MATE CHOICE IN THE WOOD FROG, RANA SYLVATICA.

Abstract

Since Darwin's (1871) introduction of the theory of sexual selection, studies of mating associations have emphasized three patterns: (1) male-male competition (intrasexual selection) for control of females (Parker, 1970; Le Boeuf, 1974; Davies and Halliday, 1979; Wells, 1979), (2) male control of resources favored by females (Thornhill, 1976; Wells, 1977a; Howard, 1978), and (3) non-random mate choice by females (intersexual selection). Later refinements of Darwin's theory have focused on the differential investment of males and females in reproduction (Bateman, 1948), individual behaviors which maximize reproductive success (Trivers, 1972), and the ecological factors influencing patterns of mate choice (Orians, 1969; Borgia, 1979). Trivers (1972) and others (Bateman, 1948; Fisher, 1958; Williams, 1975) concluded that females generally have a much greater investment in reproduction and a higher certainty of parenthood than males and would be expected to be more discriminating in their choice of potential mates. Male reproductive success is generally limited by access to females and the number of female gametes fertilized. While natural selection in both sexes favors increased genetic representation in future generations (Williams, 1975), males are expected to maximize their reproductive success by increasing the number of matings, whereas females should choose higher quality mates. The major mechanism hypothesized that underlies patterns of sexual selection is that discriminating females control the reproduction of indiscriminate males. A fourth pattern of mating associations is non-random mate choice by males. Discussions of male choice are usually restricted to monogamous mating systems where males contribute an amount of parental effort that equals or exceeds that of females (Wilson, 1975) and in cases where sex role reversal occurs (Hohn, 1967; Smith, 1976). Otherwise males are assumed to lose little in indiscriminate matings (Bateman, 1948; Trivers, 1972; Williams, 1975). Few studies have examined the role of male choice in mating systems. This paper reports the importance of both male-male competition and male discrimination in the mating pattern of the wood frog, Rana sylvatica. Results of field observations and experiments performed with natural populations in woodland ponds, and of laboratory mate-choice experiments using males and females of varying ages, body sizes and geographic origins are presented. Experiments were designed to assess (1) relative importance of male and female choice, (2) effects of age and size on male reproductive success and (3) presence of non-random, assortative mating.