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Most bird studies of female signalling have been confined to species in which females display a male-ornament in a vestigial form. However, a great deal of benefit may be gained from considering phenotypic traits that are specific to females. This is because (1) sex-specific traits may signal sex-specific qualities and (2) females may develop a male-ornament not because they are selected to do so, but because fathers transmit to daughters the underlying genes for its expression (genetic correlation between the sexes). We investigated these two propositions in the barn owl Tyto alba, a species in which male plumage is lighter in colour and less marked with black spots than that of females. Firstly, we present published evidence that female plumage spottiness reflects parasite resistance ability. We also show that male plumage coloration is correlated with reproductive success, male feeding rate and heart mass. Secondly, cross-fostering experiments demonstrate that plumage coloration and spottiness are genetically correlated between the sexes. This implies that if a given trait value is selected in one sex, the other sex will indirectly evolve towards a similar value. This prediction is supported by the observation that female plumage coloration and spottiness resembled that of males, in comparisons at the level of Tyto alba alba populations, Tyto alba subspecies and Tyto species. Our results therefore support the hypothesis that sex-specific traits signal sex-specific qualities and that a gene for a sex-specific trait can be expressed in the other sex as the consequence of a genetic correlation between the sexes.

Brighter red three-spined stickleback, Gasterostetus aculeatus, males have been shown to be preferred by females in the laboratory but in the field, these males did not receive more eggs. Instead, they had heavier eggs in their brood. We investigated the hypothesis that sexual selection for red coloration in male sticklebacks acts through mate choice by preferred males, who can afford to be choosy, for high-quality females which lay heavier eggs. We assume here that heavier eggs provide a direct fitness advantage. In simultaneous choice tests males were presented with two females differing in size. The number of zigzags directed to and the time spent orienting to each female were measured. After the test the females laid eggs, which we counted and weighed. Bigger (i.e longer and heavier) females laid significantly more and heavier eggs than smaller females. For all 23 males pooled together, the preferred female was the bigger of the two in 17 cases, laid more eggs in 18 cases,but laid heavier eggs in only 13 cases. When bright and dull males were analysed separately, we found that bright but not dull males spent more time oriented to the bigger female, and to the female that laid more eggs. Females preferred by bright males tended to lay heavier eggs than nonpreferred females, although this result was not quite significant. We conclude that in nature this preference for bigger females results in brighter males receiving on average heavier eggs. Assuming higher survival of bigger offspring, we propose that this can explain how brightness can be sexually selected in spite of brighter males not receiving more eggs. (C) 1998 The Association for the Study of Animal Behaviour

Most bird studies of female signalling have been confined to species in which females display a male-ornament in a vestigial form. However, a great deal of benefit may be gained from considering phenotypic traits that are specific to females. This is because (1) sex-specific traits may signal sex-specific qualities and (2) females may develop a male-ornament not because they are selected to do so, but because fathers transmit to daughters the underlying genes for its expression (genetic correlation between the sexes). We investigated these two propositions in the barn owl Tyto alba, a species in which male plumage is lighter in colour and less marked with black spots than that of females. Firstly, we present published evidence that female plumage spottiness reflects parasite resistance ability. We also show that male plumage coloration is correlated with reproductive success, male feeding rate and heart mass. Secondly, cross-fostering experiments demonstrate that plumage coloration and spottiness are genetically correlated between the sexes. This implies that if a given trait value is selected in one sex, the other sex will indirectly evolve towards a similar value. This prediction is supported by the observation that female plumage coloration and spottiness resembled that of males, in comparisons at the level of Tyto alba alba populations, Tyto alba subspecies and Tyto species. Our results therefore support the hypothesis that sex-specific traits signal sex-specific qualities and that a gene for a sex-specific trait can be expressed in the other sex as the consequence of a genetic correlation between the sexes.

Brighter red three-spined stickleback, Gasterostetus aculeatus, males have been shown to be preferred by females in the laboratory but in the field, these males did not receive more eggs. Instead, they had heavier eggs in their brood. We investigated the hypothesis that sexual selection for red coloration in male sticklebacks acts through mate choice by preferred males, who can afford to be choosy, for high-quality females which lay heavier eggs. We assume here that heavier eggs provide a direct fitness advantage. In simultaneous choice tests males were presented with two females differing in size. The number of zigzags directed to and the time spent orienting to each female were measured. After the test the females laid eggs, which we counted and weighed. Bigger (i.e longer and heavier) females laid significantly more and heavier eggs than smaller females. For all 23 males pooled together, the preferred female was the bigger of the two in 17 cases, laid more eggs in 18 cases,but laid heavier eggs in only 13 cases. When bright and dull males were analysed separately, we found that bright but not dull males spent more time oriented to the bigger female, and to the female that laid more eggs. Females preferred by bright males tended to lay heavier eggs than nonpreferred females, although this result was not quite significant. We conclude that in nature this preference for bigger females results in brighter males receiving on average heavier eggs. Assuming higher survival of bigger offspring, we propose that this can explain how brightness can be sexually selected in spite of brighter males not receiving more eggs. (C) 1998 The Association for the Study of Animal Behaviour