Ontogenetic constraints and diet shifts in Perch (Perca fluviatilis): mechanisms and consequences for intra‐cohort cannibalism

1. In many populations, sufficient size variation to allow for cannibalism may develop not only among age cohorts but also within them. Here, we used data on resource dynamics, consumer body size distribution and gape size limitation to unravel mechanisms promoting cannibalism within cohorts of young‐of‐the‐year (YOY) perch (Perca fluviatilis). 2. Perch are strongly gape limited when feeding on large zooplankton during early ontogeny. As a consequence, only initially large fish were able to shift to feeding on abundant large invertebrates, necessary to sustain fast growth. 3. We suggest that a combination of high initial size variation and exclusive access to resources for individuals with an initial size advantage is a prerequisite for the development of a size distribution sufficient for intra‐cohort cannibalism to occur. 4. During the time when cannibalism was observed, growth of the largest individuals in YOY perch cohorts was faster than that of smaller individuals. However, the energy gain from cannibalism did not increase growth rate enough to reach a size necessary to feed on more abundant size classes of victims, and therefore, the effect of cannibalism on overall cohort density was minor. 5. In addition to a high energy gain from cannibalism allowing for fast growth, strong resource limitation and slow growth rates of small individuals (i.e. potential victims) are a prerequisite not only for the development of intra‐cohort cannibalism but also for its persistence.     

Is Contest Experience a Trump Card? The Interaction of Residency Status, Experience, and Body Size on Fighting Success in Misumenoides formosipes (Araneae: Thomisidae)

Game theory models predict that individuals in contests adjust their strategy appropriately to the current value of the contested resource and the resource holding potential (RHP) of their opponent. In the current investigation, I examined interactively operating, multiple contest asymmetries on dyadic disputes for precopulatory guarding positions in the crab spider Misumenoides formosipes. In contests between equally sized adult males with no previous contest experience, residents had clear advantages in fighting success over intruders. Asymmetries in experience predicted outcome when tested against residency status, and experience operating in concert with residency status predicted resolution when tested against size asymmetries. Data from this investigation suggest that crab spiders learn strategies through experience rather than rely solely on the assessment of their opponent's RHP before determining contest effort.     

Intra‐cohort cannibalism and size bimodality: a balance between hatching synchrony and resource feedbacks

Cannibalistic interactions generally depend on the size relationship between cannibals and victims. In many populations, a large enough size variation to allow for cannibalism may not only develop among age‐cohorts but also within cohorts. We studied the implications of variation in hatching period length and initial cohort size for the emergence of cannibalism and bimodal size distributions within animal cohorts using a physiologically structured population model. We found that the development of size bimodality was critically dependent on hatching period length, victim density and the presence of a feedback via shared resources. Cannibals only gained enough energy from cannibalism to accelerate in growth when victim density was high relative to cannibal density at the onset of cannibalism. Furthermore, we found that the opportunity for early hatchers to initially feed on an unexploited resource increases the likelihood both for cannibalism to occur and size bimodality to develop. Once cannibals accelerated in growth relative to victims size bimodality, reduced victim numbers and relaxed resource competition resulted. Thus, in addition to that cannibals profited from cannibalism through energy extraction, their potential victims also benefited as the resource recovered due to cannibal thinning. To ensure recruitment success, it can be critical that a few individuals can accelerate in growth and reach a size large enough to escape size‐dependent predation and winter starvation. Hence, within‐cohort cannibalism may be a potentially important mechanism to explain recruitment variation especially for cannibalistic species in temperate climates with strong seasonality. However, the scope for size bimodality to develop as a result of cannibalism may be limited by low victim densities and size and food‐dependent growth rates.     

