Anne-Katrin Eggert

Intraspecific brood parasitism in the burying beetle, Necrophorus vespilloides (Coleoptera: Silphidae)

Abstract Burying beetles, Necrophorus , fight for carcasses suitable for reproduction, and usually only one female or one pair cares for the larvae. In fights observed between female N. vespilloides , the winner was determined after only a few fights. Females losing fights did not abandon the carcass immediately. Instead, they stayed to lay eggs of their own. In laboratory experiments, in 60% of cases some of the losers larvae were cared for by the winner and survived to adulthood. Intraspecific brood parasitism is likely to occur in the field as well, since on 17% of the carcasses used for reproduction by N. vespilloides , two females were still present some time after egg laying had begun. From the laboratory experiments, it can be inferred that these two females had fought with one another. Further laboratory experiments showed: (1) a positive correlation between the parasites duration of stay near the carcass and her chances of parasitizing the other females brood; and (2) a negative effect of parasitism on the caring females own reproductive success.

FEMALE CONTROL OF SPERM TRANSFER AND INTRASPECIFIC VARIATION IN SPERM PRECEDENCE: ANTECEDENTS TO THE EVOLUTION OF A COURTSHIP FOOD GIFT

Manipulation of ejaculates is believed to be an important avenue of female choice throughout the animal kingdom, but evidence of its importance to sexual selection remains scarce. In crickets, such manipulation is manifest in the premature removal of the externally attached spermatophore, which may afford females an important means of postcopulatory mate choice. We tested the hypothesis that premature spermatophore removal contributes significantly to intraspecific variation in sperm precedence by (1) experimentally manipulating spermatophore attachment durations of competing male Gryllodes sigillatus and (2) employing protein electrophoresis to determine the paternity of doubly mated females. The relative spermatophore attachment durations of competing males had a significant influence on male paternity, but the pattern of sperm precedence deviated significantly from the predictions of an ideal lottery. Instead, paternity data and morphological evidence accorded best with a model of partial sperm displacement derived here. Our model is similar to a displacement model of Parker et al. in that sperm of the second male mixes instantaneously with that of the first throughout the displacement process, but the novel feature of our model is that the number of sperm displaced is only a fraction of the number of sperm transferred by the second male. Regardless of the underlying mechanism, female G. sigillatus can clearly alter the paternity of their offspring through their spermatophore‐removal behavior, and employ such cryptic choice in favoring larger males and those providing larger courtship food gifts. We discuss how female control of sperm transfer and intraspecific variation in sperm precedence may be important precursors to the evolution of gift giving in insects.

Joint breeding in female burying beetles

SummaryBurying beetles (Nicrophorus) exhibit advanced parental care, by feeding and guarding their offspring on buried vertebrate carrion. Till now, interactions between two conspecific females on a carcass have been thought to be mostly competitive, and parental care was thought to be provided by single females or male-female pairs exclusively. Here we demonstrate that cooperative brood care occurs in this species, and that its incidence is contingent on carcass size. Small carcasses are usually monopolized by one female; typically the larger of two female combatants secures the carcass for her offspring (Figs. 1 and 2). On large carcasses fights still occur, but in most cases both females stay on the carcass long enough to provide care for the brood. The use of genetic markers revealed that the maternity of offspring is shared evenly among joint breeders (Figs. 3, 4). We hypothesize that cooperative breeding is an adaptive response to a situation that arises partly as a consequence of a physical constraint.

The Gin Trap as a Device Facilitating Coercive Mating in Sagebrush Crickets

Female sagebrush crickets (Cyphoderris strepitans) feed on males’ fleshy hind wings during copulation and ingest haemolymph from the wounds they inflict. At the same time, males physically secure females using a specialized, abdominal pinching organ known as a gin trap. Although the gin trap clearly serves a reproductive role, its precise functional significance remains unknown. The objective of the present study was to determine the adaptive significance of the gin trap by independently manipulating the ability of males to provision and secure females during copulation. When the hind wings of males were left intact, there was no difference in the mating success of males with functional and experimentally disabled gin traps, respectively. However, when males’ hind wings were removed, males with functional gin traps experienced a significantly higher mating success than those whose gin traps had been disabled. We conclude that the gin trap functions as a device by which males with insufficient hind-wing material are able to force copulations upon females unwilling to accept their spermatophores.

The Coolidge effect, individual recognition and selection for distinctive cuticular signatures in a burying beetle

The ability to recognize individuals is an important aspect of social interactions, but it can also be useful to avoid repeated matings with the same individual. The Coolidge effect is the progressive decline in a males propensity to mate with the same female combined with a heightened sexual interest in new females. Although males that recognize previous partners and show a preference for novel females should have a selective advantage as they can distribute sperm evenly among the females they encounter, there are few invertebrate examples of the Coolidge effect. Here we present evidence for this effect in the burying beetle Nicrophorus vespilloides and examine the mechanism underlying the discrimination between familiar and novel mates. Burying beetles feed and reproduce on vertebrate carcasses, where they regularly encounter conspecifics. Males showed greater sexual interest in novel females (virgin or mated) than in females they had inseminated before. The application of identical cuticular extracts allowed us to experimentally create females with similar odours, and male responses to such females demonstrated that they use female cuticular patterns for discrimination. The chemical analysis of the cuticular profile revealed greater inter-individual variation in female than in male cuticular patterns, which might be due to greater selection on females to signal their individual identity.

