Marlène Goubault

Volatile emission by contest losers revealed by real-time chemical analysis

Animal interactions often involve chemical exchange but simultaneous evaluation of chemistry and behaviour has been problematical. Here we report findings from a novel method, atmospheric pressure chemical ionization-mass spectrometry (APCI-MS) coupled with manipulation of molecular-mass achieved by rearing organisms on deuterium-enhanced nutrients. This allows real-time monitoring of the occurrence and quantity of volatile chemicals released by each of two interacting individuals, in tandem with behavioural observations. We apply these methods to female–female contests in the parasitoid wasp Goniozus legneri. We show that this species emits the spiroacetal 2-methyl-1,7-dioxaspiro[5.5]undecane. Chemical release is most common in more behaviourally aggressive contests, which occur when prior resource owners successfully resist take-over by similar-sized intruder females. Volatiles released during contests are always emitted by the loser. Aggression in contests is reduced after spiroacetal release. We suggest that the spiroacetal functions as a weapon of rearguard action. We anticipate that APCI-MS, which is rapid, non-intrusive and relatively inexpensive to operate, will be widely applied in studies linking chemistry and behaviour.

The importance of offspring value: maternal defence in parasitoid contests

Parent investment theory predicts that parents should adjust their investment in offspring defence according to offspring value. For instance, parents should protect older, more valuable, offspring more intensively than younger offspring against a given risk of mortality. The benefits of protection may, however, vary with offspring age and parents should thus behave according to the harm that offspring would suffer in the absence of parental defence. Prior studies on this topic have focused mainly on birds and mammals with data on other taxa largely lacking. We used the parasitoid wasp Goniozus nephantidis to investigate the effect of offspring developmental stage on maternal defence against infanticidal conspecific females. On finding a host caterpillar, a G. nephantidis female paralyses it and lays eggs onto it approximately 1 day later, then remains with the offspring during development until the offspring pupate. If the host is encountered by a second female, classic owner–intruder contests ensue, with the loser being aggressively excluded from the vicinity of the host. We tested the effect of brood developmental stage on contest outcomes and also assessed the consequences of offspring age for their survival in the event that an intruder female wins the contest. We found that females defend younger offspring more than older offspring. Explanations for this behaviour are discussed in terms of owner–intruder asymmetries in resource value and the ‘harm to offspring hypothesis’.

Animal Contests: Analysis of animal contest data

Summary In this chapter we outline and discuss statistical approaches to the analysis of contest data, with an emphasis on testing key predictions and assumptions of the theoretical models described in Chapters 2 and 3. We use examples from an array of animal taxa, including cnidarians, arthropods and chordates, to illustrate these approaches and also the commonality of many key aspects of contest interactions despite the differing life histories and morphologies (including weaponry) of these organisms. We first deal with the analysis of contest outcomes, a useful approach for determining which traits contribute to an individuals resource holding potential (RHP). Here we outline alternative statistical approaches that treat the outcome as either an explanatory (independent) variable or as the response (dependent) variable. In both cases, we treat a single contest as one ‘experimental unit’ and consider ways in which multiple measures taken from the same experimental unit should be accounted for in the analysis. Thus, we introduce paired and repeated measures approaches for contest data and also the calculation of composite measures. We then discuss more complex mixed models, which are particularly useful for dealing with multi-party contests when multiple individuals from the same group occur in more than one observation. Having established what factors influence RHP, one might then ask questions about the roles of information-gathering and decision-making during contests. These questions are prompted by the theoretical models of dyadic contests discussed in Chapters 1 and 2, and we consider the advantages and limitations of using analysis of contest duration to distinguish between ‘mutual-’ and ‘self-assessment’ type contests. An additional tool that we can use to address this question is the analysis of escalation and de-escalation patterns, and we thus shift the focus to within-contest behavioural changes.

