**1. Introduction**

Mainland and island are two unique bodies of landmasses that hold not only the obvious different area dimension but also a timeline that portrayed dynamic changes on their geographical and ecological features. Southeast Asia that comprises mainland and the patches of island is a home for the primate species diversity with high rate of endemism and provinciality [1]. 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 *Tarsius* [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]. These functional characters support their high adaptability in predator avoidance and alternate dietary shifts when resources are limited [9].

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) [3–5, 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].

In many ecological aspects, mainland environment differs from island environment. In addition, large-sized island provides different ecological scenarios from small-sized island. Certain duration of isolation on a relatively small island may lead to limited resources, fewer predators, and reduced interspecific competition [12]. Although it is not impacted universally, the combinative geographical effects on island size and island isolation can promote gigantism in smaller insular mammal species and dwarfism in larger mammal species. It is widely known as island rule (=Foster's rule) [6, 13–18]. With the wide span of latitudinal range, primates inhabiting the Sunda Shelf region are also assumed to follow Bergmann's rule, by testing the effect of latitudinal position to body size [3–5]. This study aims to elicit the validity of ecogeographical rules affected body size and biodiversity changes of primates around Sunda Shelf throughout the geological chronology, since their appearance in Quaternary until recent.
