Parasites and Sexual Selection in Pheasants.

Abstract

This thesis presents the results of an intraspecific test of the Hamilton and Zuk theory of sexual selection using the ring-necked pheasant, Phasianus colchicus. This theory is based on several assumptions:

1. The parasites adversely affect host fitness, and susceptibility to parasites is heritable.
2. Expression of secondary sexual characters is, at least to some extent, dependent on host condition and vigour, and therefore on host ability to control parasites.
3. Females evolve appropriate discriminatory preference for secondary sexual characters that best reveal susceptibility to parasites.

Chapter one introduces the background to the theory and reviews some previous inter- and interspecific tests of the Hamilton and Zuk hypothesis. Chapter two explains the methods used in the experiments and gives an outline of the biology of the study animal and its parasites. Chapter 3 presents the results from the heritable resistance field experiment. Eight hundred pheasant chicks were divided into 16 replicates of 25 untreated birds, and 16 replicates of 25 treated birds. The untreated groups of birds were not protected from parasites, indeed they were encouraged to become infected. The untreated chicks were, as far as possible, protected from parasites. Therefore there was a significant difference in survival among the two types of birds (64 % treated, and 23 % untreated). The offspring of the untreated groups of pheasants had a significantly higher survival rate than the treated groups of birds when all were allowed to become infected with parasites by being reared on ground where their parents had been as adults. The surviving offspring of the untreated birds also weighed significantly more than the surviving offspring of the treated birds. Also females mated to untreated males produced significantly more eggs than females mated to treated males. Laboratory experiments revealed that second generation offspring of the untreated males survived significantly longer than offspring of the second generation treated males, when given standard inoculations of three species of Eimeri that were common on the experimental pheasant field, and a small number of nematode ova (chapter 4). The above experiments indicate that there are heritable differences in survival in pheasants, and that this is likely to be due, at least in part, to resistance to certain Eimeria species. The mate choice experiments discussed in chapter 5 show that female preference correlates with some male pheasant secondary sexual characters (wattle colour, wattle size, iris colour, display rate, call), and also correlates with low male pheasant parasite load. The parasite species were the nematodes Heterakis, and Capillaria; three species of coccidia, and four species of Mallophaga. Females also significantly preferred the offspring of untreated males. These offspring of untreated males should have had a higher proportion of resistant males than the offspring of the treated males. The correlations between male secondary sexual characters and parasite load are less convincing as they are only just significant. Chapter 6 presents the results of a survey of the distribution of pheasant parasites in the wild in Britain and elsewhere. The main purpose of the survey was to determine if the numbers of experimental pheasant parasites were within the range found in wild pheasants.