Females raised from birth on the trace-carotenoid diets showed a stronger preference for high-carotenoid males, compared with females raised on the high-carotenoid diets (). Statistically, this result was confirmed by the interaction between male carotenoid diet and female carotenoid diet (). While the mean strength of the female preference for carotenoid colouration varied significantly among populations (interaction between female population and male carotenoid diet; ), the sensitivity of the preference to female carotenoid intake did not (interaction between female population, male carotenoid diet and female carotenoid diet; F6,464=0.36, p=0.90). Female food intake had a significant negative effect on the sensitivity of the female preference to carotenoid intake (interaction between male carotenoid diet, female carotenoid diet and female food level; ), to the extent that the effect of female carotenoid diet on the strength of the preference was only apparent in low-food females (). However, food level, per se, had no overall effect on the strength of the female preference (interaction between female food level and male carotenoid diet; F2,464=0.40, p=0.67).
Effects of diet on the strength of the female mate preference for carotenoid coloration. Points represent least squares means (+s.e.m.) from the ANOVA in .
Table 1 Sources of variation in mate attractiveness in the mate choice tests. (Male attractiveness was measured from the responses of females to male courtship displays. Nonsignificant two- and three-way interactions were included in the model but are not shown (more ...)
Females raised on the high food level were larger (standard length, F1,683=698.36, p<0.0001) and in better condition (Ka, F1,683=68.94, p<0.0001) than their low-food counterparts. In contrast, the level of carotenoids in the diet did not affect female body size (F1,683=2.76, p=0.10) or condition (F1,683=2.40, p=0.12).
The basic results of the colour attraction tests replicate and strengthen the findings of Rodd et al. (2002)
. Both sexes were more strongly attracted to orange discs than to discs of other colours, as measured by number of pecks (Wilcoxon matched pairs tests; orange versus all other colours individually; females: all p
=127; males: all p
=128). This held even for the subset of females raised on the trace-carotenoid diet (p
=49), which demonstrates that attraction to orange objects is indeed an innate sensory bias. In an earlier study, fish used in the colour attraction tests had been exposed to multicoloured flake food and orange Artemia
nauplii, as well as to mature male guppies, which left open the possibility that attraction to orange was a learned response. In the present study, females raised on the trace-carotenoid diet had no opportunity to be rewarded for approaching orange objects (the trace-carotenoid diet is pale brown in colour). The four guppy populations tested in this study also followed the same geographic pattern as the six previously tested populations (Rodd et al. 2002
) with respect to the relationship between preference strength and orange attraction ().
In parallel to the mate preference results, females raised on the trace-carotenoid diets showed stronger attraction (as measured by the number of pecks) to orange discs, compared with females raised on the high-carotenoid diets (). In males, however, carotenoid intake had no significant effect on the sensory bias (). There was a significant effect of carotenoid diet (F1,226=4.31, p=0.04) and a significant sex-by-carotenoid diet interaction (F1,226=4.55, p=0.03). The difference between the trace- and high-carotenoid diet groups was significant for females (planned contrast t=2.96, p=0.003) but not for males (t=−0.06, p=0.9). Females were more strongly attracted to the orange discs than males were within the trace-carotenoid diet group (t=2.58, p=0.01) but not within the high-carotenoid diet group (t=−0.39, p=0.7). Neither sex nor food level had significant main effects (sex F1,226=2.44, p=0.11; food F1,226=0.92, p=0.34), nor were the interactions involving food level significant (p≥0.3). Population had a marginally significant main effect (F3,226=2.63, p=0.05) and did not interact significantly with the other factors (p≥0.2). No three-way interactions were significant (p≥0.3); the four-way interaction was removed from the model after being tested for significance (p=0.86).
Figure 3 Effects of diet on the rate at which guppies pecked orange discs. Points represent least squares means (+s.e.m.) from the ANOVA described in § 2(c).
Male guppies spend a smaller fraction of their time foraging than females do (Magurran & Seghers 1994
), and thus a sex difference in attraction to coloured objects might reflect a general sex difference in foraging motivation. Overall, however, the sexes did not differ significantly in the rate at which they pecked orange discs (p
=0.11) or coloured discs in general (Mann–Whitney test, z
=127). Thus, males and females differ specifically in regard to their sensitivity to carotenoids.