Miranda L. Dyson

Two-choice phonotaxis in Hyperolius marmoratus (Anura: Hyperoliidae): the effect of temporal variation in presented stimuli

Female painted reed frogs, Hyperolius marmoratus, were subjected to two-choide discrimination experiments to determine whether temporal overlap in the presented stimuli affects frequency preferences. The results showed that females preferred low frequency calls when the stimuli were presented alternately. Simultaneous presentation of stimuli resulted in a random response by females. When presented stimuli partially overlapped or abutted each other, the females responded significantly more often to the leading call irrespective of its frequency. These findings provide some insight into the possible effects of male vocal interactions on the ability of females actively to choose mates based on call frequency in natural breeding assemblages.

Absolute hearing thresholds and critical masking ratios in the European barn owl: a comparison with other owls

Abstract Absolute thresholds and critical masking ratios were determined behaviorally for the European barn owl (Tyto alba guttata). It shows an excellent sensitivity throughout its hearing range with a minimum threshold of −14.2 dB sound pressure level at 6.3 kHz, which is similar to the sensitivity found in the American barn owl (Tyto alba pratincola) and some other owls. Both the European and the American barn owl have a high upper-frequency limit of hearing exceeding that in other bird species. Critical masking ratios, that can provide an estimate for the frequency selectivity in the barn owls hearing system, were determined with a noise of about 0 dB spectrum level. They increased from 19.1 dB at 2 kHz to 29.2 dB at 8 kHz at a rate of 5.1 dB per octave. The corresponding critical ratio bandwidths were 81, 218, 562 and 831 Hz for test-tone frequencies of 2, 4, 6.3 and 8 kHz, respectively. These values indicate, contrary to expectations based on the spatial representation of frequencies on the basilar papilla, increasing bandwidths of auditory filters in the region of the barn owls auditory fovea. This increase, however, correlates with the increase in the bandwidths of tuning curves in the barn owls auditory fovea.

MATE CHOICE OCCURS ONLY IN SMALL CHORUSES OF PAINTED REED FROGS HYPEROLIUS MARMORATUS

ABSTRACT In two-choice discrimination experiments, females of Hyperolius marmoratus preferred the calls of lower frequency of the pair of stimuli. This preference was not shown in mating patterns observed in natural choruses, but is when females are phonotactically orienting in small choruses in an experimental enclosure. With an increase in chorus size, the mating pattern shifts from size-based, non-random (with some evidence of size-assortative) mating to random mating. This is the first time that frequency-based mate-choice by female anurans has been associated with chorus size, and hence with the sonic complexity of the acoustic environment.

Reproductive stage and history affect the phonotactic preferences of female midwife toads, Alytes muletensis

We investigated whether the reproductive stage or reproductive history of females affects their preferences for male calls that differ in frequency. We repeatedly tested gravid female midwife toads prior to and immediately after mating, and during ovulation. When females were ovulating they were much more consistent in their choice of call than when they were gravid but not ovulating. When females had recently mated they still showed positive phonotaxis, but did not reliably discriminate between alternative calls. This study is the first to test gravid female anurans repeatedly at different stages in their reproductive cycle (ovulating or not; pre- and postmating). It highlights the need to treat with caution the results from population-based preference studies in which individuals are tested only once, and in which variation in reproductive state is not taken into account. Copyright 2000 The Association for the Study of Animal Behaviour.

Selective Phonotaxis by Males in the Majorcan Midwife Toad

Males are expected to exhibit mating preferences when there is high variance in the quality of females or when males are limited to a small number of matings. In the Majorcan midwife toad, Alytes muletensis, the male performs parental care by carrying the eggs wrapped in a string around his hind legs. Both sexes possess courtship vocalizations which they use to advertise receptivity. We looked for evidence of male mating preferences in a phonotaxis arena in which males were presented with two alternative synthetic female calls. We predicted that males should prefer to mate with large females because of maternal effects on egg size, and that this preference would be manifest by selective phonotaxis toward low frequency calls. We also predicted that males should prefer high intensity calls as indicators of the energetic or motivational state of the female. Contrary to our prediction, males did not exhibit a preference for low frequency calls. The results suggest that there is stabilizing selection on call frequency and directional selection on call intensity.

