Influence of hormones and carotenoids on signalling, immunocompetence and performance in a lizard.
Year: 2012
Type: dissertation
Abstract: The animal kingdom contains a spectacular diversity in colour signals used to indicate \nquality. The challenge of understanding this diversity lies in identifying and interpreting \nconstraints on signals that maintain signal honesty. I used an integrative approach to \nmeasure the effects of potential signal modulators on whole-animal performance, \nornaments, condition and immunocompetence in the lizard P. i. wilhelmi. This approach \nattempts to remove some of the uncertainty surrounding the validity of existing handicap \nmodels. First, I investigated seasonal changes in testosterone, corticosterone and \ncarotenoids and compared these to seasonal changes in endurance, immunocompetence \nand body condition male P. i. wilhelmi. I also determined which colour patches were \npredictors of male quality by relating them to morphology, endurance, body condition \nand immunocompetence. I found some support for the immunocompetence handicap \nhypothesis so I tested whether testosterone was modulating ornaments and constraining \nsignals through its immunosuppressive properties. I tested the immunocompetencehandicap \nhypothesis in male P. i. wilhelmi while also conducting parallel studies with \nfree-ranging and captive-housed lizards to assess whether there were differences between \nthe groups that could indicate exogenous factors influencing signalling in their natural \nenvironment. While experimentally elevated testosterone did affect endurance and the \nproperties of their colour patches, immunosuppression was only evident in free-ranging \nlizards. To measure the extent of organisational effects in males I also manipulated \ntestosterone in females and found no evidence of immunosuppression although \ntestosterone did affect some aspects of colouration and endurance. Next, I tested the \nstress-linked immunocompetence handicap hypothesis by experimentally elevating \ntestosterone, corticosterone, or both at the same time. I found that corticosterone had an \nisolated effect on one colour patch, and testosterone and corticosterone had opposing \neffects on endurance that were negated when both were elevated. Different colour \npatches were affected by either testosterone or corticosterone with little overlap, and the \ncombination of the two had a different action to either hormone elevated in isolation. \nFinally, I tested the oxidative stress hypothesis of carotenoids as a limiter of signal output \ndue to their requirement in the oxidative stress response. I found that supplemented \ncarotenoids vastly improved endurance, immune response and influenced different \naspects of colouration to that of testosterone and corticosterone. My study examined four \ndifferent hypotheses of constraints on signalling involving testosterone, corticosterone, \ninteractions between testosterone and corticosterone and carotenoids in a consistent \nmanner measuring multiple indices of quality as well as multiple colour signals. This \nstudy provides a unique integrative perspective on the roles played by each factor as well \nas prompting us to re-examine our approach to understanding constraints on signalling.
Author: Helen Jane Place
Language: English
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W2222969817