Arms races in evolution—An ESS to the opponent-independent costs game

The opponent-independent cost game is an animal contest in which the bigger and more heavily-armed opponent wins a disputed resource without significant fighting costs. A strategy is a choice of investment level in armament. Increasing armament is assumed to have fitness costs that are unrelated to contests; i.e. the cost of an individual's investment in arms is independent of the strategy played by an opponent.Previous work with this model showed that no ESS exists if a strategy prescribes an arms level exactly. This is equivalent to the notion that there is no environmental variation in the arms level attained by a given strategy. If environmental variation is introduced, a pure ESS can generally exist. A strategy is assumed to prescribe an exact investment cost, but this is translated into a probability distribution of arms levels attained, rather than an exact arms level. Increasing investment increases the mean of the arms level distribution. The ESS investment level depends both on how environmental factors distribute arms levels, and on the shape of the cost function (i.e. on the way that costs increase with investment); in some instances there is no ESS. Two types of model are investigated; in one fitness is additive (benefit-cost), in the other it is multiplicative (benefit × survivorship). The multiplicative model is likely to apply to the case where contests are between males for access to females. Here the ESS investment level (an ESS degree of risk that a male sustains as a result of armament) increases as fewer individuals guard the available resources. Thus sexual size dimorphism (male/female size) and relative male armament should increase as harem size increases. The ESS investment level will also be highest if most individuals are small and poorly armed, as would often be the case where size increases throughout life.The model can be applied to coevolutionary arms races between two classes of opponent, such as prey and predator, or parent and offspring. Here the ESS is likely to be a pair of ESS arms levels, one for each class of opponent.     

Interference competition in ant-lion (Macroleon quinquemaculatus) larvae

Abstract.

• 1 The effects of density, feeding regime, and body size on interference competition in the pit-digging larvae of the ant-lion Macroleon quinquemaculatus (Hagen) were investigated in laboratory experiments.
• 2 Competition had little effect on the pit size of winners but losers constructed much smaller pits than isolated larvae. Losers were less likely to dig or maintain pits and more likely to move than winners.
• 3 Competition was much stronger between well-fed larvae than between hungry ones, and well-fed competitors showed reduced growth rates. Well-fed larvae orientated themselves so that they could throw sand into their neighbour's pit whereas hungry larvae faced away from each other. Differences in hunger level reversed the competitive advantage of larger larvae only when individuals were of similar size.
• 4 Cannibalism was density-dependent and most frequent in hungry, similar-sized, larvae; the smaller larva was usually the victim.
• 5 Displays/challenges between larvae affected the distance between pits. Body size was the main determinant of contest outcome though pit ownership and hunger level also had an effect.

     

Do damselfly larvae recognize and differentially respond to distinct categories of macroinvertebrates?

Zygopteran larvae normally encounter other aquatic macroinvertebrates that are predators, competitors, and prey and should therefore demonstrate varied responses when faced with different categories of opponent. In a laboratory experiment individual final-instarIschnura posita (Hagen) larvae were observed in interactions with six categories of invertebrate opponents. The opponent categories were a nonconspecific damselfly and a small crayfish, which represented threatening opponents because they were larger than theI. posita subject larva, and a mayfly and a stonefly, which constituted nonthreatening opponents because they were smaller than the subject larva. The levels of threat posed by conspecific larvae of final and penultimate instar were inferred by comparison to the other opponent categories. Multivariate analysis showedI. posita's response differed between the two larger opponents, but responses were statistically indistinguishable between the two smaller opponents. Larvae retreated, moved around the stalk, and struck their opponents with their lamellae more often in the presence of a crayfish than the nonconspecific zygopteran. In contrast, they assumed an S-bend posture frequently with the zygopteran. Responses toward final-instar conspecifics differed from responses toward the larger opponents. Effectively, larvae wagged their abdomens only in the presence of final-instar conspecifics and retreated and moved around the stalk less frequently in these trials. Responses toward the smaller conspecifics differed from the responses to the small opponents. Larvae struck penultimate-instar conspecifics with their lamellae more frequently than the other small opponents. Our results suggest that larval zygopteran behaviors (such as S-bend and SCS) that have previously been described as intraspecific displays are of a more general nature and used toward a variety of opponents, whereas wag is unique to intraspecific interactions inI. posita.     

Ontogenetic Behaviour Changes in Larvae of the Damselfly Ischnura verticalis (Odonata: Coenagrionidae)

Larvae of most animals go through large changes in size. Because change in size can lead to changes in ability to gain food and in predation risk, changes in behaviour are predicted to reflect this. Models consider change in amounts of the same behaviour, but different selective pressures on different larval sizes may also lead to qualitative differences in behavioural repertoire. In the damselfly Ischnura verticalis we observed ontogenetic changes in behaviour under controlled laboratory conditions. We found that frequency and duration of feeding behaviour and behaviour related to activity were increased in larger instars. Larger larvae also had higher transition probabilities to the behaviour abdomen wave, straight abdomen raise, and rotate + head out. We also found that the behavioural repertoire of damselfly larvae changed with development: some behavioural patterns were performed almost exclusively by smaller larvae (F - 4 to F - 7) while others were performed almost exclusively by larger larvae (F - 3 to F - 0).     

