A scientific team from the Doñana Biological Station has been looking at morphological changes across elevational gradients in a tropical montane mammal. This new research and other recent studies cast doubt on the validity of Bergmann's and Allen's ecogeographical rules in tropical mountains. Studies such as this provide important information to understand how species can adapt to changes in different environmental factors (such as temperature), and can thereby shed light on how montane specialists could react to different climate change scenarios.
Researchers captured more than 200 Mountain Treeshrews (Tupaia montana) a small mammal endemic to Borneo, Southeast Asia, across three elevational gradients spanning from 800 meters above sea level to over 3300 meters, with the aim to study changes in morphological traits across elevation. "The body and skull size of treeshrews decreased from the lowest elevations to middle elevations (2000 meters), and then increased from middle to higher elevations", explains Arlo Hinckley, first author of the study. "These findings show that treeshrews are following a different pattern than that predicted by Bergmann's rule". This theory, formulated in 1847, predicts that populations of endothermic species in colder climates grow larger than populations of that same species in warmer climates. In colder climates (higher latitudes/elevations) larger animals, with a lower surface-area-to volume ratio, increase their ability to conserve heat.
The results of this study also questioned, partially, Allen's rule, described in 1877, which predicts that populations of endothermic species living in warmer places have larger appendages, increasing the surface area to volume ratio and heat dissipation. Allen's rule was supported for relative tail length, which decreased with elevation, but not for ear and hindfoot length, with the former remaining constant and the latter increasing with elevation. "This evidence together with changes in presumed diet-related traits along elevation suggest that selective pressures other than temperature, such as diet, food acquisition, predation pressure, and competition, are playing a more important role shaping the morphological variation across the distribution of the Mountain Treeshrew", explains Hinckley.
The team also looked at the variation of these morphological traits across time, through the comparison of modern specimens captured in the field with historical specimens stored in natural history museums. The research found that the skull size has not changed in the last century, despite the average temperature increase and potential changes in the habitat of this species. However, treeshrews do seem to have acquired an increased zygomatic breadth (related to bite force), which may be a result of an adaptation to a different diet, perhaps associated with changes in food resources.
The importance of conducting field studies
Although this is not the first time that Bergmann's and Allen's rules are brought into question, this study is one of the first testing their validity across a tropical elevation gradient, and also the first that estimates temporal morphological changes in a Southeast Asian montane mammal. Most studies addressing such hypotheses and rules are usually conducted in Northern temperate habitats so there is an important bias in the field, and these frequently make broader generalizations based on the data generated by many field studies such as this one. Field studies are pivotal to provide new perspectives, particularly in less studied habitats, such as tropical ones.
"It is important to keep on funding these field studies, they might be less cited due to their specific focus, but that does not make them less relevant", concludes the researcher. "We must keep on estimating morphological changes in these montane mammal populations so we can better understand the effects of climatic change on these vulnerable species".
This study has been possible thanks to the cooperation of different Malaysian institutions and the funding, specimens, and support from several natural history museums, such as the Scientific Collections of the Doñana Biological Station-CSIC, the Natural History Museum of London, the Harvard Museum of Comparative Zoology, the Smithsonian National Museum of Natural History and the Field Museum of Natural History.
Arlo Hinckley, Ines Sanchez-Donoso, Mar Comas, Miguel Camacho-Sanchez, Melissa T. R. Hawkins, Noor Haliza Hasan and Jennifer A. Leonard. Challenging ecogeographical rules: phenotypic variation in the Mountain Treeshrew (Tupaia montana) along tropical elevational gradients. PLOS ONE. https://doi.org/10.1371/journal.pone.0268213