Intraspecific brain size variation between coexisting sunfish ecotypes

Abstract

Variation in spatial complexity and foraging requirements between habitats can impose different cognitive demands on animals that may influence brain size. However, the relationship between ecologically related cognitive performance and brain size is not well established. We test whether variation in relative brain size and brain region size is associated with habitat use within a population of pumpkinseed sunfish composed of different ecotypes that inhabit either the structurally complex shoreline littoral habitat or simpler open-water pelagic habitat. Sunfish using the littoral habitat have on average 8.3% larger brains than those using the pelagic habitat. We found little difference in the proportional sizes of five brain regions between ecotypes. The results suggest that cognitive demands on sunfish may be reduced in the pelagic habitat given no habitat-specific differences in body condition. They also suggest that either a short divergence time or physiological processes may constrain changes to concerted, global modifications of brain size between sunfish ecotypes.

Keywords: adaptive divergence; brain size; cognitive ecology; habitat; plasticity; sunfish.

Figure 1.

Whole brain mass variation after adjusting for other factors in the mixed-effects model. Red symbols indicate littoral habitat and blue symbols indicate pelagic habitat. (a) The relationship between adjusted brain mass and standard length (both log-transformed), along with the linear fits for each habitat. (b) The relationship between brain mass (log-transformed) and gape width residuals, along with linear fits for each habitat. In both analyses, linear fits were parallel (no interaction between habitat and standard length in (a): 0.13 ± 0.084, t₁₀₂ = 1.58, p = 0.12; and no interaction between habitat and gape width residuals in (b): 0.0028 ± 0.025, t₁₀₂ = 0.11, p = 0.91).