Male Shiners May Have Relatively Larger Brains?

I took the opportunity of a second snow day today to chug through some very detailed data we've generated about brain size in telescope shiners. Brittany, Alexandra, and I think Andrew measured the overall brain and the volume of individual brain structures from each of 10 male and female telescope shiners from each of April and June, the beginning and peak of their spawning season at Estill Fork. Looking at averages, and average ratios such as brain volume to standard length, I teased out some patterns. While female telescopes are longer and heavier on average with larger brains, in both months males had larger ratios of brain mass to net body weight (not counting gonadal mass), the volume of the optic tectum to total brain volume, and the ratio of optic tectum volume to standard length. These ratios weren't always statistically significantly different using a two-tailed t-test. We have found basically the same patterns with scarlet shiners. What's interesting is that the optic tectum is a relatively large part of the brain that processes visual information and helps the fish to move in three-dimensional space. As mammals we have them too, but they're much reduced.

The big question is, as always, what does it mean? I think it reflects the fact that male vertebrates have a much wider variation in reproductive success than females, and for stream fishes this is strongly linked to the ability to find and hold optimal position for spawning. Telescope shiners aren't strongly sexually dimorphic like scarlet shiners, and males don't guard territories to attract females. But males have to be able to show up in spawning aggregations and not be muscled out, which is largely an information processing exercise. My working hypothesis is that it's advantageous for males to have relatively larger brains, especially the optic tectum, as the result of long-term sexual selection pressures producing large-brained males. Does that mean they're smarter than females? Not exactly, but they may be more agile swimmers, which is a hard thing to quantify I suspect. Now I have to pull out our scarlet shiner data and look at it more closely, too, and try to mesh the two data sets together in a coherent fashion.

School actually starts tomorrow!