Pure Self‐Assessment of Size During Male–Male Contests in the Parasitoid Wasp Nasonia vitripennis

During contest competition, a competitor may persist in a given contest based on information regarding its own fighting ability (resource‐holding potential, RHP), or that of its opponent. Although a number of models formalize the ways in which competitors are hypothesized to use RHP‐related information to determine their persistence in contests, we focused on pure self‐assessment and mutual assessment models in this study. According to pure self‐assessment models, a competitor uses only information regarding its own RHP to determine its persistence in a contest. In contrast, according to mutual assessment models, persistence is based on information regarding a competitor's RHP relative to that of its opponent and therefore requires assessment between competitors. In this study, using size as a proxy for RHP, we tested whether the parasitoid wasp Nasonia vitripennis utilizes pure self‐assessment or mutual assessment during pairwise, male–male contests. When we examined competitors of varied sizes, we found that the losing male's size was positively related to contest duration, but the winning male's size was uncorrelated with contest duration. When we examined contests in which competitors were size‐matched, we found that the mean size of paired competitors was positively related to contest duration. These results suggest that male N. vitripennis engage in pure self‐assessment during contests.     

Optimal reproductive phenology under size‐dependent cannibalism

Intra‐cohort cannibalism is an example of a size‐mediated priority effect. If early life stages cannibalize slightly smaller individuals, then parents face a trade‐off between breeding at the best time for larval growth or development and predation risk from offspring born earlier. This game‐theoretic situation among parents may drive adaptive reproductive phenology toward earlier breeding. However, it is not straightforward to quantify how cannibalism affects seasonal egg fitness or to distinguish emergent breeding phenology from alternative adaptive drivers. Here, we devise an age‐structured game‐theoretic mathematical model to find evolutionary stable breeding phenologies. We predict how size‐dependent cannibalism acting on eggs, larvae, or both changes emergent breeding phenology and find that breeding under inter‐cohort cannibalism occurs earlier than the optimal match to environmental conditions. We show that emergent breeding phenology patterns at the level of the population are sensitive to the ontogeny of cannibalism, that is, which life stage is subject to cannibalism. This suggests that the nature of cannibalism among early life stages is a potential driver of the diversity of reproductive phenologies seen across taxa and may be a contributing factor in situations where breeding occurs earlier than expected from environmental conditions.     

Matter of Size or Matter of Residency Experience? Territorial Contest in a Green Hairstreak,Chrysozephyrus smaragdinus (Lepidoptera: Lycaenidae)

Territorial residents usually win asymmetrical owner‐intruder contests and a variety of hypotheses have been proposed to explain this phenomenon. In the butterfly Chrysozephyrus smaragdinus (Lepidoptera: Lycaenidae), male territorial residents defended their territories against intruders during numerous contests and kept them for many successive days. Field observations and experiments were conducted to examine the factors related to this superiority of residents. Forewing length did not differ between residents and intruders, suggesting that body size is not correlated with resource holding potential. Removal–replacement experiments demonstrated that residency did not serve as an arbitrary means for contest settlement, and did not support the recently presented alternative hypothesis that males with higher body temperature are more likely to win. New residents fought longer in defense of the territory as their residence duration in the territory increased. I discuss these results in light of game theory and suggest that the superiority of residents in C. smaragdinus may be based on the asymmetry of resource (territory) value for residents and intruders.     

Non‐sex‐biased Dominance in a Sexually Monomorphic Electric Fish: Fight Structure and Submissive Electric Signalling

Males and females commonly compete for limited resources. When interaction costs are similar for both sexes and there are no sexual differences in resource value estimation, a non‐sex‐biased dominance is expected. Moreover, only non‐sex‐biased assessment of contenders fighting ability (Resource Holding Potential, RHP) should influence contest decisions. To test these predictions, we evaluated non‐breeding agonistic intra‐ and intersexual dyadic interactions in the weakly electric fish, Gymnotus omarorum. During the non‐breeding season, resource value is not expected to depend on individuals’ reproductive status and should thus be equal for males and females. In addition, as G. omarorum presents no sexual differences in body size, interaction costs can be considered symmetric between sexes. We confirmed that body size differences, but not individuals’ gender, is the best predictor of dominance. We correlated RHP asymmetries with contest duration and evidenced that body size but not sex influences assessment in intrasexual and intersexual encounters. All dyads tested engaged in agonistic interactions (N = 33) in which a clear dominant emerged. The analysis of conflict phases evidenced the submissive role of electric displays. Electric organ discharge (EOD) interruptions appear early in the contest as an electric hiding attempt, whereas chirps are post‐resolution signals of subordinate status. Interestingly, the decision of interrupting the EOD was also influenced by RHP asymmetries, whereas chirping activity was influenced by the intensity of the attacks received. Our results confirm that body size is the best RHP proxy in non‐breeding intra‐ and intersexual contests of this monomorphic species and demonstrated a sequential pattern of submissive signalling by means of two different electric displays.     

