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In the Light of Evolution
the muscles related to breathing. Bass and Chagnaud further point out that in toadfishes the hindbrain vocal motor neurons lie adjacent to motor neurons innervating the pectoral fins. This finding suggests that the neural circuitry for sound production shares a long evolutionary (and developmental) history with the circuits controlling the pectoral fins and, in tetrapods, the forelimbs. This hypothesis may seem far-fetched at first; however, pectoral fins are used for sound production in a number of fishes, and forelimbs are clearly used for gestural communication in humans. If correct, the hypothesis implies a deep homology between behaviors that seem quite disparate but involve homologous neural circuits and, presumably, homologous developmental genes.
James Goodson and colleagues in Chapter 11 examine variation in neuropeptide expression across multiple brain regions involved in avian social behavior. More specifically, the paper focuses on differences in peptide expression among four emberizid songbird species, examining their correlation with seasonal changes in territoriality and/or flocking behavior. The analysis gets complicated, because variation in the degree of territoriality may be caused by reduced aggression or increased gregariousness (i.e., flocking), which likely involve different neural mechanisms. However, clever species selection allows the authors to identify one set of differences in neuropeptide expression that is most likely linked to differences in aggression and another set that correlates with differences in flocking behavior. As the authors admit, the conclusions are based on just a few species and, therefore, tentative. However, the study undeniably reveals an unexpectedly large degree of variation in peptide levels both across species and within species (i.e., seasonal variation). This variation is probably a driving force behind the variation in behavior, although it may also be a consequence. Experimental manipulations are needed to discriminate between these two hypotheses.
In Chapter 12, Lucia Jacobs develops ideas about the role of the hippocampus in navigation. She suggests that olfaction played a crucial early role in the evolution of spatial orientation, providing information about spatial gradients (in odor plumes) as well as local cue constellations (locale-specific odorant mixtures). The hippocampus became specialized to process and integrate these two kinds of information. Subsequently, these functions were extended to other sensory modalities. An interesting corollary of this hypothesis is that the size of the olfactory system should correlate more tightly with an organism’s ability to navigate by olfactory cues than with its capacity for odor discrimination. The hypothesis might also explain why olfactory brain regions scale less tightly than other regions with overall brain size. Perhaps the evolutionary shift to multimodal navigation allowed the olfactory system to be reduced. Jacobs predicts that the olfactory system should be larger in species that must