Neville I. Passmore

Time constraints and multiple choice criteria in the sampling behaviour and mate choice of the fiddler crab, Uca annulipes

Abstract Active female sampling occurs in the fiddler crab Uca annulipes. Females sample the burrows of several males before remaining to mate in the burrow of the chosen partner. Females time larval release to coincide with the following nocturnal spring tide and must therefore leave sufficient time for embryonic development after mating. Here we show how this temporal constraint on search time affects female choosiness. We found that, at the start of the sampling period (when time constraints are minimal), females selectively sample the larger males in the population. Towards the end of the sampling period (when the temporal constraints increase the costs of sampling), females are less selective. Furthermore, we suggest that the number of males sampled (and other indices of ‘‘sampling effort’’) may not be reliable indicators of female choosiness and may not reflect the strength of female mating preferences under certain conditions. Burrow quality also emerged as an important criterion in final mate choice. Burrow structure potentially influences reproductive success, and mate acceptance based on burrow structure appears to involve a relatively invariant threshold criterion. Since there is no relationship between male size and burrow quality, females are using at least two independent criteria when choosing potential mates. We envisage mate choice as a two-stage process. First, females select which males to sample based on male size. They then decide whether or not to mate with a male based on burrow features. This sampling process explains how two unrelated variables can both predict male mating success.

Dishonest signalling in a fiddler crab

Animal communication theory predicts that low–frequency cheating should be common in generally honest signalling systems. However, perhaps because cheats are designed to go undetected, there are few examples of dishonest signals in natural populations. Here we present what we believe is the first example of a dishonest signal which is used commonly by males to attract mates and fight sexual rivals. After losing their large claw, male fiddler crabs (Uca annulipes) grow a new one which has less mass, is a less effective weapon and costs less to use in signalling than an equivalent–length claw of the original form. Males with original claws do not differentially fight males with regenerated claws even though they are likely to win. Regenerated claws effectively bluff fighting ability and deter potential opponents before they fight. During mate searching, females do not discriminate against males with low–mass, regenerated claws, indicating that they are deceived as to the true costs males pay to produce sexual signals. Up to 44% of males in natural populations have regenerated claws, a level unanticipated by current signalling theory. The apparent rarity of cheating may be an artefact of the usual difficulty of detecting cheats and dishonesty may be quite common.

Synchronized courtship in fiddler crabs

The apparent paradox posed by the synchronization of mating displays by males competing to attract females has provoked considerable interest among evolutionary biologists,. Such synchronized sexual signalling has only been documented for communicationusing light flashes (bioluminescence) or sound. It has been suggested that the “fundamental reasons that might favour precise adjustments in signal timing relative to that of a particular neighbour could only be compelling for signallers using these two channels”. Here we provide the first quantitative evidence for synchronous production of a conventional visual courtship signal, the movement of a body part.

Repeatability of mate choice: the effect of size in the African painted reed frog, Hyperolius marmoratus

Abstract Female painted reed frogs were offered a choice between artificial advertisement calls differing in frequency. Repeatability of mate choice was assessed by multiple testing of females. When the difference between the stimuli was 400 Hz, almost all females, regardless of size, consistently chose the low frequency call. When the frequency difference between the calls was smaller (200 Hz), females as a group appeared to choose at random. However, individual females seemed to show repeatability of mate choice. There was also a relationship between female size and the number of times they chose the lower frequency stimuli. This suggests that the manner in which females respond to stimuli that differ in frequency may be size-dependent. Larger females may be more sensitive to variation in call frequency, and therefore more likely to express a mating preference for low frequency calls.

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.

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.

Breeding behaviour of the African frog, Chiromantis xerampelina: Multiple spawning and polyandry

Breeding was observed in the foam-nest frog, Chiromantis xerampelina, in Hoedspruit, South Africa. The population had a 3-week breeding season with a male-biased operational sex ratio and asynchronous and unpredictable female arrival. Nests were built above water and at 14 out of 15 nests, from one to seven unpaired males (‘peripheral males’) gathered around the amplexing pair during nest construction. Those peripheral males closest to the pair competed with each other, and with the amplexing male, to position their cloacae aginst the females cloaca during oviposition. It is suggested that multiple spawning was occurring. Of 30 males, 25 (83%) were observed as peripheral males, and 17 (56·7%) were observed both in amplexus and as peripheral males. Male mating success and participation at nests was unrelated to size or weight. Chorus participation was the best predictor of mating success and presence at nests. Nests were built in two to four sessions with females descending to the water between sessions. On 55% of 20 occasions amplexing males dismounted between sessions, but they had only an 11% probability of re-amplexing when the female returned to the nest. Of 15 females, 10 (66%) mated polyandrously with two or more males on one night.

Phonotaxis in the painted reed frog (Hyperolius marmoratus)

SummaryA detailed study was conducted of the three-dimensional accuracy of phonotaxis by femaleHyperolius marmoratus. This analysis involved videotape recordings of phonotactic approaches to an elevated loudspeaker through a three-dimensional grid. Females readily resolved the sound source elevation, but the jump error angles describing the precision of approach were considerably greater in this three-dimensional analysis than in the more conventional two-dimensional ground approach analysis. Extensive use was made of visual cues in elevated phonotactic approach and lateral head scanning prior to jumps, often accompanied by vertical changes in head orientation, was frequent. The ability of such small anurans to localize a sound source in both the horizontal and vertical plane is remarkable.

The effect of chorus organization on mate localization in the Painted Reed frog (Hyperolius marmoratus)

SummaryExperiments with female Painted Reed frog (Hyperolius marmoratus) indicate that temporal separation of the calls of neighbouring males does not enhance their locatability. The ability of females to locate a sound source is not impaired when identical calls are broadcast simultaneously through two spatially separated loudspeakers (n=8). We suggest that call alternation may be important in the spatial distribution of males rather than in the ability of females to locate mates.

THE SIGNIFICANCE OF MULTI-NOTE ADVERTISEMENT CALLS IN A REED FROG, HYPEROLIUS TUBERILING UIS

ABSTRACT The reed frog Hyperolius tuberilinguis is a prolonged breeder with an advertisement call that varies in complexity from one to six click notes. Call complexity increases with chorus size, but calls containing more than three notes are rare. In playback experiments to males, subjects responded by increasing the complexity of their calls, without closely matching the stimulus and rarely exceeding the stimulus in complexity. Stimuli less complex than their own evoked a reduction in complexity. Call repetition rate remained unchanged in the responses. In two-choice phonotaxis experiments, females discriminated against one-note calls, and two- and three-note calls were the most attractive. Males thus adjust their calling in the presence of neighbours to a pattern most preferred by females. Calls of higher complexity may be more easily detected or located by females in the noisy environment of a chorus.