In sexually dimorphic species, the larger sex is often assumed to exhibit greater vulnerability during the period of parental care, due to their assumed greater nutritional requirements. However, results in the literature are mixed regarding sex-biased environmental sensitivity, and it is uncertain to what extent these inconsistencies are due to the shortage of experimental studies in this area, or to flawed assumptions regarding the factors influencing the development of male and female offspring. In this thesis, long term data, along with experimental work were used to test whether habitat quality had sex-specific effects on nestling growth, survival to fledging, immune development, overwinter survival, and recruitment in a population of the great tit. I also investigated whether these sex-specific selective patterns relate to any observed bias in sex ratios. Consistent patterns were observed for greater female sensitivity to poor rearing conditions (relative to males) with regards to their growth; however, males showed greater vulnerability in poor conditions (relative to females) with respect to their post-fledging survival as well as their recruitment success. Investigation of sex allocation suggests that sex ratios become more male biased with improved habitat quality, which appear to correspond to the patterns of selection (e.g., survival and recruitment); however, the overall results suggest that some other factor was likely causing the mismatch observed between growth and survival. Previous work suggests that the development of immunity may influence short and long term fitness, and that males and females may show different priorities in how they allocate resources during development when exposed to harsh conditions (e.g., growth versus immunity). Therefore, I also explored whether rearing conditions had sex-specific effects on the development of immunity, and whether these differences correlate with the survival and recruitment of offspring. Though I was unable to detect any affect of sex, habitat, or their interaction on immune response, I did find that the survival of male and female nestlings varied depending on the habitat in which they were reared, and that those individuals with greater immune responses survived better: female nestlings survived relatively better than males in poorer quality habitats, whereas males survived better than females in good quality habitats, and the survival of male and female nestlings was positively associated with their immune response in those habitats in which they showed overall greater survival. Rearing environment had an opposite effect on the cell mediated immunity (CM I) of male and female nestlings, although these patterns were only evident among nestlings that survived overwinter. Among surviving females, CMI increased with declining rearing conditions, while having the opposite effect among surviving males. Since I found CMI to be important for the survival of nestlings, and found that male and female nestlings showed opposite effects of rearing environment on CMI, it seems plausible that differences in immune function may be at the root of the observed mismatch between results for growth, survival and recruitment. The results from these studies illustrate how sex-specific patterns of vulnerability may be more complex than is commonly assumed. Thus, finally, I examined the support in the literature for three different explanations for sex-specific vulnerability to poor rearing conditions using met a-regression. My results demonstrated that there is no support in the literature for hypotheses based on size or sex alone. However, met aregression revealed a joint influence of sexual size dimorphism and clutch size in explaining patterns of vulnerability. Overall, the results from this thesis suggest that there are many factors which can have sex-specific effects on offspring performance, and that predicting the effects within particular species may be very difficult.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:644894 |
| Date | January 2008 |
| Creators | Jones, Kristopher |
| Contributors | Sheldon, Ben |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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