Ian C.W. Hardy

Sex ratios concepts and research methods

Preface and acknowledgements Part I. Sex Ratio Theory: 1. Models of sex ratio evolution Jon Seger and J. William Stubblefield 2. Optimal sex allocation: steps towards a mechanistic theory Ido R. Pen and Franz J. Weissing Part II. Statistical Analysis of Sex Ratio Data: 3. Statistical analysis of sex ratios: an introduction Kenneth Wilson and Ian C. W. Hardy 4. Analysis of sex ratios in social insects J. J. (Koos) Boomsma and Gosta Nachman 5. Analysis of sex ratio variances and consequences of sex allocation Sven Krackow, Evert Meelis and Ian C. W. Hardy 6. Comparative analysis of sex ratios Peter J. Mayhew and Ido R. Pen Part III. Genetics of Sex Ratio and Sex Determination: 7. Sex determining mechanisms in vertebrates Sarah B. M. Kraak and Ido R. Pen 8. Sex determination in invertebrates James M. Cook 9. Sex ratio distorters and their detection Richard Stouthamer, Gregory D. D. Hurst and J. A . J. (Hans) Breeuwer Part IV. Animal Sex Ratios Under Different Life-Histories: 10. Sex ratios of parasitic Hymenoptera with unusual life-histories Paul J. Ode and Martha S. Hunter 11. Sex ratio control in arrhenotkous and pseudo-arrhenotokous mites Maurice W. Sabelis, C. J. (Kees) Nagelkerke and J. A. J. (Hans) Breeuwer 12. Aphid sex ratios William A. Foster 13. Sex ratios in birds and mammals: can the hypotheses be disentangled Andrew Cockburn, Sarah Legge and Michael C. Double 14. Human sex ratios: adaptations and mechanisms, problems and prospects John Lazarus Part V. Sex ratios in other kingdoms: 15. Sex ratios of malaria parasites and related protozoa Andrew Read, Todd G. Smith , Sean Nee and Stuart A. West 16. Sex allocation in hermaphroditic plants Peter G. L. Klinkhamer and Tom J. de Jong 17. Sex ratios in dioecious plants Tom J. de Jong and Peter G. L. Klinkhamer Part VI. Applications of sex ratios: 18. Operational sex ratios and mating competition Charlotta Kvarnemo and Ingrid Ahnestjo 19. Using sex ratios: the past and the future Steven Hecht Orzack 20. Using sex ratios: why bother? Stuart A. West and Edward Allen Herre.

Sex ratio and mating structure in the parasitoid Hymenoptera

Assumptions about mating structure have been important in the development of sex ratio theory. Fishers theory of frequency dependent selection for sex ratio equality, and Charnovs theory of sex allocation when host quality varies, assume panmixis, whereas Hamiltons theory of local mate competition assumes mating occurs only at the natal patch. An important group of organisms in stimulating the development of sex ratio theory, and in its empirical testing, have been the parasitoid Hymenoptera. These exhibit many life history variations and possess a mechanism by which females can adaptively control their progeny sex ratio. Although many parasitoid species may conform closely to both the assumptions and predictions of the above models, others do not

