Variation among individual females in mate choice behaviour may have important consequences for both sexual selection and mate choice evolution [1, 2]. Until recently, however, few studies have addressed the possibility or importance of within-population differences in mate choice behaviour. Most studies have focussed on the modal or mean expression of female choice, and thus the net sexual selection operating on the male population as a whole. As a result, the extent of variation in mate choice and the underlying genetic and environmental basis of such variation remains poorly understood. Here we present the results of a laboratory study in which we manipulated diet and thus somatic condition of female guppies (Poecilia reticulata), and tested the effects that this had on various components of female choice behaviour.
In this paper, we consider two distinct components of female mate choice behaviour: responsiveness and preference functions. Responsiveness is the willingness of females to respond positively to males and thus to engage in active mate choice. It is an aspect of female choosiness [1–3]. Preference functions are measures of the male ornamental and display trait values that females favour [1–5]. The distinction between responsiveness and preference functions allows the question of how willing females are to respond to male courtship to be considered separately from which male trait values females prefer. It is conceivable that very different patterns of variation may be manifest at these two levels [3, 4], and that the consequences may be quite distinct [1, 4]. The responsiveness and preference functions of a female, together with any constraints on their expression, contribute to the eventual outcome of mate choice: which male/s the female mates with.
Female responsiveness may influence the strength of sexual selection in three possible ways. If females that are less responsive are more likely to mate at random than highly responsive females, then low responsiveness will be associated with weak sexual selection. If, however, low female responsiveness raises the threshold level of attractiveness required for a male to obtain a mating , then low female responsiveness will be associated with strong sexual selection. Which of these two possibilities pertain will depend on the nature of the mating system and the precise meaning of responsiveness in the species concerned. Last, in many species, including guppies, sneak copulations by males can circumvent female choice [7, 8]. Reductions in responsiveness to male courtship are often associated with an increase in the importance of sneak copulations relative to courtship-choice copulations [7, 9, 10], and thus a reduction in the sexual selection imposed by mate choice.
Variation in preference functions might result in different females preferring different males [3, 5, 11]. This too will decrease the variance in male mating success and thus the opportunity for sexual selection. Variation in preference functions can also change the nature of sexual selection. For example, it may lead to frequency-dependent sexual selection on male traits  even if these traits are not preferred on average . This makes variation in female mate choice a potential mechanism for generating and maintaining polymorphism in male secondary sexual traits within populations [1, 2].
Several studies have shown that environmentally and socially imposed costs can affect female mate choice within populations. Female sticklebacks become less choosy close to spawning time or when forced to swim against a current when sampling males [13–15]. Female fiddler crabs (Uca annulipes) also become less choosy under time constraints . Increased predation risk causes female crickets (Gryllus integer) to choose less attractive males than they choose in the absence of predators . Parasite-infected females make fewer inspections of males before mating than uninfected females do in two fish species: bullies  and guppies .
The expression of costly mate choice may be dependent on female physiological condition [1, 20]. If so, condition-dependence could account for some of the observed variation in female mate choice behaviour [1, 20]. Variation in female (and male) condition within populations is likely to be common, due to stochastic environmental variation (e.g. seasonal and spatial variation in food availability) and heritable variation in resource acquisition [21, 22]. Several studies support the idea that components of mate choice may be condition-dependent. Female sticklebacks (Gasterosteus aculeatus) from groups with a high average condition factor prefer redder males, whereas those reared under 'simulated winter conditions' prefer orange males . Female stalk-eyed flies (Cyrtodiopsis dalmanii) under transient food stress have weaker preferences for males with large eye-spans than unstressed females . Food supplementation results in a decrease in responsiveness of female fireflies (Photinus ignitus) to simulated male flashes , whereas food restriction results in lower responsiveness and reduced probability of mating in female cockroaches (Nauphoeta cinerea) .
Studies of mate choice in female guppies have demonstrated significant levels of repeatable variation among females within populations in the extent to which they prefer particular male traits [3, 26–28]. The only study to dissect the components of mate choice  found some differences among females in preference functions, and that these differences lead individual females to prefer different males. However, most of the phenotypic variation, and the only significant levels of heritable variation were in responsiveness . Here we experimentally manipulate the condition of female guppies by controlling food intake over four weeks. We then test the hypotheses that: 1) females in good condition are more sexually responsive and 2) express stronger preference functions than females in poor condition. We also test whether females in good and poor condition prefer the same individual males. Our approach allows us not only to estimate whether female condition influences mate choice behaviour, but also to resolve which components of choice it influences.