Testing Evolutionary Brain Size Change in Bats
In his classic text on the evolution of the brain in vertebrates, Jerison (1973) demonstrated with numerous examples that brains have increased in size over time in most mammal lineages. He thus confirmed a general trend that had been progressively recognized since the time of Marsh (1874). More recent studies have confirmed Jerison’s findings for primates (Martin, 1990), cetaceans (Marino et al., 2004) and carnivores (Finarelli & Flynn, 2007). It was originally thought that the development in brain size was related to body surface area (Jerison, 1973), but then it was shown that brain size may be correlated to maternal metabolic turnover instead (Martin, 1981). This alternative interpretation was proposed because of the high energetic cost of brain tissue. However, brain size may vary by a factor of five relative to body weight. This wide range of variation cannot be explained by variation in adult basal metabolic rate alone, so the brain size of mammals must be affected by other things such as the early developmental period. Recognition of this led to formulation of the Maternal Energy Hypothesis (Martin, 1996). Larger brains may therefore be due to increase in the resources the mother provides to the developing offspring, according to the mode of reproduction. However, on an evolutionary scale, brain size usually increases due to new adaptations and generally beneficial characteristics.
This interpretation was challenged by Safi et al. (2005) with respect to bats. These authors proposed that evolution of the brain in different bat lineages had involved reduction as well as enlargement in brain size. The suggestion that bat brains may become smaller in evolution was specifically linked to the energetic cost of maintaining a large brain. However, that conclusion was based on a theoretical analysis of brain size in living bats and made no mention of the need to test it with reference to the fossil record. In fact, the major problem in this paper is that brain size in fossil bats is a totally neglected area. How could it be possible to test for an evolutionary trend without looking at the ancestors?
The paper by Safi et al. goes on to claim that, among bats, fast flyers have undergone a decrease in brain size relative to their ancestors because they needed reduced weight to forage in open spaces. They needed flight efficiency, and the energetic cost of a large brain would be a disadvantage, so sensory needs were relaxed. By contrast, in bats that have large wings for maneuvering in highly structured habitats, brain size would increase over time. These bats would need better spatial memory and hearing in order to fly in dense habitats.
When phylogeny and body mass were corrected for in a phylogenetic least squares approach (PGLS), wing size had a positive correlation with brain size, showing that wing size can predict encephalization after phylogeny has been corrected for....