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.

Molecular phylogenetics supports the origin of an endemic Balearic shrew lineage (Nesiotites) coincident with the Messinian Salinity Crisis.

The red-toothed shrews (Soricinae) are the most widespread subfamily of shrews, distributed from northern South America to North America and Eurasia. Within this subfamily, the tribe Nectogalini includes the fossil species Nesiotites hidalgo recorded from the Late Pleistocene to Holocene of the Balearic Islands (Western Mediterranean). Although there is a consensus about the close relationship between the extinct red-toothed shrew genera Nesiotites and Asoriculus based on morphology, molecular data are necessary to further evaluate the phylogenetic relationships of the Balearic fossils. We obtained a near complete mitochondrial genome of N. hidalgo, allowing the first molecular phylogenetic analysis of this species. Analyses based on 15,167 bp of the mitochondrial genome placed N. hidalgo as close relative to the extant Himalayan shrew (Soriculus nigrescens), and a combined analysis using molecular and morphological data confirm that N. hidalgo and Asoriculus gibberodon are sister-taxa with S. nigrescens as the immediate outgroup. Molecular clock and divergence estimates suggest that the split between N. hidalgo and its closest living relative occurred around 6.44 Ma, which is in agreement with the previously proposed colonisation of the Balearic Islands from mainland Europe by nectogaline shrews during the Messinian Salinity Crisis (5.97-5.33 My ago). Our results highlight that it is possible to retrieve genetic data from extinct small mammals from marginal environments for DNA preservation. Additional finds from the fossil record of Soricinae from the Eurasian Late Miocene/Early Pliocene are needed to shed further light on the still confusing taxonomy and paleobiogeography of this clade.

The case of an insular molarless black rat: Effects on lifestyle and mandible morphology.

We report a specimen of an insular black rat (Rattus rattus) from Illa den Colom (Menorca, Western Mediterranean) displaying a singular dental characteristic. It has no molar teeth but displays regular incisors. Its mere occurrence as a regular adult rat is puzzling and we attempted to evaluate what diet and morphological changes in jaw shape were promoted by the total lack of molars, and allowed the successful survival of this specimen. Two approaches were performed: first, bone tissue was analysed to obtain delta(15)N and delta(13)C values in order to estimate dietary preferences of the rat. Second, the shape of the jaw was analysed through elliptic Fourier analysis, using outlines as markers of diet. The values for C and N fractionation (-19.89 per thousand and 10.06 per thousand, respectively) suggest that the molarless rat included animal food in the diet and not exclusively plant material as observed in other mainland rat populations. The morphometric analysis in which the shape of the molarless mandible falls into the range of omnivorous groups leads to a similar conclusion. The adult age of the specimen suggests that it fed efficiently enough with its incisors to allow a normal growth. Although displaying a lack of molar teeth, no deep changes in remodelling jaw morphology can be observed and its shape falls into the variation of regular murines. The molarless rat exemplifies that special ecological features on small islands allow the survival of aberrant morphotypes.

Body shape and life style of the extinct Balearic dormouse Hypnomys (Rodentia, Gliridae): new evidence from the study of associated skeletons.

Hypnomys is a genus of Gliridae (Rodentia) that occurred in the Balearic Islands until Late Holocene. Recent finding of a complete skeleton of the chronospecies H. morpheus (Late Pleistocene-Early Holocene) and two articulated skeletons of H. cf. onicensis (Late Pliocene) allowed the inference of body size and the calculation of several postcranial indexes. We also performed a Factorial Discriminant Analysis (FDA) in order to evaluate locomotory behaviour and body shape of the taxa. Using allometric models based on skull and tooth measurements, we calculated a body weight between 173 and 284 g for H. morpheus, and direct measurements of articulated skeletons yielded a Head and Body Length (HBL) of 179 mm and a Total Body Length of 295 mm for this species. In addition to the generally higher robustness of postcranial bones already recorded by previous authors, H. morpheus, similar to Canariomys tamarani, another extinct island species, displayed elongated zygopodium bones of the limbs and a wider distal humerus and femur than in an extant related taxon, Eliomys quercinus. Indexes indicated that Hypnomys was more terrestrial and had greater fossorial abilities than E. quercinus. This was also corroborated by a Discriminant Analysis, although no clear additional inference of locomotory abilities could be calculated.