Philip W. Bateman

Leave it all behind: a taxonomic perspective of autotomy in invertebrates

Autotomy is defined herein as the shedding of a body part, where (1) the loss of the body part is defensive (autotomy helps prevent the whole animal from being compromised and is in response to external stimuli); (2) shearing occurs by an intrinsic mechanism along a breakage plane (there has been selection for certain body parts to be pulled off easily); and (3) the loss is controlled ‐ the animal moves away from the trapped limb, the loss is under some form of central control (neural or hormonal), or the body part is detached quickly.

Male size and sequential mate preference in the cricket Gryllus bimaculatus

Abstract In crickets, females show preferences for male traits such as body size and either reject outright, take longer to mate with, or remove the spermatophores of nonfavoured males sooner, interrupting sperm transfer. In most mate choice studies, however, empirical tests so far have concentrated on choice between simultaneously presented males, which may not reflect the natural situation. We exposed sequentially to potential mates females that ranged from completely naive (virgin) to twice mated. Naive females did not remove the spermatophores of smaller males sooner, but tended (nonsignificant) to reject small males outright. With increasing experience, there was a general decline in spermatophore retention time, but large males were favoured: females either increased retention time when large males succeeded small males or maintained it for the same duration. There were more rejections when small males succeeded large or small males, but fewer when large males succeeded small males. Females appeared to ‘bet hedge’ at the first copulation, rather than be unable to judge male size/quality; but with increasing experience, their spermatophore retention and rejection behaviour clearly indicated a favouring of large males and an active discrimination against small males. There was a first-male advantage in terms of sperm transfer, but subsequent female behaviour compromised any advantage gained by a small first mate.

Experimental alteration of litter sex ratios in a mammal

Adaptive theory predicts that mothers would be advantaged by adjusting the sex ratio of their offspring in relation to their offsprings future reproductive success. Studies investigating sex ratio variation in mammals, including humans, have obtained notoriously inconsistent results, except when maternal condition is measured around conception. Several mechanisms for sex ratio adjustment have been proposed. Here, we test the hypothesis that glucose concentrations around conception influence sex ratios. The change in glucose levels resulted in a change in sex ratios, with more daughters being born to females with experimentally lowered glucose, and with the change in glucose levels being more predictive than the glucose levels per se. We provide evidence for a mechanism, which, in tandem with other mechanisms, could explain observed sex ratio variation in mammals.

Estimating Brown Hyaena Occupancy Using Baited Camera Traps

Conservation and management of brown hyaenas (Hyaena brunnea) is hampered by a lack of information on abundance and distribution, which is difficult and labour-intensive to obtain. However, occupancy surveys offer a potentially efficient and robust means of assessing brown hyaena populations. We evaluate the efficacy of camera trapping for estimating brown hyaena occupancy, and the effect of environmental variables and lures on detection probability. We estimated population density in Pilanesberg National Park, South Africa, at 2.8/100 km2, occupancy at 1.0 and model-averaged detection probability at 0.1. Using a fish lure increased detection probability to 0.2 and significantly increased encounter rates. We also found that brown hyaenas are more likely to be detected in areas of scrub or woodland rather than grassland. Our results suggest that 13 camera sites would be needed to achieve an occupancy estimate with S.E. of 0.05, and a minimum of 16–34 sampling occasions (with and without the fish lure) should be used in comparable study areas. We conclude that camera trapping is a viable method of estimating brown hyaena occupancy at local and landscape scales and capture—recapture analysis is also possible at a local scale.

Males are selective too: mating, but not courtship, with sequential females influences choosiness in male field crickets (Gryllus bimaculatus)

Mate choice experiments have generally focused on female choice; few have considered that males can also be selective. We examined courtship in male field crickets sequentially introduced to four females of differing size. Large (L) and small (S) females were introduced in order of either LSLS or SLSL. We demonstrate that naive males invest equally (courtship effort) in the first female they meet, regardless of her size, but show greater courtship effort when they subsequently encounter large compared with small females. Moreover, we demonstrate that males show this discriminatory behavior when they are permitted to mate with females (i.e. use a spermatophore) but are less choosy when the female is removed before spermatophore transfer.

Direct and indirect costs of limb autotomy in field crickets, Gryllus bimaculatus

Autotomy, found in multiple taxa, is the voluntary shedding of limbs and other appendages in response to predation, intraspecific conflicts and infection or damage. In a wild population of the field cricket, we found that 36% of 110 individuals of both sexes were missing at least one limb. We examined the speed to autotomize an entrapped limb and the costs of this autotomy in both sexes of a first-generation laboratory population of known age. We induced autotomy of either the two hindlegs or the two middle legs over 2 subsequent days. We found no sex differences in speed of autotomy, but hindlegs were autotomized faster than middle legs, and first legs were shed faster than second legs. Escape speed was significantly reduced by the loss of either middle or hindlegs, and number of jumps in the escape sprint was reduced. We found no significant effect of loss of two legs on male–male competitive ability, but loss of two legs in females significantly reduced mating ability by pairs. Males with autotomized middle legs showed reduced longevity, but neither sex showed a significant effect of autotomy on body mass. Gryllus bimaculatus, therefore, appears to incur both direct and indirect fitness costs from limb autotomy. This study has implications for understanding the general biology of crickets and contributes to our understanding of the role of autotomy as an evolutionary mechanism.

