Bird-song - Evolution of HVC and repertoire size
Devoogd et. al. (1993) discusses how levels of song repertoire and number of syllables in a song relate to the volume of brain nuclei. They find that there is a correlation between the high vocal center and the number of song types. Do the same species always have the same repertoire? In any case, the researchers note that the total number of syllables in the complete repertoire is the best index of learning. Apparently, brevity is no sign of intelligence among avian species. There are two neural paths associated with song systems: the caudal projections [song production] and rostral projections [song acquisition]. The data showed that the differences in the numbers of songs species sing are associated with the relative size of the HVC.
Airey et. al. (2000) sought to determine if the volume of song-control nuclei was heritable. They started with three premises: male behavior determines female preference, behavior is intimately linked to brain anatomy, and the differences in behavior must be heritable. Material and Methods: (p. 2100 – “Birds were supplied with … hard-boiled chicken egg.”) Evolvability, or the proportional response for measured traits, was greater for the high vocal center in the species studied. Most importantly, the researchers find that there is “a moderate heritability for HVC size.” It is also correlated (in size) to the RA. Song production shows more evolvability for nuclei than song acquisition.
Spencer et. al. (2005) show that canaries, which have been exposed to malaria, have decreased reproductive fitness. This is due to a decline in repertoire. Previous studies linked song complexity to parasite loads. The study also sought whether coritcosterone (a stress hormone) had increased as a response to the introduction of the parasite. But, levels of corticosterone were not found to be higher in the parasitized birds. The study gives strong evidence to the developmental stress hypothesis, that is, physiological stressors will reduce reproductive fitness. Especially if they act against the HVC, in certain avian species.
Half of the world's human population is infected with Toxoplasma: Mind control by parasitesEO Wilson and James Watson on Charlie Rose
posted by Eric Brandon at 2:20 AM
1 Comments:
Devoogd et al. 1993
This study conducts a comparison of song complexity/repertoire size and brain structure across 41 oscine species using independent contrasts. They find a correlation between HVC volume (relative to the telencephalon) and repertoire size, but not between relative HVC and the number of syllable types per song. The relative volume of area X is not correlated with either measure of song variation (p >= 0.10??). However, residual HVC and Area X volumes are significantly and positively correlated with each other. A~B and B~C but not A~C? Do these analyses, combined with their first paragraph of the discussion, support the conclusion that the HVC is the structure related to these song attributes, which is in turn influencing the volume of other nuclei, "superimposing shifts in an entire neural system"? Maybe this is more convincing in light of other articles. I'm not convinced from their analysis that the song attributes examined are completely independent (r=0.30, 0.12>p>0.10 --it's close anyway); what would be a suitable measure of song complexity, as it is expected to affect brain structure? Should song complexity (measured as the number of syllable types per song type) be left out of the picture? If we were to redo this study, how could we best summarize song 'elaboration'?
Airey et al. 2000
In this paper, the plot thickens as the authors report, for zebra finches, that the HVC nucleus is at least moderately heritable and quite "evolvable" (and genetically correlated with RA), and HVC, as part of the posterior pathway is much more heritable than the Area X nucleus and the anterior pathway, which is probably more affected during development (i.e. parasite loads?). Higher heritability paves the way for strong selection pressures on song attributes, as in female choice for complicated songs in males. A short discussion on additive genetic variance, evolvability and selection gradient terms might be useful to put everyone on the same page for discussion (especially me).
Spencer et al. 2005
Despite a few shortcomings in the methods (biases in measurements of repertoire size in favor of expected result, improper controls with Columba blood and antigen responses), this paper is really fascinating. Male canaries infected with Plasmodium parasites develop simpler songs and reduced HVC structures. But from what we have read in the last two papers, why don't they measure anterior and posterior pathway brain nuclei?! Wouldn't we expect a larger effect on the anterior (Area X, etc) than the posterior (HVC) if HVC is more genetically determined? Certainly though, HVC is affected during development, perhaps in concert with the entire song neural system or directly attributable to effects on efferent nuclei?
A couple of side notes...I wonder what HVC and secondary nuclei look like in suboscine birds that do not learn their songs? Would we expect parasites (or general stressors) to have little effect on their song development compared to oscines? Also, Plasmodium infections are probably (definitely) cyclic or dynamic over the course of an individual's life. Could these parasite loads affect individual song nuclei "maintenance" (for lack of a better word) throughout the course of its life, beyond developmental stages?
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