Influence of hormones and carotenoids on signalling, immunocompetence and performance in a lizard.
Date
2012-09-18
Authors
Place, Helen Jane
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Abstract
The animal kingdom contains a spectacular diversity in colour signals used to indicate
quality. The challenge of understanding this diversity lies in identifying and interpreting
constraints on signals that maintain signal honesty. I used an integrative approach to
measure the effects of potential signal modulators on whole-animal performance,
ornaments, condition and immunocompetence in the lizard P. i. wilhelmi. This approach
attempts to remove some of the uncertainty surrounding the validity of existing handicap
models. First, I investigated seasonal changes in testosterone, corticosterone and
carotenoids and compared these to seasonal changes in endurance, immunocompetence
and body condition male P. i. wilhelmi. I also determined which colour patches were
predictors of male quality by relating them to morphology, endurance, body condition
and immunocompetence. I found some support for the immunocompetence handicap
hypothesis so I tested whether testosterone was modulating ornaments and constraining
signals through its immunosuppressive properties. I tested the immunocompetencehandicap
hypothesis in male P. i. wilhelmi while also conducting parallel studies with
free-ranging and captive-housed lizards to assess whether there were differences between
the groups that could indicate exogenous factors influencing signalling in their natural
environment. While experimentally elevated testosterone did affect endurance and the
properties of their colour patches, immunosuppression was only evident in free-ranging
lizards. To measure the extent of organisational effects in males I also manipulated
testosterone in females and found no evidence of immunosuppression although
testosterone did affect some aspects of colouration and endurance. Next, I tested the
stress-linked immunocompetence handicap hypothesis by experimentally elevating
testosterone, corticosterone, or both at the same time. I found that corticosterone had an
isolated effect on one colour patch, and testosterone and corticosterone had opposing
effects on endurance that were negated when both were elevated. Different colour
patches were affected by either testosterone or corticosterone with little overlap, and the
combination of the two had a different action to either hormone elevated in isolation.
Finally, I tested the oxidative stress hypothesis of carotenoids as a limiter of signal output
due to their requirement in the oxidative stress response. I found that supplemented
carotenoids vastly improved endurance, immune response and influenced different
aspects of colouration to that of testosterone and corticosterone. My study examined four
different hypotheses of constraints on signalling involving testosterone, corticosterone,
interactions between testosterone and corticosterone and carotenoids in a consistent
manner measuring multiple indices of quality as well as multiple colour signals. This
study provides a unique integrative perspective on the roles played by each factor as well
as prompting us to re-examine our approach to understanding constraints on signalling.