When thinking about the evolution of nervous systems, there is sometimes a tendency to think of a sort of manifest destiny of nervous systems. It’s part of a larger tendency to see everything in evolution as part of a “march of progress,” but somehow, I think there’s a greater tendency to think about neural evolution as a tale of ever increasing complexity than bones or livers or what have you.
Examples of simplification are nice examples of exceptions. Vestigial organs have always been seen as powerful examples indicating that structures have an evolutionary history. A new paper by Lehmann and colleagues looks at simplification of grasshopper ears.
They examined four species, which ranged from animals with full wings to hoppers that are flightless. (A wingless species, Peripodisma tymphii, is shown.)Wings matter for hearing, not directly, but because one of the major selective pressures on flying insects is the desire to not be eaten by a bat – and many bats echolocate. So if you give up on flying, you are less likely to be eaten by a bat.
Perhaps the biggest difference between the four species is in the size of the eardrum: or, to use the more precise name, the tympanum. The authors claim in the discussion that there seems to be a relationship between wing length and tympanum size, which you would predict based on the considerations above. But they don’t plot it anywhere in the results.
The sensitivity of the ears differs in the four species. Slightly. All tend to be most sensitive to sounds in the 4-5 kHz range, and can usually hear sounds at higher and lower frequencies if they’re loud enough. As might be expected, though, the wingless species shown above, P. tymphii, was the least sensitive to ultrasound – the range that bats use to echolocate.
The sensory neurons, however, do not seem to be noticeably different across the four species. This suggests that differences in sensitivity is probably caused by tweaking the physiology of the neurons, not by loss of cells (although they don’t do close counts of the neurons).
Given that neurons are often thought of as being energy hungry, it’s perhaps a little surprising that the periphery changes more then the neurons do. It will be hard to judge these kinds of evolutionary tales until we get a better handle on what the costs of developing and maintaining small neural circuits is.
Lehmann G, Berger S, Strauß J, Lehmann A, & Pflüger, H. 2010. The auditory system of non-calling grasshoppers (Melanoplinae: Podismini) and the evolutionary regression of their tympanal ears Journal of Comparative Physiology A. DOI: 10.1007/s00359-010-0560-2