Coccidian oocyst parasitism in the blue-black grassquit: influence on secondary sex ornaments and body condition

Abstract

Female choice for conspicuous secondary sexual traits, which often decrease male survival, is greatly debated in the sexual selection literature. The parasite-mediated sexual selection hypotheses suggest that parasite-resistant males should show greater expression of secondary sexual traits. Males in better condition should also have more extravagant ornamentation. According to these hypotheses, females that favour more conspicuous secondary sexual traits are choosing less parasitized males and/or those in better condition and should have offspring with better chances of surviving. We assessed interactions between the expression of secondary sexual traits, parasite load and body condition in the blue-black grassquit, Volatinia jacarina. We trapped and banded males and sampled intestinal parasites (coccidian oocysts). We considered plumage (percentage of blue-black coverage and coloration of feathers on rump and breast), display characteristics (frequency of events and height/frequency of leaps) and body condition (body mass and size, haematocrit and total plasma protein levels). There was a negative correlation between the number of oocysts and two secondary sexual traits, blue-black coverage and leap frequency. A negative correlation was also found between oocysts and two measures of body condition, body mass and size index. We found no correlation between body condition and secondary sexual traits. Results support parasite-mediated but not condition-dependent sexual selection. There were no correlations between body condition and secondary sexual traits, so parasites may affect secondary sexual traits directly.