Logistic analysis of animal contests

Animal behaviourists have long been interested in contests between pairs of animals competing for a resource. Contest outcomes are predicted to be influenced by factors such as the costs of contest behaviour and asymmetries in fighting ability, ownership status and resource value (e.g. Maynard Smith & Parker 1976; Hammerstein 1981; Leimar & Enquist 1984; Enquist & Leimar 1987). Animal contests are sometimes studied simply in order to test these predictions, but also within the context of numerous research topics (e.g. the evolution of clutch size, parental care, patch exploitation, mating strategies and sexual selection) to elucidate mechanisms by which individuals’ properties, such as body size, relate to reproductive success (e.g. Davies 1978; Crespi 1986; Hardy & Blackburn 1991; Rosenberg & Enquist 1991; Benton 1992; Petersen & Hardy 1996; Choe & Crespi 1997; Foster 1997; Schneider & Lubin 1997; Schuett 1997; Tobias 1997; Whitehouse 1997). Analyses of factors influencing the outcome of contests have employed a variety of statisical techniques, but many of these do not make the most efficient possible use of the data. Contests usually have unequivocal binary outcomes: a contestant either wins or loses (e.g. Petersen & Hardy 1996; Schuett 1997; Tobias 1997) and hence can be analysed using logistic regression, a type of generalized linear modelling specifically designed for the evaluation of binary data. Despite the suitability and wide accessibility of this technique, it has been relatively seldom used to analyse contest data. Amongst the numerous empirical studies of animal contests, we can find very few which have employed logistic analyses (Beaugrand et al. 1991; O’Neill et al. 1993; Petersen & Hardy 1996; Stamps & Krishnan 1997; Sneddon et al. 1997; Van Dyck et al. 1997; Neat et al. 1998; Field & Calbert, in press). Here we illustrate the workings and appropriateness of logistic regression, with particular reference to two recent studies of animal contests published in Animal Behaviour. Our primary aim is to provide worked examples and to encourage analysis of future data on contests using this technique. We hope this may facilitate comparisons among different studies, and also between empirical results and theoretical predictions.

Possible factors influencing vertebrate sex ratios: an introductory overview

Abstract Sex ratio theory is one of the more developed areas of evolutionary ecology, yet adaptive explanations for sex ratio variation among vertebrates are still the subject of much debate and uncertainty, partly because it can be difficult to infer process from pattern and adaptive and non-adaptive explanations may be confounding. In this introductory overview, I consider factors which may influence vertebrate sex ratios. I first consider ‘theoretical factors’, i.e. the adaptive value of the control of offspring sex ratio to parents. I then examine ‘empirical factors’ which influence parental ability to control offspring sex ratio, focusing on potential constraints imposed by sex determination mechanisms. Finally, I outline some of the evidence for the influence of these factors on sex ratios.

The importance of being larger : parasitoid intruder-owner contests and their implications for clutch size

One of the most important relationships in parasitoid behavioural ecology is that between adult female size and fitness. However, surprisingly few studies have attempted to assess size-related components of fitness beyond fecundity and longevity. This paper provides a more complete appraisal of the size–fitness relationship in the bethylid waspGoniozus nephantidisby investigating the influences of female size and ownership-status on the outcome of owner–intruder contests for paralysed hosts. Both influenced contest outcomes: larger females tended to win, but outcome was modified in favour of owners when intruders were only moderately larger. Aggressive behaviour during contests is described. The size–fitness relationship, based on fecundity and longevity as correlates of fitness, has previously been used to estimate optimal clutch size inG. nephantidisand a large discrepancy was found between observed and predicted clutch sizes. Here it is argued that because female fitness is determined, at least in part, by body size relative to competitors, an evolutionarily stable strategy approach is required to calculate optimal clutch size when host ownership contests occur. Clutch sizes could be influenced greatly and this provides a candidate explanation for the previously reported discrepancy.

Aggression by polyembryonic wasp soldiers correlates with kinship but not resource competition.

Kin selection theory predicts that individuals will show less aggression and more altruism towards relatives. However, recent theoretical developments suggest that with limited dispersal, competition between relatives can override the effects of relatedness. The predicted and opposing influences of relatedness and competition are difficult to approach experimentally because conditions that increase average relatedness among individuals also tend to increase competition. Polyembryonic wasps in the family Encyrtidae are parasites whose eggs undergo clonal division to produce large broods. These insects have also evolved a caste system: some embryos in a clone develop into reproductive larvae that mature into adults, whereas others develop into sterile soldier larvae that defend siblings from competitors. In a brood from a single egg, reproductive altruism by soldiers reflects clone-level allocation to defence at the cost of reproduction, with no conflict between individuals. When multiple eggs are laid into a host, inter-clone conflicts of interest arise. Here we report that soldier aggression in Copidosoma floridanum is inversely related to the genetic relatedness of competitors but shows no correlation with the level of resource competition.