POLYANDRY AND FITNESS OF OFFSPRING REARED UNDER VARYING NUTRITIONAL STRESS IN DECORATED CRICKETS

Abstract.— Females, by mating with more than one male in their lifetime, may reduce their risk of receiving sperm from genetically incompatible sires or increase their prospects of obtaining sperm from genetically superior sires. Although there is evidence of both kinds of genetic benefits in crickets, their relative importance remains unclear, and the extent to which experimentally manipulated levels of polyandry in the laboratory correspond to those that occur in nature remain unknown. We measured lifetime polyandry of free‐living female decorated crickets, Gryllodes sigillatus, and conducted an experiment to determine whether polyandry leads to an increase in offspring viability. We experimentally manipulated both the levels of polyandry and opportunities for females to select among males, randomly allocating the offspring of experimental females to high‐food‐stress or low‐food‐stress regimes to complete their development. Females exhibited a high degree of polyandry, mating on average with more than seven different males during their lifetime and up to as many as 15. Polyandry had no effect on either the developmental time or survival of offspring. However, polyandrous females produced significantly heavier sons than those of monandrous females, although there was no difference in the adult mass of daughters. There was no significant interaction between mating treatment and offspring nutritional regimen in their effects on offspring mass, suggesting that benefits accruing to female polyandry are independent of the environment in which offspring develop. The sex difference in the extent to which male and female offspring benefit via their mothers polyandry may reflect possible differences in the fitness returns from sons and daughters. The larger mass gain shown by sons of polyandrous females probably leads to their increased reproductive success, either because of their increased success in sperm competition or because of their increased life span.

Carcass maintenance and biparental brood care in burying beetles: are males redundant?

1. Burying beetles inter small vertebrate carcasses that ultimately serve as a food source for their developing young. The male remains with the female on the carcass after the brood has been produced, purportedly to aid in the feeding and protection of larvae. However, numerous laboratory experiments have failed to demonstrate a beneficial effect of the male on the growth and survival of offspring.

Female-coerced monogamy in burying beetles

The reproductive interests of the sexes often do not coincide, and this fundamental conflict is believed to underlie a variety of sex-specific behavioral adaptations. Sexual conflict in burying beetles arises when a male and female secure a carcass that can support more offspring than a single female can produce. In such a situation, any male attracting a second female sires more surviving offspring than he would by remaining monogamous, whereas the females reproductive success decreases if a rival female is attracted to the carcass. Monogamously paired males on large carcasses do in fact attempt to attract additional females by means of pheromone emission, whereas males on small carcasses do not. Females physically interfere with male polygynous signaling using various behavioral tactics. We demonstrate that such interference leads to a significant decrease in the amount of time that males spend signaling, according females a means by which to impose monogamy on their mates.

Starving the competition: a proximate cause of reproductive skew in burying beetles (Nicrophorus vespilloides)

Proximate mechanisms underlying reproductive skew are obscure in many animals that breed communally. Here, we address causes of reproductive skew in brood-parasitic associations of burying beetles (Nicrophorus vespilloides). Male and female burying beetles feed and defend their larvae on buried carcasses. When several females locate the same small carcass, they engage in violent physical altercations. The subordinate then acts as an intraspecific brood parasite, laying eggs, but not providing care. The dominant female largely monopolizes access to the carcass; she alone provides parental care and her share of the brood is much larger than the subordinates. On larger carcasses, subordinates have greater access to the carcass than on small ones, and reproductive skew is reduced. Differential fecundity, ovicide and larvicide have been suggested as causes of skew on small carcasses. Here, we report the results of the experiments pertaining to the first two of these potential mechanisms. Ovicide did not significantly contribute to reproductive skew on small carcasses, but differential fecundity did. Fecundity differences were due to dominance status, not body size per se. Fecundity differences disappeared when supplemental food was available, suggesting that reduced access to the carcass limits fecundity by causing nutritional deficiencies. Supplemental food prevented such nutritional deficiencies and allowed subordinates to produce as many eggs as dominants. Apparently, aggressive behaviour by dominants functions in the context of reproductive competition, limiting subordinate reproduction by preventing food intake on the carcass.

Linear models for assessing mechanisms of sperm competition: the trouble with transformations.

Abstract.— Although sperm competition is a pervasive selective force shaping the reproductive tactics of males, the mechanisms underlying different patterns of sperm precedence remain obscure. Parker et al. (1990) developed a series of linear models designed to identify two of the more basic mechanisms: sperm lotteries and sperm displacement; the models can be tested experimentally by manipulating the relative numbers of sperm transferred by rival males and determining the paternity of offspring. Here we show that tests of the model derived for sperm lotteries can result in misleading inferences about the underlying mechanism of sperm precedence because the required inverse transformations may lead to a violation of fundamental assumptions of linear regression. We show that this problem can be remedied by reformulating the model using the actual numbers of offspring sired by each male, and log‐transforming both sides of the resultant equation. Reassessment of data from a previous study (Sakaluk and Eggert 1996) using the corrected version of the model revealed that we should not have excluded a simple sperm lottery as a possible mechanism of sperm competition in decorated crickets, Gryllodes sigillatus.