Encountering competitors reduces clutch size and increases offspring size in a parasitoid with female-female fighting

Understanding the size of clutches produced by only one parent may require a game-theoretic approach: clutch size may affect offspring fitness in terms of future competitive ability. If larger clutches generate smaller offspring and larger adults are more successful in acquiring and retaining resources, clutch size optima should be reduced when the probability of future competitive encounters is higher. We test this using Goniozus nephantidis, a gregarious parasitoid wasp in which the assumption of size-dependent resource acquisition is met via female–female contests for hosts. As predicted, smaller clutches are produced by mothers experiencing competition, due to fewer eggs being matured and to a reduced proportion of matured eggs being laid. As assumed, smaller clutches generate fewer but larger offspring. We believe this is the first direct evidence for pre-ovipositional and game-theoretic clutch size adjustment in response to an intergenerational fitness effect when clutches are produced by a single individual.

Previous Experience and Contest Outcome: Winner Effects Persist in Absence of Evident Loser Effects in a Parasitoid Wasp

The experience of a previous conflict can affect animals’ performance during a later contest: a victory usually increases and a defeat usually decreases the probability of winning a subsequent conflict. These winner and loser effects could result from a reassessment by contestants of their perceived fighting abilities. Game-theoretic models based on this assumption predict that a loser effect can exist alone or in the presence of a winner effect, but a winner effect cannot persist alone, at least when contestants are young and without experience of contest. Moreover, when both effects coexist, the loser effect is expected to be of a greater magnitude and last longer than the winner effect. To date, these predictions have been supported by empirical evidence. Here we show for the first time that a winner effect can exist in the absence of any evident loser effect in a parasitoid wasp, Eupelmus vuilleti, when fighting for hosts. This finding consequently raises questions about the possible mechanisms involved and challenges the main assumption of previous theoretical models. We suggest an alternative explanation for the evolution of only winner effects that is based on the modification of contestants’ subjective value of the resource rather than on a reestimation of their fighting abilities.

The effect of direct interspecific competition on patch exploitation strategies in parasitoid wasps

The presence of competitors may affect the pay-off of individuals’ foraging strategies. They should therefore modify their resource exploitation decisions accordingly. In such a direct competition situation, theory predicts that individuals should stay longer on a resource patch than when foraging alone. However, models predicting patch residence time focus on intraspecific competition without agonistic interactions. Here, we investigate the patch use strategies of females of two parasitoid species, Eupelmus vuilleti and Dinarmus basalis, attacking the same host, Callosobruchus maculatus, knowing that D. basalis is more aggressive and can exclude E. vuilleti during pairwise contests for single hosts. Our results showed that time allocation and oviposition strategies differed in relation to the species and type of competition (i.e., presence/absence of competitor, simultaneous/sequential female introduction or resident/intruder female). Eupelmus vuilleti females tended to wait in the patch surroundings for D. basalis females’ departure to return and exploit hosts parasitized by the opponent (after destruction of her eggs). In contrast, D. basalis females tended to self-superparasitize and stay motionless near the hosts. After detecting an E. vuilleti female entering the patch, they attacked and chased her permanently from the patch. Females of both species spent less time in the patch when faced with a competitor than when alone. This study is the first to test the influence of direct interspecific competition and arrival order on patch exploitation strategies in parasitoid species, and highlights the necessity to include agonistic behaviors in theoretical models predicting optimal patch residence time in competitive situations.

A combined approach to heat stress effect on male fertility in Nasonia vitripennis: from the physiological consequences on spermatogenesis to the reproductive adjustment of females mated with stressed males.

In recent years, several studies have shown a decline in reproductive success in males in both humans and wildlife. Research on male fertility has largely focused on vertebrates, although invertebrates constitute the vast majority of terrestrial biodiversity. The reduction of their reproductive capacities due to environmental stresses can have strong negative ecological impacts, and also dramatic consequences on world food production if it affects the reproductive success of biological control agents, such as parasitic wasps used to control crop pests. Here Nasonia vitripennis, a parasitic wasp of various fly species, was studied to test the effects of 24h-heat stress applied during the first pupal stage on male fertility. Results showed that only primary spermatocytes were present at the first pupal stage in all cysts of the testes. Heat stress caused a delay in spermatogenesis during development and a significant decrease in sperm stock at emergence. Females mated with these heat-stressed males showed a reduce sperm count stored in their spermatheca. Females did not appear to distinguish heat-stressed from control males and did not remate more frequently to compensate for the lack of sperm transferred. As a result, females mated with heat-stressed males produced a suboptimal lifetime offspring sex ratio compared to those mated with control males. This could further impact the population dynamics of this species. N. vitripennis appears to be an interesting biological model to study the mechanisms of subfertility and its consequence on female reproductive strategies and provides new research perspectives in both invertebrates and vertebrates.