Male mate choice in the Botswana armoured ground cricket Acanthoplus discoidalis (Orthoptera: Tettigoniidae; Hetrodinae). Can, and how, do males judge female mating history?

In species of many taxa, males alter their investment in ejaculates depending on perceived levels of male–male competition and female mating history/age. Males of the tettigoniid Acanthoplus discoidalis produce a large sperm ampulla and associated spermatophylax, resulting in a long refractory period for males. Female A. discoidalis mate once before laying eggs, and then mate and lay eggs in a random order. The males, therefore, only have a high confidence of paternity when mating with virgins. Male A. discoidalis were studied to determine if they alter their investment in sperm count, spermatophylax mass and water content of spermatophylaces when mating with virgin and non-virgin females. No difference in investment in any spermatophore characteristics between virgins and non-virgins was found, but males do reject non-virgins more often, preferring to transfer spermatophores to virgin females. Males seem to prefer females of a lower mass, where higher mass may reflect increasing female age (and hence likelihood of previous matings) through advanced development of eggs. Outright rejection, rather than changes in spermatophore investment may be because of constraints in tailoring large spermatophores that develop over relatively long periods of time between matings.

Body temperature daily rhythm adaptations in African savanna elephants (Loxodonta africana)

The savanna elephant is the largest extant mammal and often inhabits hot and arid environments. Due to their large size, it might be expected that elephants have particular physiological adaptations, such as adjustments to the rhythms of their core body temperature (T(b)) to deal with environmental challenges. This study describes for the first time the T(b) daily rhythms in savanna elephants. Our results showed that elephants had lower mean T(b) values (36.2 +/- 0.49 degrees C) than smaller ungulates inhabiting similar environments but did not have larger or smaller amplitudes of T(b) variation (0.40 +/- 0.12 degrees C), as would be predicted by their exposure to large fluctuations in ambient temperature or their large size. No difference was found between the daily T(b) rhythms measured under different conditions of water stress. Peak T(b)s occurred late in the evening (22:10) which is generally later than in other large mammals ranging in similar environmental conditions.

Quantity versus quality: how does level of predation threat affect Cape ground squirrel vigilance?

How individuals balance time spent satisfying nutritional needs and time spent being vigilant to protect against potential predation has received abundant attention from researchers. Evidence indicates that both social conditions and predator risk affect how individuals perceive environmental threat, however, the relative influence of each type of risk remains unclear. The risk allocation hypothesis considers how individuals respond to predation risk over time by adapting their investment in high-quality or low-quality vigilance. Here we use the risk allocation hypothesis as a guide to examine how different risk factors influence vigilance quantity and quality in Cape ground squirrels, Xerus inauris. We tested how the social conditions of age, sex, distance to a safe refuge, distance from a nearest neighbour and predator risk affect the amount of time (quantity) individuals spend vigilant. Individuals were more vigilant in areas with more potential predators and when they were further from a safe refuge or nearest neighbour. We then tested whether these risk factors that affected vigilance quantity also affected vigilance quality by measuring exclusive vigilance (high-cost, high-quality) and shared-foraging–vigilance (low-cost, low-quality). Individuals in high-predation risk areas invested more time in high-cost vigilance behaviour than in low-cost vigilance/foraging behaviour compared with individuals in low-predation risk areas. Since squirrels invested in high-cost vigilance significantly more than in low-cost vigilance regardless of distance to a refuge or other squirrels, we suggest that combined foraging with vigilance compromises vigilance to lower its quality.

Size matters: genital allometry in an African mole-rat (Family: Bathyergidae)

Typically, sexually selected traits show positive allometry and high coefficients of variation (CV). To date, many studies on the allometry of genitalia have focused on insects. In addition, studies have largely ignored the potential for sexual selection on female genitalia, despite male and female structures presumably co-evolving. Insects tend to show negative allometry in both male and female genitalia, while in contrast, the few studies carried out in mammals (males only) show positive allometry. Reasons for these differences between the taxa still remain unclear. However, in mammals, three main mechanisms have been proposed for genital evolution, namely, sperm competition, female cryptic choice and sexual conflict. In the first such study that we are aware of, we examined intra-specific genital allometry in both males and females of a mammal, the subterranean solitary Cape dune mole-rat, Bathyergus suillus. We found positive allometry occurring in male genitalia, which is consistent with previous vertebrate studies. Similarly, we found that female genitalia also exhibited positive allometry further supporting the notion of co-evolution of male and female genitalia. Although it is difficult to distinguish between the forces or mechanisms determining this directional selection, we suggest that several reproductive advantages are incurred as a result of positive allometric relationship of the genitalia in B. suillus and such advantages are also likely in other subterranean mammals. Our study further highlights the differences in genital allometry across taxa.