All by myself? Meta‐analysis of animal contests shows stronger support for self than for mutual assessment models

Since the 1970's, models based on evolutionary game theory, such as war of attrition (WOA), energetic war of attrition (E‐WOA), cumulative assessment model (CAM) and sequential assessment model (SAM), have been widely applied to understand how animals settle contests. Despite the important theoretical advances provided by these models, empirical evidence indicates that rules adopted by animals to settle contests vary among species. This stimulated recent discussions about the generality and applicability of models of contest. A meta‐analysis may be helpful to answer questions such as: (i) is there a common contest rule to settle contests; (ii) do contest characteristics, such as the occurrence of physical contact during the fight, influence the use of specific contest rules; and (iii) is there a phylogenetic signal behind contest rules? To answer these questions, we gathered information on the relationship between contest duration and traits linked to contestants' resource holding potential (RHP) for randomly paired rivals and RHP‐matched rivals. We also gathered behavioural data about contest escalation and RHP asymmetry. In contests between randomly paired rivals, we found a positive relationship between contest duration and loser RHP but did not find any pattern for winners. We also found a low phylogenetic signal and a similar response for species that fight with and without physical contact. In RHP‐matched rivals, we found a positive relationship between contest duration and the mean RHP of the pair. Finally, we found a negative relation between contest escalation and RHP asymmetry, even though it was more variable than the other results. Our results thus indicate that rivals settle contests following the rules predicted by WOA and E‐WOA in most species. However, we also found inconsistencies between the behaviours exhibited during contests and the assumptions of WOA models in most species. We discuss additional (and relatively untested) theoretical possibilities that may be explored to resolve the existing inconsistencies.     

Contest dynamics for food and reproductive resources are defined by health condition in a dung beetle

Male dung beetles compete to obtain food and females, and early resource recognition and appropriation increase the probability of mating. The outcome of such encounters is expected to be defined by self‐ and the opponent’s health condition. In this study, we analyzed the effect of body condition and immune defense on the contest dynamics between males that rolled a food ball with a partner female (owner males) and intruder males of dung beetle Canthon cyanellus. Body condition was measured as body size, body dry mass, lipid mass, and muscle mass; immune defense was estimated via phenoloxidase activity. Owner males with higher lipid mass contacted the food ball significantly earlier than owner males with lower lipid mass. Individuals with lower phenoloxidase activity started to roll food balls earlier than individuals with higher phenoloxidase activity. Owner males that had higher body dry mass, compared to female partners, began to roll the food ball significantly earlier than male–female pairs with lower differences in dry mass. Heavier males won significantly more contests than lighter males. Our results suggest that the health condition is a key factor related to the dynamics and outcome of male–male contests for resources and females in C. cyanellus. Consequently, differences in individual condition are main determinants of contest outcomes in dung beetles.     

Do weaponless males of the hermit crab <Emphasis Type="Italic">Pagurus minutus</Emphasis> give up contests without escalation? Behavior of intruders that lack their major cheliped in male–male contests

In dyadic contests, theoretical studies have predicted that weaker contestants are less likely to engage in fights to minimize the cost of aggression. Since the major cheliped of decapod crustaceans is critically important as a weapon, contestants without a major cheliped should be more likely to give up the contests. We therefore examined whether loss of the major cheliped by the hermit crab Pagurus minutus would affect their decision to escalate male–male contests over guarded females. Intruders without a major cheliped showed no difference in the frequency of escalation compared with intact intruders, and the decision to give up was affected by the body size difference between the contestants. After escalation, compared with intact intruders, intruders without a major cheliped had significantly decreased success of takeover of a female from opponents, suggesting a strong disadvantage of losing their major cheliped. Although the decision of weaponless intruders to escalate seems irrational, several factors, such as poor accuracy of resource holding potential assessment, the influence of body size, and a high benefit to cost ratio of male–male contests, may have affected their behavior.     

