It has been argued that females should be able to choose parasite-resistant mates on the basis of the quality of male secondary sexual characters and that such signals must be costly handicaps in order to evolve. To a large extent, handicap hypotheses have relied on energetic explanations for these costs. Here, we have presented a phenomenological model, operating on an intraspecific level, which views the cost of secondary sexual development from an endocrinological perspective. The primary androgenic hormone, testosterone, has a dualistic effect; it stimulates development of characteristics used in sexual selection while reducing immu- nocompetence. This "double-edged sword" creates a physiological trade-off that influences and is influenced by parasite burden. We propose a negative-feedback loop between signal intensity and parasite burden by suggesting that testosterone-dependent signal intensity is a plastic re- sponse. This response is modified in accordance with the competing demands of the potential costs of parasite infection versus that of increased reproductive success afforded by exaggerated signals. We clarify how this trade-off is intimately involved in the evolution of secondary sexual characteristics and how it may explain some of the equivocal empirical results that have surfaced in attempts to quantify parasite's effect on sexual selection.

Parasites, bright males, and the immunocompetence handicap

Theories of animal signalling emphasize the importance of costliness-to be effective, signals must be dependable; to be dependable, signals must carry costs-and carotenoid-based signals are a favoured example. The traditional view that carotenoids are costly because they are scarce still carries weight. However, biomedical research has led to alternative views on costliness, mainly related to beneficial, but also to detrimental, effects of carotenoids. Recent improvements in our understanding of carotenoids suggest that the relative importance of these mechanisms will soon be determined, leading to a fresh outlook on cost-based signalling.

Costly sexual signals: Are carotenoids rare, risky or required?

This article describes the selective recognition and noncovalent dimerization of N-terminal aromatic peptides in aqueous solution by the synthetic host compound, cucurbit[8]uril (Q8). Q8 is known to bind two aromatic guests simultaneously and, in the presence of methyl viologen, to recognize N-terminal tryptophan over internal and C-terminal sequence isomers. Here, the binding of Q8 to aromatic peptides in the absence of methyl viologen was studied by isothermal titration calorimetry (ITC), 1H NMR spectroscopy, and X-ray crystallography. The peptides studied were of sequence X-Gly-Gly, Gly-X-Gly, and Gly-Gly-X (X = Trp, Phe, Tyr, and His). Q8 selectively binds and dimerizes Trp-Gly-Gly (1) and Phe-Gly-Gly (4) with high affinity (ternary K = 109−1011 M-2); binding constants for the other 10 peptides were too small to be measured by ITC. Both peptides bound in a stepwise manner, and peptide 4 bound with positive cooperativity. Crystal structures of Q8·1 and Q8·42 reveal the basis for selective recognition a...

/pdf/sequence-specific-recognition-and-cooperative-dimerization-30lz7unthb.pdf

Sequence-specific recognition and cooperative dimerization of N-terminal aromatic peptides in aqueous solution by a synthetic host.

Genetics and evolution of colour patterns in reptiles.

When naturally infected with the malarial parasite Plasmodium mexicanum, Western fence lizards, Sceloporus occidentalis, exhibit numerous reproductive pathologies. Infected males display fewer courtship and territorial behaviors, have altered sexually dimorphic coloration, and have smaller testes. Infected animals store less fat by late summer, which, for females, reduces clutch size in the spring We describe hormonal alterations associated with malarial parasitism that may underlie this reproductive inhibition. We caught lizards in the field and bled them either immediately or 1 h after capture for measurement of basal and stress levels of steroid hormones. Compared to uninfected lizards, infected lizards had lower levels of basal plasma testosterone (24 vs. 38 ng/mL) and higher levels of corticosterone (18 vs. 8 ng/mL) following 1 h of capture and confinement. Infected animals also had lower levels of plasma glucose (243 vs. 270 mg/dL). When we experimentally elevated plasma corticosterone of uninfected...

Hormonal Alterations and Reproductive Inhibition in Male Fence Lizards (Sceloporus occidentalis) Infected with the Malarial Parasite Plasmodium mexicanum

Hamilton and Zuk (1982) proposed that the quality of male showy traits reflects genetically-based resistance to parasites and can be used by females to select mates that are less prone to parasitic attack. The hypothesis requires that a particular state of a variable showy trait should be associated with parasite infection. We tested this idea with a population of western fence lizards, Sceloporus occidentalis, infected with the malarial parasite, Plasmodium mexicanum. Ventral color pattern is strongly dimorphic in fence lizards and varies greatly among males in this population. Malaria-infected males exhibited significantly more black and less pale on their ventral surface than did noninfected males of similar body size. This difference was not a function of differing ages of infected and noninfected animals of the same body size. However, logistic regression demonstrated that females using male ventral color as a gauge of infection status would only marginally improve their chance of choosing a noninfected lizard over random selection of mates.

/pdf/parasites-and-showy-males-malarial-infection-and-color-3o41nc29xa.pdf

Parasites and showy males: malarial infection and color variation in fence lizards.

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Parasites and showy males: malarial infection and color variation in fence lizards.