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Apomys, a Philippine endemic genus of forest mice, occurs throughout most oceanic portions of the archipelago and is its most speciose mammal genus, with 18 species currently recognized. Recent extensive surveys of mammals on Mindoro Island have produced specimens that document the presence of three genetically and morphologically distinct candidate species of Apomys (subgenus Megapomys) previously unknown. These three, plus one previously described relative from Mindoro, constitute a clade of well-supported, reciprocally monophyletic units based on cytochrome b sequence data, all of which are strongly supported using BPP species delimitation. Data from three nuclear genes show less divergence, but species delimitation analyses are consistent with results from cytochrome b. These four taxa are easily diagnosed on the basis of pelage and cranial morphology. Each of the four species occurs allopatrically, though two occur along a single elevational gradient. In this paper, we formally describe the three new species. We estimate that the common ancestor of the four species arrived on Mindoro from Luzon roughly 4.7 Ma, with initial diversification beginning roughly 2.7 Ma, and increasing to the current four species about 1.3 Ma. The three new species increase the number of mammals currently recognized as endemics on Mindoro from nine to twelve. This is a remarkably high number of endemic mammals from an island of its size, and reflects Mindoro’s status as a geologically old island permanently isolated from other oceanic islands in the Philippines by deep water, while also corroborating Mindoro as the smallest island within which endemic speciation by small mammals is known to have occurred.   

Abstract Trabecular bone is modelled throughout an animal’s life in response to its mechanical environment, but like other skeletal anatomy, it is also subject to evolutionary influences. Yet the relative strengths of factors that affect trabecular bone architecture are little studied. We investigated these influences across the Philippine endemic murine rodent clade Chrotomyini. These mammals have robustly established phylogenetic relationships, exhibit a range of well-documented substrate-use types, and have a body size range spanning several hundred grammes, making them ideal for a tractable study of extrinsic and intrinsic influences on trabecular bone morphology. We found slight differences in vertebral trabecular bone among different substrate-use categories, with more divergent characteristics in more ecologically specialized taxa. This suggests that the mechanical environment must be relatively extreme to affect trabecular bone morphology in small mammals. We also recovered allometric patterns that imply that selective pressures on bone may differ between small and large mammals. Finally, we found high intrataxonomic variation in trabecular bone morphology, but it is not clearly related to any variable we measured, and may represent a normal degree of variation in these animals rather than a functional trait. Future studies should address how this plasticity affects biomechanical properties and performance of the skeleton. 

Abstract Arboreal locomotion allows access to above-ground resources and might have fostered the diversification of mammals. Nevertheless, simple morphological measurements that consistently correlate with arboreality remain indefinable. As such, the climbing habits of many species of mammals, living and extinct, remain speculative. We collected quantitative data on the climbing tendencies of 20 species of murine rodents, an ecologically and morphologically diverse clade. We leveraged Bayesian phylogenetic mixed models (BPMMs), incorporating intraspecific variation and phylogenetic uncertainty, to determine which, if any, traits (17 skeletal indices) predict climbing frequency. We used ordinal BPMMs to test the ability of the indices to place 48 murine species that lack quantitative climbing data into three qualitative locomotor categories (terrestrial, general and arboreal). Only two indices (both measures of relative digit length) accurately predict locomotor styles, with manus digit length showing the best fit. Manus digit length has low phylogenetic signal, is largely explained by locomotor ecology and might effectively predict locomotion across a multitude of small mammals, including extinct species. Surprisingly, relative tail length, a common proxy for locomotion, was a poor predictor of climbing. In general, detailed, quantitative natural history data, such as those presented here, are needed to enhance our understanding of the evolutionary and ecological success of clades.