Brain Development And Evolution In Social Insects
How do brains evolve? If species differ in the size and structure of their brains, are there patterns to these differences? Can these patterns tell us something about the evolutionary forces that affect brain architecture?
Most animal lineages sport concentrated and highly organized nervous system tissue, usually located near the front of their bodies (brains). Animals’ brains are key to their evolutionary success. Brains are centers of information processing — analyzing and integrating input from the different senses — as well as playing roles in learning, memory, and regulating internal bodily functions like temperature and breathing. Particular brain regions often serve special functions, such as regions that process different sensory inputs. There may be separate brain regions for recognizing and responding to smells versus sights, for example.
But brains come at a steep price. Neurons, the information processing cells that make up much of brain tissue, are highly metabolically active. Neurons consume more energy per unit weight than most other cell types. Investment in costly brain tissue likely comes at the expense of other bodily functions, so evolutionary biologists expect the amount of brain tissue to be strictly limited.
One approach to understanding how brain size and structure evolve is to measure and compare overall brain size, or the relative sizes of functionally distinct brain regions, among species. This kind of comparison is especially powerful when we have a good understanding of how species are related to each other. We can then ask whether particular changes in brain size or architecture are consistently associated with changes in the ecology or behavior of species: is brain structure most similar among close relatives, or alternatively, does brain evolution track changes in species’ environments?
A recent paper by O’Donnell and Bulova used comparative studies of social insects — paper wasps — to explore how species differences in size, ecology, and behavior relate to brain size and structure. Paper wasps and their close relatives are attractive subjects for brain evolution studies: species relationships are well-understood, and wasps vary in social organization from solitary species to some of the largest, most complex animal colonies known. Among social species, nest mates are divided among behaviorally distinct reproductive queens and sterile workers.
Although they are small, wasp species do range in body and brain size; species analyzed in this study spanned an 18-fold difference in total brain volume. Their brain size and structure can be measured using special neuroanatomical methods and microscopy. Paper wasp brains are divided into separate regions that process odors and vision, and they even have a “higher processing” brain center that governs learning and memory: the mushroom bodies.
The study first showed that wasp body size is an important factor in brain evolution. As expected, larger species have larger brains, but brain size does not increase in pace with body size: the smallest species have much larger brains relative to their body sizes. This pattern is seen across many animal lineages. It suggests there is some minimal brain size that is needed to perform basic nervous system functions.
In some ways, paper wasp species are relatively homogeneous. Most species are generalized predators on soft-bodied insects; caterpillars are frequently favored. This rather limited menu stands in marked contrast to ants, social insect cousins of paper wasps. Different ant species get their protein from diverse insect prey, bird droppings, and even freshly cut leaves. However, a dramatically distinct paper wasp lifestyle did arise in the New World tropics: nighttime activity.
The genus Apoicacontains the only night-flying social wasps in the Americas; comparing their brains to other paper wasps proved fascinating. Apoicahad much smaller peripheral visual brain regions and larger mushroom body visual regions than expected based on their body size. Nocturnality evolved independently in the Southeast Asian social wasp genus Provespa, inviting future brain structure comparisons. Interestingly, nest-bound paper wasp queens have slightly reduced peripheral visual processing brain regions compared to their foraging, light-exposed workers.
Sociality is also important to wasp brain investment. Queens, stay-at-home moms that are reproductively active and socially dominant, typically have a greater investment in the higher processing brain regions (mushroom bodies) than their workers. Within a colony, the demands of maintaining social dominance may require greater brain investment. However, solitary species make greater mushroom body investment than their social relatives. The reasons for this are unclear, but it may be related to the fact that members of an organized, cooperative wasp colony can share information. This may reduce the cognitive demands on each individual, permitting reduced brain tissue investment. There is some evidence to support the idea that increases in cultural information sharing have been associated with decreased individual brain investment in humans.
This research is presented in the article entitled, Development and evolution of brain allometry in wasps (Vespidae): size, ecology and sociality, recently published in a special issue focusing on physiology the journal Current Opinion in Insect Science. The work was conducted by Sean O’Donnell and Susan Bulova from Drexel University.