Since Darwin, scientists have been theorizing as to why there is variation in brain size between species and individuals. Does a larger brain, in say humans, indicate advanced cognitive abilities and complex language processing? Or is a smaller brain, such as the Olive-backed thrush’s, adapted to weigh less to accommodate lengthy flights?
In psychology, the field of phrenology has generally been dissolved, and with it, the idea that variations in brain size could indicate differences in intelligence, creativity or personality between humans. In the field of biology, however, scientists are discovering that brain variation across species might actually be linked to ecological competence. In this case, ecological competence describes the efficiency of a species to engage in ecological processes—such as flexible foraging abilities or advanced spatial memory for migration.
Take birds for example. Earlier research found denser neurons in the hippocampus of migratory dark-eyed juncos. Since this brain region is known for controlling long-term memory and spatial navigation, a more compact set of neurons in the hippocampus could indicate that the juncos’ brain physically adapted alongside migratory practices.
In addition, a study published this week in PLoS ONE describes the relationship between brain size and migration length in birds. That is, the smaller the brain, the longer the migration (or vice versa). Daniel Sol and colleagues used methods in phylogenetics, the study of evolutionary relatedness across species, to compare brain size in migratory and resident birds.
The researchers gathered data on brain volume and migratory distance for 600 passerine species in regions ranging from tropical to arctic. They analyzed the data to determine if brain size and migration evolved simultaneously, or if instead, changes in one trait produced changes in another trait. Finally, they evaluated whether the migration-brain association was caused, directly or indirectly, by ecological factors.
They found that the migratory birds tended to have smaller brains than their resident relatives. While there likely are other contributing factors, the scientists propose that the longer migration routes led to the development of smaller brains—an ecological selection that possibly balances the costs of an energy-intensive flight.
This ecological competence-brain connection extends to landbound animals as well. In one study, researchers found numerous comparisons between ungulates—such as horses, goats and antelope—and social and ecological factors.
Also using phylogenetics, Susanne Shultz and R.I.M. Dunbar compared overall brain size and neocortex size to habitat and group dynamics. They found that both habitat and group social dynamics predicted larger overall brain size. In other words, animals with larger brains tended to belong to more sociable groups and/or live in confined or mixed habitats.
When it comes to habitat, Shulz and Dunbar suggest two possible explanations: sensory abilities and predator awareness might be heightened in smaller habitats where the threat would be harder to escape, and species living in mixed habitats would need better spatial memory to navigate the terrain. For group dynamics, “An unstable herd of several thousand antelope may not present the same cognitive demands as a smaller stable group, where individuals interact on a daily basis,” say the authors.
However, the questions remain: did larger brains evolve to handle complex ecological and social processes, or did brain size and ecological factors determine the group size and social capabilities of the species?
It appears the science of explaining brain size as it relates to an animal’s behaviors, social patterns, cognitive capacity or ecological influences is an underdeveloped one. Researchers like those listed here are tapping into the methods of phylogenetics to explore the connection between physical adaptations and ecological influences, but this task includes many factors and variables. As Sol and colleagues describe in the paper, historical evidence is a key piece to the puzzle:
[P]revious studies have mostly focused on documenting advantages and/or costs of the brain under present ecological conditions. These studies have yielded a number of important discoveries such as that larger brains are associated with enhanced ecological opportunism, stronger social relationships, occupation of more variable climates, higher survival in novel environments and less pronounced population decline when the habitat changes.
In the absence of historical evidence, however, these findings are by themselves insufficient to understand the evolutionary pressures that have favored the diversification in brain size. This is because the observation that a certain variable is associated with differences in brain size does not necessarily imply that this is the cause of such differences; rather, it may be a consequence.
Cristol, D. (2003). Migratory dark-eyed juncos, Junco hyemalis, have better spatial memory and denser hippocampal neurons than nonmigratory conspecifics Animal Behaviour, 66 (2), 317-328 DOI: 10.1006/anbe.2003.2194
Sol, D., Garcia, N., Iwaniuk, A., Davis, K., Meade, A., Boyle, W., & Székely, T. (2010). Evolutionary Divergence in Brain Size between Migratory and Resident Birds PLoS ONE, 5 (3) DOI: 10.1371/journal.pone.0009617
Shultz, S., & Dunbar, R. (2006). Both social and ecological factors predict ungulate brain size Proceedings of the Royal Society B: Biological Sciences, 273 (1583), 207-215 DOI: 10.1098/rspb.2005.3283