SUCCESS BREEDS SUCCESS IN MATING MALE REED FROGS (HYPEROLIUS MARMORATUS)

Studies of the distribution of mating success among males in frog choruses typically seek to identify specific phenotypic attributes that confer a higher mating success on certain individual males. These attributes invariably relate to competition among males: either direct competition in the form of aggression, or competition to attract and be chosen by females. In this paper, we present evidence that an additional factor may operate in frog choruses. We show that individual males who mate on a given night enjoy a higher probability of being successful on the next night, and we suggest that this is because successful mating enables males to conserve energy.

The effect of rainfall on interclutch interval in painted reed frogs (Hyperolius marmoratus)

Body size, frequency of mating, clutch size, hatching success, and interval between successive matings were determined during 1982 and 1983 for a population of painted reed frogs, Hyperolius marmoratus , in southern Africa. Females mated once or twice during a single breeding season, which lasted 4 mo. Larger females produced larger egg clutches and had a greater probability of breeding twice. Absolute clutch size varied considerably, though hatching success was uniformly high and not related to the body sizes of mated pairs or the ability to produce more than one clutch. The interval between successive matings, 15-105 d, was dependent on the amount of rain prior to oviposition. Apparently females have some control over time of mating and oviposition in relation to rainfall patterns.

Animal Contests: Contests in amphibians

Studies of contests among amphibians are heavily biased towards acoustic contests in frogs: in these, males compete to be attractive to females or to defend territories required for some aspect of reproduction. While the calls of frogs are species-specific and appear to be highly stereotyped, these studies have revealed a high degree of plasticity that enables males to vary their calls in response to their immediate circumstances. Because calling is energetically expensive, males must face trade-offs between increasing their immediate calling effort and conserving energy for future mating opportunities. In some species, they also trade-off between repelling rival males and attracting females; this is because females are averse to aggressive calls. There is a great deal less known about contests among salamanders and caecilians, primarily because these animals are much harder to observe and because their primary means of communication, olfaction, is harder to manipulate experimentally than the acoustic signals of frogs. Despite many unanswered questions, a great deal has been learned about aggressive behaviour in amphibians, and these studies have made important contributions towards a general understanding of animal contests. This chapter discusses the diversity of issues related to contest behaviour in amphibians and highlights how these organisms remain fruitful subjects for future studies of animal contests

PHONOTAXIS BY FEMALE MAJORCAN MIDWIFE TOADS, ALYTES MULETENSIS

Female choice based on male call characteristics has been experimentally demonstrated in a number of species of anurans. In the Majorcan midwife toad, the male performs parental care by carrying the eggs in a string around his hind legs until they are ready to hatch. Both sexes produce calls to advertise sexual receptivity and both show phonotaxis towards the calls of the opposite sex. We used two-choice phonotaxis experiments to investigate whether females assess male caretaking ability using variation in call characteristics. Both call frequency and call duration are good indicators of male size, a characteristic that potentially influences the ability of males to successfully brood a clutch. Variation in call repetition rate and call intensity may also reflect differences in male caretaking ability, given the high energetic demands of calling behaviour in anurans. We predicted that females should prefer to mate with larger males and that this would be manifest in selective phonotaxis to low frequency and/or long duration calls. We also predicted that they would prefer calls produced at a fast rate and a high intensity. Contrary to our predictions females did not prefer lower frequency calls, longer calls or louder calls. They did however, selectively approach calls produced at a faster rate. These results are discussed in the context of sexual selection theory.

Some Determinants of the Mating System in a Population of Painted Reed Frogs (Hyperolius Marmoratus)

Hyperolius marmoratus shares its breeding site with 12 synchronously breeding species. Males call from elevated perches to attract females. Observations of captive populations suggest that males may adopt alternative mating strategies when chorus densities are high. Similar strategies were not observed in natural choruses. The operational sex ratio (OSR) was always male-biased and also highly variable. Mating was polygynous, only 25% of the male population actually mated. Females mated either once or twice. Variance in mating success was much greater among males, an obvious consequence of a skewed OSR. Female choice was probably not a causal factor because male mating success was correlated with neither size nor age. Size-based variation in the male advertisement call does exist, but females cannot exploit this potential vocal cue because of their impaired discriminatory ability in a complex sonic environment and the potential risk of mismating. Although the mating system may best be described as lek-like, with males forming organised choruses through competitive interactions, the variance in male mating success is probably a consequence of stochastic factors.