Rule reversal: Ecogeographical patterns of body size variation in the common treeshrew (Mammalia, Scandentia)

Abstract

There are a number of ecogeographical “rules” that describe patterns of geographical variation among organisms. The island rule predicts that populations of larger mammals on islands evolve smaller mean body size than their mainland counterparts, whereas smaller‐bodied mammals evolve larger size. Bergmann's rule predicts that populations of a species in colder climates (generally at higher latitudes) have larger mean body sizes than conspecifics in warmer climates (at lower latitudes). These two rules are rarely tested together and neither has been rigorously tested in treeshrews, a clade of small‐bodied mammals in their own order (Scandentia) broadly distributed in mainland Southeast Asia and on islands throughout much of the Sunda Shelf. The common treeshrew, Tupaia glis, is an excellent candidate for study and was used to test these two rules simultaneously for the first time in treeshrews. This species is distributed on the Malay Peninsula and several offshore islands east, west, and south of the mainland. Using craniodental dimensions as a proxy for body size, we investigated how island size, distance from the mainland, and maximum sea depth between the mainland and the islands relate to body size of 13 insular T. glis populations while also controlling for latitude and correlation among variables. We found a strong negative effect of latitude on body size in the common treeshrew, indicating the inverse of Bergmann's rule. We did not detect any overall difference in body size between the island and mainland populations. However, there was an effect of island area and maximum sea depth on body size among island populations. Although there is a strong latitudinal effect on body size, neither Bergmann's rule nor the island rule applies to the common treeshrew. The results of our analyses demonstrate the necessity of assessing multiple variables simultaneously in studies of ecogeographical rules.

Generalized evidence for Bergmann’s rule: body size variation in a cosmopolitan owl genus

“…Nevertheless, many exceptions have been reported (e.g. Ashton & Feldman, 2003;Freeman, 2017;Medina et al, 2017;Nunes et al, 2017;Olalla-Tárraga & Rodríguez, 2007;Olalla-Tárraga et al, 2006;Sargis et al, 2018;Slavenko & Meiri, 2015), and there is still a vibrant debate about which taxonomic level should the rule be applied to according to Bergmann's original formulation and whether interspecific and intraspecific patterns represent two different phenomena or follow the same rules (Blackburn et al, 1999;Meiri, 2011;Meiri & Thomas, 2007;Olson et al, 2009;Salewski & Watt, 2017;Watt et al, 2010). Although many studies investigated Bergmann's rule by comparing phylogenetic closely related species, or even species assemblages, recent theoretical studies suggested that this rule should be stronger at the intraspecific level (Meiri & Thomas, 2007;Watt et al, 2010;Meiri, 2011;Salewski & Watt, 2017;but see Olson et al, 2009).…”

Skull shape and Bergmann's rule in mammals: hints from Old World porcupines

“…Interspecific competition (e.g. due to resource limitation, predation or parasitism), as well as niche displacement may be major drivers of body size in small to medium‐sized mammals, obscuring the metabolic effects of climate, as well as confounding and integrating the patterns predicted by the Bergmann's rule (Dayan & Simberloff, ; Sargis et al ., ).…”

“…). The divergence of Hystrix species from this rule is common to many rodents and small mammals, but the reason behind this pattern is still unclear (Meiri & Dayan, ) and may be due to the effect of multiple environmental variables (Sargis et al ., ; Stanchak & Santana, ). Interspecific competition (e.g.…”

Mainland versus Island Adaptation: Paleobiogeography of Sunda Shelf Primates Revisited

“…Since the emergence of primates in the region during Quaternary to recent, 13 genera have been taxonomically recognized: Homo, Pongo, Hylobates, Symphalangus, Nomascus, Hoolock, Macaca, Trachypithecus, Presbytis, Simias, Nasalis, Nycticebus, and Tar s i u s [2]. With the high variability on body mass and body size, Southeast Asian primates, both the mainland and island populations, remain enigmatic when confronted toward ecogeographical "rules," resulting positive [3], contradictive [4,5], and inconsistent results [6]. However, given their peculiarity in adaptive functional characters among other mammal taxa and their close evolutionary trajectory to human [7], primates share similarities showing their capability in grasping object [2] for faster food procurement and high occasional flexibility in locomotion (e.g., arboreal quadrupedalism, terrestrial quadrupedalism, and bipedalism) [2,8].…”

“…Lying over a wide range of latitude and various sizes of islands, the Southeast Asian region is frequently subjected for the studies of primate insularity that involved spatial factors (e.g., island size, latitude, and island-mainland distance) [3][4][5]10] and temporal factors (e.g., isolation duration and geological chronology) [5]. Insularity on primates is an interesting phenomenon that invites many reports, linking to their ecomorphological complex (body size and body shape) [6] and biodiversity changes [11].…”