Anthony Arak

Female mate selection in the natterjack toad: active choice or passive atraction?

SummaryVariation in seasonal mating success among male natterjack toads (Bufo calamita) was influenced by the number of nights that males spent at the pond and by male body size. Large males produced louder and lower frequency calls than small males, and maintained larger acoustic territories. After arriving at the pond, one half of all observed females mated with the first male they encountered. The remainder visited several males before initiating amplexus, but no criteria could be identified that females might have used when deciding whether to accept or reject a male. Movements between several males seem to be best explained by low female responsiveness to male advertisement calls on cold nights which were nonoptimal for oviposition. Females attempted to reject non-calling males both before and after amplexus, but this may be a mechanism to avoid mismating with males of the common toad (Bufo bufo), an explosive breeder that utilised the same pond. In two-choice playback experiments using synthetic advertisement calls, females showed no preference for calls based on their frequency. Females preferred calls of intermediate pulse repetition rate equivalent to those produced by a male at the same body temperature. Pulse rate is thus potentially subject to stabilishing selection and may be an important character for species-recognition. Females preferred fast rather than slow call rates, but only when the alternative rates were extreme. They also preferred calls which they perceived at the highest sound pressure level, but did not discriminate between absolute sound pressure levels of alternative stimuli at different distances. Since females that delay mating and oviposition may suffer predation, it is suggested that female preference for loud, rapidly repeated calls may be adaptive in the sense of minimizing the costs of locating conspecific males, rather than maximizing the probability of obtaining a high quality mate. Competition between males to maintain large acoustic territories and produce calls that can be easily detected by females would seem to be a sufficient mechanism to explain the evolution of the striking calls produced by male natterjacks.

Callers and satellites in the natterjack toad: evolutionarily stable decision rules

Abstract Male natterjack toads, Bufo calamita , sometimes called to attract females and sometimes behaved as silent satellites, attempting to intercept females attracted by callers. Small males with low intensity calls most frequently adopted the satellite tactic and preferred to parasitize the loudest callers. Because body size and call intensity were highly correlated with male mating success, small males with weak calls were at a reproductive disadvantage, and probably achieved greater success by exploiting the displays of larger, louder males than by calling themselves. The behaviour adopted by individual males in a given situation was predicted by a game theory model which assumed that males used an evolutionarily stable decision rule when choosing between alternative mating tactics. However, the model provided close agreement with observed behaviour only when it assumed that males have imperfect information about the behaviour of competitors. It is concluded that male toads adopt a conditional strategy which depends on their attractiveness to females. By switching between mating tactics appropriately, males maximized expected mating success within the constraint of the information available to them. The decision rule of the model qualitatively corresponds to a proximate mechanism that males appeared to use when switching between tactics. The model offers insights into the behaviour of other species in which males display in groups to attract females.

The wartbiter spermatophore and its effect on female reproductive output (Orthoptera: Tettigoniidae, Decticus verrucivorus)

SummaryMale wartbiters Decticus verrucivorus transfer elaborate spermatophores to females during copulation. The spermatophore is attached externally to the females genitalia and consists of two parts: a large, gelatinous, sperm-free portion, the spermatophylax, eaten by the female after mating; and a sperm-containing ampulla, eaten after the spermatophylax has been consumed. Since females take longer to eat larger spermatophylaxes, the duration of ampulla attachment is positively correlated with spermatophylax size. Two series of experiments were carried out, one in which the size of the spermatophylax consumed by females and the duration of ampulla attachment were manipulated in concert and another in which they were manipulated independently. Some females were also maintained on a protein-free diet and either supplied with or deprived of spermatophylax material. The amount of protein in the diet, but not the amount of spermatophylax material consumed, influenced female longevity, lifetime fecundity, and egg weight. When females were deprived of the spermatophylax, an experimental increase in the duration of ampulla attachment induced longer periods of unreceptivity in females after mating, a more rapid onset of oviposition, and an increased oviposition rate. Consequently, the number of eggs laid by females during their nonreceptive refractory periods increased significantly with increasing duration of ampulla attachment up to 180 min; however, there was no significant increase in the number of eggs laid beyond 180 min of ampulla attachment. This closely corresponds to an ampulla attachment duration of 188 min expected when a male transfers a natural spermatophylax of mean size to the female. These results suggest that the amount of ejaculate transferred to the female, and not the amount of spermatophylax material per se, is the factor controlling female receptivity and oviposition behavior. Since we did not detect any effect of spermatophylax consumption on female fecundity or egg weight, we conclude that the spermatophylax of D. verrucivorus functions primarily as a sperm protection device rather than as a form of male parental investment.

Choice of singing sites by male bushcrickets (Tettigonia viridissima) in relation to signal propagation

SummaryThe spatial dispersion of singing male bushcrickets (Tettigonia viridissima) in a marshland habitat was found to be significantly clumped. Males clustered in patches of taller vegetation, buth within such clusters, males were regularly spaced with a mean distance of 6 m between nearest-neighbours. Males occupied perches on plants that were, on average, 0.3 m above the top of surrounding vegetation. Physical and acoustical interactions between males were observed more frequently when males were singing from higher sites. Excess attenuation of the male song was found to increase with frequency but decreased markedly with increasing elevation of the singing male above the ground. The maximum detection range of the song, realized when the insect was singing > 1 m above the surrounding vegetation, was estimated as 60 m for the fundamental frequency (10 kHz), 38 m for the 1st harmonic (20 kHz) and 14 m for the second harmonic (30 kHz). By contrast, when males sang from the middle of dense reed beds, the estimated detection distance was only 8 m, 6 m and 4 m for each frequency band, respectively. Males could have increased the detection range of their songs almost three fold by singing from higher positions than those usually observed in the field. This suggests that there may be a cost of singing at higher elevations such as an increased risk of predation and/or increased aggression from neighbours. We suggest the spacing strategy adopted by males reflects a compromise between maximizing the range over which their songs can be detected and accurately localized by females and minimizing interference from competing males.

The adaptive significance of acoustic spacing in male bushcrickets Tettigonia viridissima: a perturbation experiment

SummaryA perturbation experiment was carried out in which the spacing between singing male bushcrickets, Tettigonia viridissima, was artificially manipulated. The experiment entailed releasing virgin females into an area in which caged males were either spaced evenly or in which a proportion of the males were clumped. There was large variation among males in the proportion of time spent singing (range 39–91% of total singing time). Singing activity was correlated with male body weight, but was not influenced by male spacing. Out of a total of 108 females released during the experiment, the majority (N=90) moved from their release points onto the cage of one of the closest singing males. More distant signalers sometimes attracted females when one of the closest males did not sing, or sang very little, during the period in which females were moving. When clumped, males were less successful in attracting females than when regularly spaced within the experimental area. Therefore, within a homogeneous habitat in which females are randomly distributed, male mating success will be maximized when males space out as far as possible from their competitors. As predicted, a regular dispersion of signaling males is the pattern observed in the preferred microhabitats of this species in nature.