An exploration of the social brain hypothesis in insects

An exploration of the social brain hypothesis in insects

The "social brain hypothesis" posits that the cognitive demands of sociality have driven the evolution of substantially enlarged brains in primates and some other mammals. Whether such reasoning can apply to all social animals is an open question. Here we examine the evolutionary relationships between sociality, cognition, and brain size in insects, a taxonomic group characterized by an extreme sophistication of social behaviors and relatively simple nervous systems. We discuss the application of the social brain hypothesis in this group, based on comparative studies of brain volumes across species exhibiting various levels of social complexity. We illustrate how some of the major behavioral innovations of social insects may in fact require little information-processing and minor adjustments of neural circuitry, thus potentially selecting for more specialized rather than bigger brains. We argue that future work aiming to understand how animal behavior, cognition, and brains are shaped by the environment (including social interactions) should focus on brain functions and identify neural circuitry correlates of social tasks, not only brain sizes.

As intelligence dominates on instinct, the mushroom bodies and the antennal lobes become considerably larger relative to the overall brain volume, as we see when comparing cockchafers to locusts, ichneumons, carpenter bees, solitary bees, and finally social bees, where the mushroom bodies represent one-fifth of the brain and one-fortieth of the body; whereas in cockchafers, they represent less than one-thirty-third thousandth of the body.

THE SOCIAL BRAIN HYPOTHESIS

There have been suggestions that the cognitive challenges of managing social relationships in groups of increasing size have driven the evolution of large brains, with more neurons and enhanced information-processing capabilities, and that this trend is at the root of human intelligence. This "social brain" hypothesis has received some support from correlations between measures of brain size and proxies for social complexity in various mammals. In anthropoid primates, for instance, the ratio of neocortex to total brain size increases with species' typical group size. At the individual level, the volumes of the amygdala and the prefrontal cortex also appear to vary with the size of the social network. However, despite these observations, many anthropologists and neuroscientists are concerned that this hypothesis may be too simplistic to account for the complex evolution of animal brains. Some authors question whether many of the forms of social cognition found in group-living animals have a unique social component (as solitary animals also acquire information from other individuals), or whether differences in brain size result from changes in cognitive capacity unrelated to sociality.

The link between sociality and brain area size was originally proposed for social insects long before the idea emerged in primate research, by Dujardin, who observed that the mushroom bodies (structures of the insect brain involved in learning and memory; Figure one A) are substantially enlarged in honeybees. Dujardin suggested that these brain areas were the seat of insect "intelligence," and since then, a relationship between large mushroom bodies, advanced cognition, and sociality has been assumed, although rarely explicitly tested. Insects provide a unique opportunity to further explore these questions since they exhibit an unparalleled diversity of social forms - from temporary aggregations to permanent colonies containing millions of individuals working together as a "superorganism" - and have evolved an impressive range of cognitive skills despite their miniature nervous systems.

Here we explore the evolutionary relationships between sociality, cognition, and brain size in insects. We review comparative studies of brain anatomy testing predictions of the social brain hypothesis. We illustrate how some major behavioral innovations of social insects may in fact require relatively simple and computationally inexpensive forms of cognition. We then argue that the cognitive demands of sociality should be investigated in terms of the sensory information being used, the computational challenges for defined social-cognitive tasks, and the neural networks they require, not just brain sizes.