Size‐induced phenotypic reaction norms in a parasitoid wasp: an examination of life‐history and behavioural traits

The amount of resources available during development often affects body size, causing phenotypic variation in life‐history traits and reproductive behaviours. However, past studies have seldom examined the reaction norms of both life‐history and behavioural traits versus body size. We measured the phenotypic plasticity of several life‐history (age‐specific egg load, egg size, longevity) and behavioural (oviposition rate, host marking rate, walking speed) traits of the egg parasitoid Telenomus podisi Ashmead (Hymenoptera: Scelionidae) in response to body size variation. We predicted that life‐history traits would show more evidence of size compensation than behavioural traits, resulting in fewer positively‐sloped size versus trait reaction norms among the former. As predicted by life‐history models, smaller wasps appear to shift resource allocation towards early‐life reproduction, having a similar egg load to large individuals 9 days after emergence. Surprisingly, longevity was unaffected by body size. However, egg size, the number of offspring produced during oviposition bouts, and the rate of subsequent egg synthesis were greater for larger individuals. In addition, as predicted, the reaction norms of behavioural traits versus body size were all positively sloped. Thus, despite possible adaptive compensatory plasticity of life‐history traits by small individuals, behavioural constraints directly related to body size would contribute to maintaining a positive size–fitness relationship.     

Subjective and objective components of resource value additively increase aggression in parasitoid contests

Two major categories of factors are predicted to influence behaviour in dyadic contests; differences in the abilities of the contestants to acquire and retain resources (resource holding potential), and the value of the contested resource (resource value, RV; which comprises objective and subjective components). Recent studies indicate that subjective components affect contest behaviour in several animal taxa but few have simultaneously investigated objective RV components. We find that both an objective (host size) and a subjective (contestant age) component of RV affect contest intensity in the parasitoid wasp Goniozus legneri. These additively influence aggressiveness, with a larger effect from the subjective component than the objective component. The greater influence of subjective RV adds weight to the recent surge of recognition of this RV component''s importance in contest behaviour.     

Effect of Population Density and Female Body Size on Number and Size of Offspring in a Species with Size‐Dependent Contests over Resources

When size‐dependent contests over resources influence reproductive success, the trade‐off between number and size of offspring depends on the frequency of contests. Under these circumstances, clutch size should decrease and offspring size should increase as contests become more frequent. We tested these predictions with the burying beetle Nicrophorus pustulatus through manipulation of rearing densities. Burying beetles reproduce on small vertebrate carcasses, a rare but high quality food source for the larvae. Large beetles are more likely to win contests over carcasses and gain exclusive access to a carcass. The winner of a contest kills eggs and larvae already present on a carcass. As a result of the rarity of carcasses, burying beetles are unlikely to breed more than once. As predicted, brood size of N. pustulatus decreased with increasing rearing density. Despite a negative correlation between brood size and larval mass, larval mass did not increase with increasing rearing density. This may be due to the special biology of N. pustulatus which can use snake eggs for reproduction. Potentially larger supply of resources and generally small population densities of N. pustulatus may weaken selection on body size and thus the correlation between brood size and larval mass. As size‐dependent constraints can limit reproductive phenotypes, we examined whether female size influenced reproductive phenotype. Small females produced larger broods with smaller, but more variable, offspring than large females. Mechanical constraints of egg size seem an unlikely explanation for the differences because burying beetles can compensate for small egg size through parental care. Energetic constraints may impact small females because body mass and brood size of small females decreased with increasing density. Yet, at all density levels small females produced larger, not smaller, broods than large females. The larger and more variable broods of small females seem to be in agreement with a bet‐hedging strategy.     

