Winston J. Bailey

Some costs of reproduction for male bushcrickets, Requena verticalis (Orthoptera: Tettigoniidae): allocating resources to mate attraction and nuptial feeding

SummaryThe cost of reproductive effort is known to result in a trade-off between current and future reproduction. Similarly, trade-offs in energy allocation may occur between components of reproductive effort, mating and parental effort, within a single reproductive episode. We investigated the energy allocated to mating effort (calling to attract females) and parental effort (donation of spermatophore nutrients at mating) by male bushcrickets, Requena verticalis, under two dietary regimes. Males provided with a low quality diet reduced the daily energy allocated to calling activity while maintaining their investment in spermatophores. Males provided with a high quality diet did not allocate more resources per day to their spermatophores but stored excess energy for future reproduction. Thus, on a per day basis, males appear to hold constant their investment in the spermatophore at the cost of reduced mating effort when resources are limited. Males on both diets, however, increased the size of their spermatophore donations when the interval between female encounters was increased. One explanation for this pattern could be a frequency-dependent optimization of spermatophore size.

Song power, spectral distribution and female phonotaxis in the bushcricket Requena verticalis (Tettigoniidae: Orthoptera): active female choice or passive attraction

The role of sound as a cue for female choice in the bushcricket Requena verticalis was investigated in both the field and laboratory. Females oriented to the closer of two calling males in a two-male field trial. Preference appeared to depend on the power of the higher frequency components (circa 28 kHz) of the calls spectrum, as well as the variance in chirp rate. Recording the songs of small and large males, and testing females on a two-speaker laboratory arena, revealed that when absolute call intensities were held at the same level, females could not distinguish between males from different size classes. However, where a consistent speaker preference occurred to a recording of either a large or small male, preference was shown towards the speaker producing a song with the greater power in the high frequency part of the spectrum. When the call of a single male was duplicated and presented as a choice to the female at different intensities, females were able to differentiate the call with an intensity difference of 1 dB in calls where the low frequency was more pronounced than higher frequencies, and with a 3 dB difference where this balance was less pronounced. Acuity, as expressed by the distance the female travelled before reaching the speaker, was improved where higher frequencies were more pronounced.

Success in territorial defence by male tarantula hawk wasps Hemipepsis ustulata: the role of residency

1. Some male tarantula hawk wasps Hemipepsis ustulata defend landmark shrubs growing on mountain tops, while others only visit a succession of territory sites, generally fleeing from territorial males. To determine whether residency asymmetries contribute to the short duration of most interactions between territorial males and visiting intruders, established residents were held captive for varying periods while other males replaced them at their territories.

Mating System, Mate Choice and Ultrasonic Calling in a Zaprochiline Katydid (Orthoptera: Tettigoniidae)

We describe sexual communication and the mating system of an undescribed genus of zaprochiline katydid. Males produce an ultrasonic calling song described as a series of simple pulses at a carrier frequency near 50 kHz. Zaprochilines contrast with most other katydids in having a narrow diet consisting of pollen. Males in nature exhibited a resource-defence mating system by calling in choruses near flowers which were attractive to females. We determined whether females discriminated among calling males as mates. An early-season sample of male calls showed a correlation between song frequency and both body size and the size of the glands that produce a large, nutritious spermatophore which is eaten by the female. The prediction that larger males would have a mating advantage over small individuals was supported in laboratory experiments. However, examination of paired males in nature failed to reveal any evidence of such a mating advantage. Furthermore, in other laboratory experiments it was found that females preferred high frequency call models, representative of small males, over low frequency calls. It is suggested that females do not prefer calls of large males because only in the early part of the season does male song give information on the size of the male and his spermatophore glands. In addition, spermatophore nutrients may be less important to the nutrition of female zaprochiline than other katydiyds, due to the high-protein pollen diet of this species. Since higher frequencies of calls show relatively greater attenuation with distance, the high frequency calls preferred by females may be perceived as males calling closeby.

Sexual differences in auditory sensitivity: mismatch of hearing threshold and call frequency in a tettigoniid (orthoptera, tettigoniidae: Zaprochilinae)

SummarySexual dimorphism of the ear of an undescribed species of zaprochiline tettigoniid is described. The internal trachea, dedicated to hearing in other tettigoniids, is unmodified in the male but fully developed in the female. The external auditory spiracle is also lost in the male. In contrast, there is no difference between the sexes in the number of sensilla within the hearing organ. The male is 10 dB less sensitive than the female. The characteristic frequency of the hearing organ at 35 kHz does not match the carrier frequency of the males call at 51 kHz. As a result of this mismatch the female is remarkably insensitive to the males call (threshold at 75 dB SPL), and the male is even less sensitive (thresholds⩾80 dB SPL). In nature this provides a maximum hearing range of the male of less than 50 cm.

