> Can eye
>colour be used to monitor short-term changes in health? It is a brilliant
>idea. Does anyone know of any published information on this topic?
>
>Dr. Wm. James Davis
>e-mail
>
>
Hi all,
Yes, it appears eye color can indicate health status and this may be
used as a basis for female choice. "In red jungle fowl, females preferred
males with more brilliantly colored eyes and combs, both
carotenoid-dependent traits and both influenced by nematode gut parasites
(Zuk at al., 1990). Several studies of fisjes have shown the influence of
carotenoid-derived skin coloration on female choice, with females often
preferring the more intensely colored male [cites omitted]." Marlene Zuk,
"The Role of Parasites in Sexual Selection: Current Evidence and Future
Directions", Advances in the Study of Behavior 21: 39-69 (1992). The idea
that parasites, through observable effects on plumage characters, eye
brightness, or other characteristics, and female choice of less-effected
males could result in sexual selection for exaggeration of those
characteristics has been around since William Hamilton and Marlene Zuk
proposed the connection in 1982 (cite below).
The problem they were addressing is why we continue to see high levels of
sexual selection today, that is, why is mate choice so often heavily skewed
toward a few members of the opposite sex. If females choose males on the
basis of evidence of good health, the genetic variation responsible for
differences among males in these traits will quickly go to fixation, such
that all the males have the same genes for these traits, at which point
there is no reason for the females to choose based on those traits anymore.
But host-parasite interactions may lead to cycles of coadaptation, thus
ensuring a constant source of heritable fitness variation. This is really
a variation on Zahavi's "handicap principle" (1975). That principle is
that males with exaggerated characteristics "were showing their ability to
survive despite such encumbrances and, by implication, must therefore be
extremely fit....For the notion to work, female preference must come to
emphasize signs of health or vigor that cannot be faked (because they are
costly to produce), and those signs are then elaborated by males..." Zuk
at 40. As an aside, when Zahavi first published his ideas, they were not
well accepted, something as a graduate student at the time in evolutionary
biology, I never understood. I am happy to say that his ideas have gained
wide acceptance in recent years.
One final note, the Hamilton-Zuk hypothesis makes two predictions. (1)
Within species, the showiest males are the least parasitized, and (2) among
species, the showier species will have the heavier parasite loads. While
(1) may seem obvious, remember that H-Z were explaining how high levels of
sexual selection can be maintained, and how (1) leads to (2).
Zuk, at al., "Parasites and mate choice in red jungle fowl", Am. Zool. 30:
235-244 (1990).
Hamilton and Zuk, "Heritable true fitness and bright birds: a role for
parasites?", Science 218: 384-387 (1982).
von Schantz, et al., MHC genotype and male ornamentation: genetic evidence
for the Hamilton_Zuk model", Proc. R. Soc. London B 263: 265-271(1996).
Moller, "Immune defence, extra-pair paternity, and sexual selection in
birds", Proc. R. Soc. London B 26_: 561-566(1997).
Agnew and Koella, "Virulence, parasite mode of transmission, and host
fluctuating asymmetry", Proc. R. Soc. London B 26_: 9-15(1997).
Mark Mulhollam
Minneapolis, MN
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