Sexual cannibalism in Nephila plumipes as a consequence of female life history strategies

The evolution of sexual cannibalism has been modelled as both an adaptive and nonadaptive female strategy. Recent evidence from several species suggests a connection between female foraging and sexual cannibalism, but the precise benefits for females have remained obscure. Here, we investigate the difference between cannibalistic and noncannibalistic female Nephila plumipes by removing the potential nutritional benefit of cannibalism. Courting and mating males that were killed by a female were immediately removed so that the female could not consume them. Nevertheless, cannibalistic females gained more mass from maturation to oviposition and produced larger first clutches than noncannibalistic females, although cannibalistic females matured at a smaller size and mass than noncannibalistic females. In juvenile instars, mass gain was generally smaller in females that moulted in a good condition but intermoult intervals were shorter. However, the time from maturity to oviposition was not shorter in females that matured in a good condition. Male behaviour did not differ according to the risk of cannibalism. We suggest that sexual cannibalism in N. plumipes is a side‐effect of an increased foraging vigour of females that matured at a smaller size and body mass. Selection pressure on males to avoid cannibalism may be weak because of limited mating opportunities.     

Physiologically structured models – from versatile technique to ecological theory

A ubiquitous feature of natural communities is the variation in size that can be observed between organisms, a variation that to a substantial degree is intraspecific. Size variation within species by necessity implies that ecological interactions vary both in intensity and type over the life cycle of an individual. Physiologically structured population models (PSPMs) constitute a modelling approach especially designed to analyse these size‐dependent interactions as they explicitly link individual level processes such as consumption and growth to population dynamics. We discuss two cases where PSPMs have been used to analyse the dynamics of size‐structured populations. In the first case, a model of a size‐structured consumer population feeding on a non‐structured prey was successful in predicting both qualitative (mechanisms) and quantitative (individual growth, survival, cycle amplitude) aspects of the population dynamics of a planktivorous fish population. We conclude that single generation cycles as a result of intercohort competition is a general outcome of size‐structured consumer–resource interactions. In the second case, involving both cohort competition and cannibalism, we show that PSPMs may predict double asymptotic growth trajectories with individuals ending up as giants. These growth trajectories, which have also been observed in field data, could not be predicted from individual level information, but are emergent properties of the population feedback on individual processes. In contrast to the size‐structured consumer–resource model, the dynamics in this case cannot be reduced to simpler lumped stage‐based models, but can only be analysed within the domain of PSPMs. Parameter values used in PSPMs adhere to the individual level and are derived independently from the system at focus, whereas model predictions involve both population level processes and individual level processes under conditions of population feedback. This leads to an increased ability to test model predictions but also to a larger set of variables that is predicted at both the individual and population level. The results turn out to be relatively robust to specific model assumptions and thus render a higher degree of generality than purely individual‐based models. At the same time, PSPMs offer a much higher degree of realism, precision and testing ability than lumped stage‐based or non‐structured models. The results of our analyses so far suggest that also in more complex species configurations only a limited set of mechanisms determines the dynamics of PSPMs. We therefore conclude that there is a high potential for developing an individual‐based, size‐dependent community theory using PSPMs.     

Declining body size in an endangered large mammal

Body size affects almost every aspect of the biology of a species. According to the ‘resource rule’, decreasing resource availability (e.g. prey density) will lead to a reduction in body size or, alternatively, a decline in mass‐independent energy expenditure. In the present study, we provide a test of this hypothesis, assessing the effect of significantly decreasing prey density on endangered African wild dog (Lycaon pictus) body size and energy expenditure over a 20‐year period. As predicted from the ‘resource rule’, decreasing resource availability resulted in energetic re‐allocation: wild dogs' body size decreased significantly (both shorter and slimmer), whereas our fitness‐related measure of energy expenditure (i.e. litter size) remained constant over time. A phenotypic change of up to 17% within 20 years, as found in the present study, appears to be unprecedented in a nonharvested large mammal, thus advancing the emerging field of eco‐evolutionary dynamics. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 8–12.     