Female phonotaxis and frequency discrimination in the bushcricket Requena verticalis

ABSTRACT. The male bushcricket, Requena verticalis, calls with a signal containing two predominant frequencies, 16 kHz and 28 kHz. A synthesized call, made from a template of the natural call, was played to females under conditions of a two‐speaker trial on a flat arena. Orientation pathways to a speaker emitting only a 16 kHz signal were more circuitous than the pathways made by females orientating to a speaker emitting only a 28 kHz signal. Females preferred a signal with both carrier frequency peaks present within the song to a signal with only a 16 kHz or 28 kHz carrier frequency, when the signal containing a double peak was kept at equivalent absolute intensity to that with a single peak. Females chose signals containing a more powerful high frequency peak over a signal in which both peaks were balanced. For the higher peak, they were able to differentiate between frequencies with a separation of 8 kHz but not of 4 kHz. They were unable to differentiate between frequencies with a separation up to 8 kHz in the lower peak when one frequency was held at 16 kHz; however, when this frequency was held at 18 kHz, females were able to distinguish between frequencies with a difference of 4 kHz. (N.B. 18 kHz is 2 kHz above the mean value for this frequency within the natural population.) We conclude that females are choosing males on the amount of power in the higher frequency range of their song and that this may be equivalent to a close calling male in the field.

FEMALE-FEMALE COMPETITION IN KATYDIDS: SEXUAL SELECTION FOR INCREASED SENSITIVITY TO A MALE SIGNAL?

In contrast to studies of sex‐specific weaponry and other sexually selected traits, there has been no examination of Darwins (1871, p. 418) suggestion that elaborations or enlargements of “the organs of sense” function to enhance mating success. In certain katydids the size of thoracic spiracles, which are a main input into the hearing system, determines auditory sensitivity of females. Here we present evidence that sexual dimorphism in the spiracle size of a pollen katydid, Kawanaphila nartee, is a result of sexual selection on females competing to locate nuptial‐gift giving males. In field experiments in which female K. nartee were attracted to a calling male, we show a pairing advantage to females with larger auditory spiracles. The spiracle‐size advantage was not a correlated result of a larger body size or mass of winners. Finally, there was no spiracle‐size advantage or body‐mass advantage for mating females in a later stage of competition when experimental females struggled for access to a silent male. We suggest that research on the detection of displays has lagged behind work on the displays themselves; the focus has been on the species specificity of signal perception rather than on the fitness consequences of variation in the ability to detect cues from mates or predators.

Acoustic satellite behaviour in the Australian bushcricket Elephantodeta nobilis (Phaneropterinae, Tettigoniidae, Orthoptera)

Male and female Elephantodeta nobilis duet with the female responding to the males long and complex call. The duetting males call consisted of four parts, described here as parts A, B, C and D. We found that the female replied 570 ms after the males D pulse, which followed the extended part B and short click of part C. Noncalling males were attracted to the duet and often used satellite tactics by inserting a volley of clicks 200 ms before the alpha males D pulse. Satellite males used part C of the alpha male song to cue their own call and this inserted call induced females to reply earlier compared with the alpha male call alone. Alpha males often extended their calls with additional D-type calls and so we examined the effectiveness of these calls as countermeasures to satellite calling. There was no influence of this alpha strategy on the satellites propensity to call although more calls from the alpha male did cause the female to reply more frequently. We also examined the effect of relative intensity of alpha and satellite calls on the females reply. Reduced satellite intensity increased the variance in the timing of the female response. Finally, we tested the effectiveness of the satellites call on female phonotaxis within a two-speaker arena. Although females preferred the alpha male they were nevertheless attracted to the satellite calls regardless of the latters relative intensity. We discuss the possible role of satellite calling as a novel conditional strategy. Copyright 2000 The Association for the Study of Animal Behaviour.

The sex in short supply for matings varies over small spatial scales in a katydid (Kawanaphilanartee, Orthoptera: Tettigoniidae)

Abstract In katydids such as Kawanaphilanartee, a female bias in the operational sex ratio (OSR) results in female competition for mates and male choice of mates. Previous work showed that the excess of sexually active females occurs when food availability is low, in part because less food increases the propensity of females to mate as they forage for the large edible spermatophores produced by males. In this study with K.nartee, a pollen-feeding species, we estimate natural variation in numbers of sexually active males and females by assessing male calling activity and the propensity of females to respond to experimental calling males. We found an excess of sexually active males at a site with many flowers and an excess of sexually active females at a site with few flowers about 900 m away. Between-site differences in gut masses of calling males were consistent with the hypothesis that pollen availability controls OSR. Finally, at a third site where flowers were at first scarce, we found that the initial excess in sexually active females changed to an excess of sexually active males after a clump of grass-trees flowered. The mean gut mass of all sampled males from this site increased after flowering. The large variation in OSR that we document for K. nartee highlights the importance of identifying the appropriate spatial and temporal scales over which OSRs are measured in studies of factors controlling sexual selection.

An experimental study of defensive stridulation in Mygalopsis ferruginea redtenbacher (Orthoptera: Tettigoniidae)

The defence response of Mygalopsis ferruginea Redtenbacher is described and the survival value of the acoustic components is tested experimentally. Stridulating insects repelled a reptilian predator more effectively than those whose stridulatory organs had been removed. The defensive behaviour is discussed in relation to the insects activity.