Non-binomial sex allocation and brood sex ratio variances in the parasitoid Hymenoptera

Much theoretical and empirical work suggests that in many situations brood sex ratio can have important fitness consequences. The development of sex ratio theory and its empirical testing have, however, largely focussed on the mean sex ratio. Here 1 focus on the fitness consequences of sex ratio variance. Many species of parasitoid Hymenoptera lay broods of eggs and I use these as examples to show that variances either lesser or greater than binomial can be selected for and are observed. If broods of offspring are confined to each other for mating opportunities, low brood sex ratio variances (precise sex ratios) can be highly advantageous over binomial variances

Brood guarding in a bethylid wasp

Abstract. 1 Atypical of the parasitoid Hymenoptera, female Goniozus nephantidis Muesebeck remain with offspring until they pupate. This behaviour will only have evolved if the consequent reduction in fecundity is outweighed by fitness returns. 2 G.nephantidis competes for hosts with conspecific and non‐conspecific parasitoids. The effectiveness of G.nephantidis at deterring superparasitism and multiparasitism is tested. Brood survivorships were compared when G.nephantidis and Bracon hebetor Say intrude on hosts with the mother present and absent and with offspring at different developmental stages. 3 Laying by the intruder reduced brood survivorship. Guarding reduced oviposition on unparasitized hosts by intruding females, and prevented superparasitism of hosts with egg broods. Hosts with larval broods were rarely superparasitized. Multiparasitism was common except on hosts with guarded larval broods, but even here survivorship was reduced. 4 Competitive asymmetries determined the outcome of contests for possession of host resources. 5 The costs of remaining seem at least partially offset by the prevention of oviposition by competing parasitoids.

Spiteful soldiers and sex ratio conflict in polyembryonic parasitoid wasps

The existence of spiteful behaviors remains controversial. Spiteful behaviors are those that are harmful to both the actor and the recipient, and they represent one of the four fundamental types of social behavior (alongside selfishness, altruism, and mutual benefit). It has generally been assumed that the conditions required for spite to evolve are too restrictive, and so spite is unlikely to be important. This idea has been challenged in recent years, with the realization that localized competition can relax the conditions required for spite to evolve. Here we develop a theoretical model for a prime candidate for a spiteful behavior, the production of the sterile soldier caste in polyembryonic wasps. Our results show that (a) the biology of these soldiers is consistent with their main role being to mediate conflict over the sex ratio and not to defend against competitors and (b) greater conflict will occur in more outbred populations. We also show that the production of the sterile soldier caste can be classed as a spiteful behavior but that, to an extent, this is merely a semantic choice, and other interpretations such as altruism or indirect altruism are valid. However, the spite interpretation is useful in that it can lead to a more natural interpretation of relatedness and facilitate the classification of behaviors in a way that emphasizes biologically interesting differences that can be empirically tested.

Brood sex ratio variance, developmental mortality and virginity in a gregarious parasitoid wasp

Females of the parasitoid wasp Goniozus nephantidis paralyse host caterpillars and lay a clutch of up to 18 eggs onto the host integument. The known biology of G. nephantidis suggests that matings occur exclusively between siblings from the same brood. This leads to the prediction that brood sex ratios should be highly female-biased and have low variance. Sex ratios are indeed female-biased, with the mean proportion of males equal to 0.093. However, while sex ratio variance is significantly less than binomial, many broods contain no males at emergence. During development 28% of G. nephantidis offspring die. Male mortality offers a potential explanation for all-female (= virgin) broods. For the clutch sizes and mortality observed, theory predicts that <10% of females will emerge from all-female broods but the empirical value is much higher. The prediction that the prevalence of virginity decreases with increasing clutch size is, however, supported. We consider alternative explanations for the observed proportion of all-female broods, but this appears to be neither an artefact of the laboratory environment nor due to incorrect assumptions about G. nephantidis life history. Although its reproductive biology has been much investigated and its sex ratio matches some theoretical predictions, we conclude that a fuller understanding of G. nephantidis sex ratio requires a deeper knowledge of its field biology.