The ontogeny of territoriality during maturation

Territoriality drives the evolution of many mating systems, yet has remained an extremely difficult trait to measure in the wild. Classic studies rely on the theoretical framework of resource holding potential (RHP) as a predictor of success in territory acquisition. However, mounting evidence suggests that an individual's RHP may change over short time scales. Previous studies suggest that RHP is best understood by considering two categories of territoriality, resource defending and resource usurping potential (RDP and RUP, respectively). In a population of the side-blotched lizard, Uta stansburiana, blue-throated males defend territories near their natal site (RDP) while mature orange-throated males use their RUP to sequester high quality territories from defending territorial males. We tested differences in territoriality by releasing pairs of maturing male lizards onto experimentally altered territories that had improved thermal qualities owing to the addition of rock piles. Dyads of males competed for these thermal resources and the females that were released on rock piles. Early in the season, when throat colors were not yet fully expressed, large male body size predicted contest victories irrespective of throat color. This pattern changed however, with the onset of the breeding season and maturation of throat color. Orange males tended to usurp territories from blue males within 2 weeks of contest initiation. Large male body size still influenced these contests, but after one more week, throat color was the sole factor explaining variance in territory ownership. We demonstrate the ontogeny of territoriality relating to body size and throat color during maturation, and suggest a novel approach to assessing territoriality and aggression in the wild.     

Female and male Texas cichlids (Herichthys cyanoguttatum) do not fight by the same rules

Selection usually acts differently on males and females duringintrasexual competition for resources and/or mates. Nevertheless,agonistic behavior has been examined both theoretically andempirically mostly in males. Our research questions whethermales and females follow the same rules of engagement in intrasexualcontests as predicted by the sequential assessment model (SAM).The SAM predicts negative correlations between contest intensityand duration and the magnitude of asymmetry in resource holdingpower (RHP) between the contestants, such that the most escalatedcontests are those between similarly endowed individuals. Westaged male and female intrasexual contests with varying degreesof body size asymmetry under a round robin design using themonogamous Texas cichlid fish (Herichthys cyanoguttatum) asa study case. We used Mantel's matrix analysis to compare howthe behavioral content, duration, structure, and outcome ofmale and female contests were affected by the relative bodysize of the contestants. In the case of males, relative sizein each contest predicted outcome, duration, and frequency ofconventional and escalated behaviors according to prevailingtheory. Female contest structure and outcome, however, werenot predicted by the relative size of contestants. We discussour results in terms of other asymmetries that might be importantin structuring female contests, and we propose potential approachesto study female–female aggression.     

Abundance–body mass relationships in size-structured food webs

In communities sharing a common energy source, the energetic equivalence hypothesis predicts that numerical abundance (N) scales with body mass (M) as M^?0.75. However, in size‐structured food webs all individuals do not share a common energy source, and the energy available (E) to larger individuals is constrained by inefficient energy transfer through the food chains that support them. This is expected to lead to steeper scalings of N with M. Here, we formalize and test an existing model for predicting abundance–body mass scaling, where the decline in E with M is calculated from the mean predator–prey body mass ratio (from size‐based nitrogen stable isotope analysis) and trophic transfer efficiency. We show that the steep predicted scalings of abundance and body mass (N scales as M^?1.2, B scales as M^?0.2) in a marine food web are consistent with empirical estimates and can be attributed to the small predator–prey body mass ratio (106 : 1). As a previous study has shown that environmental stability may favour low predator–prey mass ratios and long food chains, we predict that steeper abundance–body mass relationships will be found in more stable environments.     

Boldness as a consistent personality trait in the noble crayfish, <Emphasis Type="Italic">Astacus astacus</Emphasis>

Consistent individual differences in behavioural responses to perceived predation risk may have extensive ecological and evolutionary implications. We studied the repeatability of boldness across time and its relation to resource holding potential in the noble crayfish, Astacus astacus L., using predator-naïve immature individuals. We followed individual’s shelter use both with and without exposure to the chemical and physical cues of predators, and with and without the presence of a conspecific. In addition, we examined if armament, i.e. relative chelae size, would be correlated with individual differences in behaviour. Individuals showed repeatable behaviours across time and context. Individuals that occupied the shelter in competitive dyadic tests also spent more time in the shelter during individual control observations, suggesting that boldness is a personality trait that does not necessarily relate positively to high resource holding potential in the noble crayfish. The relative size of chelae did not correlate with any of the measured behavioural variables. Our results suggest that boldness can be considered as individually consistent and ecologically important personality trait in the noble crayfish.