Strategic sperm allocation in response to perceived sperm competition risk in a lekking insect

Sperm competition has been demonstrated to be a strong selective force shaping male mating behaviours and ejaculate traits, particularly in highly polyandrous species. Its effect, however, is expected to be limited in lek mating systems, where female mating decisions appear relatively unconstrained and thus female remating rate is generally low. Hence, testing for male adaptations to sperm competition in lek mating species is of particular importance in order to understand whether and how sperm competition, as a selective force, may also have shaped male phenotype in such systems. Here, we investigated copulation behaviour and strategic sperm allocation in the lekking lesser wax moth, Achroia grisella, a well-established model for studying precopulatory sexual selection as males produce elaborate ultrasonic courtship songs targeted by female choice. Recent evidence suggests that females do sometimes remate, thus creating a selective potential for sperm competition. To test the hypothesis that sperm competition also selects for male adaptations in ejaculate traits in lek mating systems, we staged experimental matings after males had experienced either a competitive (one competitor present) or a noncompetitive (no competitor present) social environment during early adulthood. Males increased sperm transfer rate and thereby allocated higher proportions of available sperm reserves during virgin copulations when experiencing the elevated sperm competition risk environment. Our study provides the first evidence of strategic sperm allocation in relation to sperm competition risk in a lekking insect, demonstrating that sperm competition might represent an important selective force in such mating systems.

Modeling the evolution of winner and loser effects: A survey and prospectus.

The evolution of winner or loser effects-higher probabilities of winning after winning or of losing after losing-has received remarkably little attention from theoreticians, even though such effects are widespread across the animal kingdom. We review game-theoretic models that regard such winner and loser effects as outcomes of a strategic response. We show that these models have been well supported by the empirical literature in the past, but are not designed to address some recent observations. In the light of this recent progress on the empirical front, we identify factors that newer theory must be developed to explore.

Deuterium marking of chemical emissions: detectability and fitness consequences of a novel technique for insect behavioural studies

Behavioural studies often need to employ marking techniques to identify and track individuals. Marks ideally have no influence on behaviour or fitness of marked individuals or on the organisms with which they interact. A newly developed internal marking technique uses deuterium to manipulate the chemical emissions of bethylid wasps that are parasitoids of lepidopteran larvae. Wasps are deuterated indirectly by injecting deuterium oxide (heavy water) into the host on which they feed as larvae. Adult wasps emit a volatile chemical under stressful conditions and the emissions of deuterated and undeuterated wasps can be clearly and readily distinguished by measuring the ratio of deuterated/undeuterated molecules using real‐time mass spectrometry. Deuterated female wasps are, however, detectably disadvantaged in dyadic contests for host resources, a component of female fitness. In this study, we evaluated potential side‐effects of this deuterium marking technique on three further correlates of female Goniozus legneri Gordh (Hymenoptera: Bethylidae) fitness: longevity, lifetime fecundity, and offspring developmental mortality. We found that deuterium enrichment has no overall adverse effects on these parameters. We further showed that injecting hosts with different concentrations of deuterium results in distinguishably different volatile emissions of adult wasps, such that at least three individuals can be simultaneously identified by their chemical signatures. We also showed that deuterium marking remains effective beyond a wasps death: deuterated and undeuterated wasps remain readily distinguishable after storage. We discuss potential applications of deuterium marking for the laboratory and field study of chemical communication, dispersal, and mating structure in parasitoid wasps and other insects.