical for resolving the long debated origin of the Hippopotamidae, as part of the wider question of cetacean affinities within artiodactyls. We performed the first detailed comparative description, character by character, of kenyapotamines, using the following putatively related taxa: Hippopotaminae, Suoidea, and Anthracotheriidae. The development of an improved nomenclature for the dentition facilitates comparisons amongst a wide array of cetartiodactyls. This has permitted the first assessment of the phylogenetic position of Kenyapotaminae using a cladistic analysis. This work provides an emendation of kenyapotamine taxonomy and diversity. Kenyapotaminae are indeed closely related to Hippopotaminae and should be kept within Hippopotamidae, the late Miocene kenyapotamines not necessarily being the forerunners of the first known hippopotamines. Hippopotamidae are deeply nested within anthracotheriids, with crown bothiodontines as sister group, and any close affinities with Suoidea should be rejected. This reinforces a scenario linking cetaceans to Hippopotamoidea (Hippopotamidae + Anthracotheriidae), possibly via other early Palaeogene artiodactyls.. RÉSUMÉMorphologie et phylogénie des premiers hippopotamidés (Cetartiodactyla, Hippopotamidae, Kenyapotaminae).-Les plus anciens hippopotamidés, attibués à la sous-famille Kenyapotaminae, sont connus essentiellement par les restes dentaires de deux espèces, connues dans le Miocène moyen du Kenya et à la base du Miocène supérieur d'Afrique orientale et septantrionale. Les affinités exactes des Kenyapotaminae doivent être connues pour résoudre le long débat sur l'origine des Hippopotamidae, partie intégrante de la question plus large des relations entre cétacés et artiodactyles. Nous avons effectués la première comparaison détaillée, caractère par caractère, des kényapotaminés avec les Hippopotaminae, Suoidea, et Anthracotheriidae, grâce au développement d'une *Corresponding authors.

Body size reduction in mammals is usually associated with only moderate brain size reduction as the brain and sensory organs complete their growth before the rest of the body during ontogeny1,2. On this basis “phyletic dwarfs” are predicted to have a higher relative brain size than “phyletic giants”1,3. This trend has been questioned, however, in the special case of dwarfism of mammals on islands4. Here we show that the endocranial capacities of extinct dwarf species of hippopotamus from Madagascar are up to 30% smaller than those of a mainland African ancestor scaled to equivalent body mass. These results show brain size reduction is much greater than predicted from an intraspecific ‘late ontogenetic’ model of dwarfism where brain size scales to body size with an exponent of 0.35. The nature of the proportional change or grade shift2,5 observed here indicates that selective pressures upon brain size are potentially independent from those on body size. This study demonstrates empirically that it is mechanistically possible for dwarf mammals on islands to evolve significantly smaller brains than would be predicted from a model of dwarfing based on the intraspecific scaling of the mainland ancestor. Our findings challenge our understanding of brain-body allometric relationships in mammals and suggest that the process of dwarfism could in principle explain small brain size, a factor relevant to the interpretation of the small-brained hominin found on the Island of Flores, Indonesia6.

The fossil record of the Hippopotamidae can shed light on three major issues in mammalian evolution. First, as the Hippopotamidae are the extant sister group of Cetacea, gaining a better understanding of the origin of the Hippopotamidae and of their Paleogene ancestors will be instrumental in clarifying phylogenetic relationships within Cetartiodactyla. Unfortunately, the data relevant to hippopotamid origins have generally been ignored in phylogenetic analyses of cetartiodactyls. In order to obtain better resolution, future analyses should consider hypotheses of hippopotamid Paleogene relationships. Notably, an emergence of the Hippopotamidae from within anthracotheriids has received growing support, leading to reconciliation between genetic and morphological evidence for the clade Cetancodonta (Hippopotamidae + Cetacea). Secondly, full account needs to be taken of the Hippopotamidae when studying the impact of environmental change on faunal evolution. This group of semi-aquatic large herbivores has a clear and distinct ecological role and a diverse and abundant fossil record, particularly in the African Neogene. We examine three major phases of hippopotamid evolution, namely the sudden appearance of hippopotamines in the late Miocene (the "Hippopotamine Event"), the subsequent rampant endemism in African basins, and the Pleistocene expansion of Hippopotamus. Each may have been influenced by multiple factors, including: late Miocene grass expansion, African hydrographical network disruption, and a unique set of adaptations that allowed Hippopotamus to respond efficiently to early Pleistocene environmental change. Thirdly, the fossil record of the Hippopotamidae documents the independent emergence of adaptive character complexes in relation to semiaquatic habits and in response to insular isolation. The semiaquatic specializations of fossil hippopotamids are particularly useful in interpreting the functional morphology and ecology of other, extinct groups of large semiaquatic herbivores. Hippopotamids can also serve as models to elucidate the evolutionary dynamics of island mammals.

Hominids display marked body size dimorphism, suggestive of strong sexual selection, yet they lack signif icant sex differences in canine size that are commonly associated with intrasexual competition in primates. We resolve this paradox by examining sex differences in hominoid facial morphology. We show that chimpanzees, but not gorillas, exhibit clear sexual dimorphism in face width, over and above that expected based on sex differences in body size. We show that this facial dimorphism, expressed as an index, is negatively correlated with canine dimorphism among anthropoid primates. Our f indings suggest that a lack of canine dimorphism in anthropoids is not owing to weak sexual selection, but rather is associated with strong sexual selection for broader face width. Enlarged cheekbones are linked with attractiveness in humans, and we propose that the evolution of a broad face and loss of large canines in hominid males results from mate choice.