Predation on females or males: who pays for bright male traits?

Bright colour patterns are found on male guppies, Poecilia reticulata, in regions of low predation but not on males in regions of high predation (Haskins et al. 1961). These data have been used to make the claim that bright male guppies get more matings, but are consumed more often by predators, thereby suggesting that coloration results from a balance between natural and sexual selection (Endler 1980). Female preference for male coloration also varies with the local intensity of predation: females from risky locales prefer duller males (Breden & Stoner 1987). It is assumed that female preference evolved through indirect, rather than direct selection on female behaviour (Breden & Stoner 1987; Stoner & Breden 1988; Houde & Endler 1990). Male animals are often the focus of work in behavioural ecology, with females relegated to less important roles. However, new insights may be gained by looking at the effects of a trait on the fitness of both sexes (Rosenqvist & Berglund 1992; Ahnesjo et al. 1993; Berglund et al. 1993). Although bright male guppies are frequently detected by predators (at long range), this need not lead to more male deaths. Guppies are commonly found in mixed-sex groups (Haskins et al. 1961) and predators that detect bright males from a distance might not necessarily attack them once they are closer. Crenicichla alta (a pike cichlid) is a stalking/ambush predator that can detect guppies from at least 1·2 m, but attacks at distances of 5–40 cm (Seghers 1973). Thus, detection and attack will often be separated in time and space. Once a group has been detected by a predator we would expect females to suffer a higher attack rate, because they are larger, and hence more profitable, given equivalent capture rates and handling times. We performed the following experiment to see which sex pike cichlids attack when within striking distance of pairs of female and male guppies. We presented pairs of guppies to three commercially purchased pike cichlids (10–15 cm standard length, SL) that had experience capturing guppies. The predators used were all identified as members of the C. saxitalis species complex (W. Liebel, personal communication), of which C. alta is a member. We used only 39 pairs of guppies in the experiment as we wished to use the fewest number of fish possible because the experiment resulted in the death of the prey. The guppies were from a Quare River population that is sympatric with pike cichlids introduced by J. Endler (personal communication) in the 1970s. The experiments were performed in a small climate-controlled room. The temperature remained at approximately 21)C and the lights were set on a 12:12 h light:dark cycle, providing 800 lx illumination at the water surface during the day. The female guppies used ranged from 20 to 26 mm SL (mean=23 mm), and the males ranged from 13 to 19 mm SL (mean=17 mm). The females weighed 0·23–0·54 g (mean=0·35 g) and the males weighed 0·06–0·18 g (mean=0·12 g). Mixed-sex pairs of fish were removed from stock tanks and placed in a circular, white, Plexiglas arena (22 cm diameter, 10 cm deep) to acclimatize. After 2 h we opened an opaque gate between the attack arena and an adjoining predator holding compartment (20#10#10 cm) remotely, and videotaped interactions between predator and prey from above. In 29 of 39 trials, the predator left its compartment and attacked one or both of the guppies. When given a choice, pike cichlids first attack female guppies (Fig. 1). We found both sexes were captured with equal facility (mean attacks/kill were 2·22 and 2·18 for females and males, respectively; ANOVA: F28=0·004, P>0·1), while handling times were unmeasurably short. Thus, with equivalent capture rates and handling times, we conclude that females were the more profitable prey because they were on average three times as