Sexual selection and intra-female competition in the green poison-dart frog, Dendrobates auratus

Observations on the mating and parental behaviour of the green poison-dart frog were used to test two hypotheses for the function of female-female competition in this species. The hypothesis that females compete to monopolize as many mates as possible was not supported because: (1) male territoriality was associated with competition for females; (2) intra-female competition for mates was not more common or intense than intra-male competition; (3) females were more selective about mating than were males; (4) estimated offspring production by individual females did not exceed the parental capacity of a single male. The hypothesis that females compete to monopolize the parental care of particular males was consistent with observations that: (I) females competed for males, but were more selective about mating than were males; (2) females remained near particular males and attempted to keep other females from mating with those males; (3) females may have used courtship to prevent males from mating with other females. Both hypotheses were consistent with evidence that: (I) females are large compared with males in D. auratus, relative to closely related species with low male parental investment; (2) females destroyed the eggs of other females when they located them. Williams (1966) and Trivers (1972) argued that relative parental investment is the primary determinant of the mating strategies of males and females in many species of animals. They argued that the sex that supplies the larger share of parental investment (investment in particular offspring that limits the parent’s potential to produce other offspring) will become a limiting resource for which members of the other sex compete. Females provide most parental investment in most species that have been studied, and intra-male competition exceeds intra-female competition for mates, whereas females are often selective compared with males. In species where male parental investment significantly exceeds that of females, sex role reversal should occur, that is, females compete for a limited number of available males and males are choosier than females about whom they mate with. Citing observations of males performing parental care (Eaton 1941) and of more than one female following a single calling male (Dunn 1941), Trivers (1972) suggested that sex role reversal might occur in the green poison dart frog. Wells (I 978) studied D. auratus in the field and in captivity in Panama, and suggested that his observations were consistent with the hypothesis of sex role reversal; that small clutch size and large time investment by males in parental care may have caused receptive males to become rare compared with receptive females, causing female-female competition and female courtship of males. This hypothesis will be referred to as the sex role reversal hypdthesis. Parental investment by males can cause competition for mates among females, even if male parental investment does not exceed female parental investment. Variance in the quality or quantity of parental care provided by males may lead to selection for female competition for those males able to provide the highest quality parental care (Wittenberger 1979; Petrie 1983, 1986). Female competition for males should also be favoured by selection if the quality or quantity of parental care provided by any particular male declines as his number of mates increases (Yasukawa & Searcy 1982; Halliday 1983; Hrdy & Williams 1983). Males may achieve higher reproductive success by being polygynous even if the quality or quantity of parental care they can provide per offspring declines with increasing offspring number. For any particular female the quality of a male would decrease as the number of other females mating with that male increased. This is analogous to the effect of decline in territory quality in the polygyny threshold model (Orians 1969). In this situation 01989 The Association for the Study of Animal Behaviour