Brain size dependent on food supply?
At least in Southeast Asian orangutans, brain size development may depend on the quality and quantity of the food available. This is the conclusion of a study by Andrea Taylor from Duke University and Carel van Schaik from the University of Zurich after comparing the skulls of the two orangutan species Pongo pygmaeus from the island of Borneo and Pongo abelii from Sumatra.
In particular, females of the subspecies Pongo pygmaeus morio from eastern Borneo had a significantly smaller brain volume than their relatives on the neighboring island – determined from a total of 226 museum skeletons. At the same time, these smaller orangutans live in the most nutrient-poor regions of Borneo, where food shortages are more common and the animals' preferred fruits are generally poorer in nutrients. On the other hand, the large-headed primates on the neighboring island inhabit rainforests that grow on relatively young soil and thus ensure a much better supply. In addition, the regularly occurring Pacific El Niño events, in contrast to East Borneo, hardly affect plant growth and thus also the livelihood of the monkeys.
The sex differences in Pongo pygmaeus morio are also due to the stress on the females through pregnancy and the rearing of the offspring, according to the researchers. However, since the development and operation of the brain is energy-intensive, the animals reacted to the often inadequate supply conditions by reducing the size of their skulls. However, other physical characteristics can possibly also be attributed to this nutritional situation: These orangutans have stronger jaws than their conspecifics from other regions in order to be able to consume tougher and harder fruit varieties.