Role assessment,reserve strategy,and acquisition of information in asymmetric animal conflicts

It was formerly argued that alternative evolutionarily stable strategies (ESSs) are possible for animal contests characterized by some asymmetry that can be perceived with perfect accuracy. Where roles A and B refer to the asymmetry between opponents, ESSs are: ‘fight when A, retreat when B’, and vice versa. Either can be an ESS, but only if the ‘reserve strategy’ (=what an animal does when it fights) is sufficiently damaging. We examine the ‘war of attrition’ (winner = opponent that persists longer). In a population at either ESS, reserve strategy is never normally shown; it is therefore subject to drift unless the selective action of rare individuals which break the convention is considered. These could arise either by mutation or by mistakes in role assessment. When mutations and mistakes simply specify that occasionally an animal fights when it ‘should’ retreat, selection adjusts reserve strategy to a level where only one ESS (the ‘commonsense’ ESS) is possible, if the asymmetry is relevant to payoff. Thus for asymmetries in fighting ability or resource value, the individual with the lower score will retreat. However, we are particularly concerned with cases where both payoff-relevant aspects (fighting ability and resource value) are asymmetric. If opponents sustain contest costs at rates K_A and K_B, and their resource values are V_A and V_B, an ‘optimal assessor’ strategy defined by the interaction between the two asymmetries, is a unique ESS. It obeys the rule ‘fight on estimating role A, where V_A/K_A>V_B/K_B; retreat in B’. If mistakes can occur in both roles, but are very rate, the ESS is not fundamentally altered though there will be infinitesimal tendencies for persisting in role B. Selection to improve assessment abilities intensifies as abilities improve, but is weak if roles A and B are rather similar. Over a range of similarity between roles, an ‘owner wins’ convention may be adopted if ownership correlates positively with role A and an individual cannot tell when it would otherwise pay him to break the convention. We also examine a contest in which information about roles can be acquired only during a contest itself, and at a cost. Much depends on the rate at which information is acquired relative to the rate at which costs are expended, and on whether contests normally escalate in intensity, remain at the same level, or de-escalate. Selection favours short contests when costs are high relative to resource value, where the outcome of a round contains much information about fighting ability, and where the actual disparity in fighting ability is large.     

Individual Recognition Affects Aggression and Dominance Relations in Rainbow Trout,Oncorhynchus mykiss

In this study, the hypothesis that individual recognition is used as a cue to reduce the cost of contesting resources in rainbow trout Oncorhynchus mykiss, was addressed. The predictions were that the second contest between familiar individuals should be settled with less aggression, and lower probability of status reversal, than a contest between strangers. To test these predictions, similar-sized juvenile rainbow trout were subjected to two dyadic dominance contests in one of four treatment groups: in half of the pairs, the initially subordinate and dominant individuals were staged against an unfamiliar opponent (of opposite rank to their own) in a second contest, while in the other the initial pair was reunited for the second contest. Further, in half of the pairs, contestants were separated for 3 d between contests, while in the other the second contest was staged immediately after the first. Levels of aggression in contests between familiar individuals were lower than in contests between strangers, which supports the hypothesis that individual recognition reduces aggression. Dominance status was reversed in 31% of the 62 tested pairs. Although the probability of status reversal was not significantly affected by individual recognition, the degree of change in competitive success appeared to be lower in familiar pairs. Separation interacted with familiarity so that the effect of individual recognition generally was less pronounced when pairs were separated between contests, suggesting that the ability to remember opponents is time/limited. In natural streams, salmonids may use individual recognition to reduce the cost of contesting resources within groups, to reduce aggression between territory neighbours and to distinguish ‘cheaters’ from honest signallers.     

Payoffs and strategies in territorial contests: ESS analyses of two ecotypes of the spiderAgelenopsis aperta

Summary Game-theoretic analyses were completed on the territorial contest behavior of two populations of a desert spider that exhibit markedly different levels of within-species competition. Numerical payoff matrices were constructed from field data collected on the behavior and demography of each population. Payoffs were expressed in terms of expected future egg production. Three behavior patterns that a spider might exhibit following assessment of its weight relative to that of its opponent and the value of the site were considered: withdraw, display, or escalate. The model predicts for the more harsh grassland habitat an evolutionarily stable strategy (ESS) that makes ownership decisive in settling contests between opponents with small weight differences, whereas it otherwise assigns victory to the heavier opponent. Whereas the empirical data collected for this grassland population closely approximates the predicted ESS, that for a population occupying a more favorable riparian habitat deviates significantly. The ESS prediction for this latter population is that an intruding spider will withdraw from a contest if it is similar in weight to the web-owner. Withdrawal is common in this population, but so are display and threat and these actions were not predicted. We hypothesize that gene flow from surrounding habitats is preventing the riparian population from completely adapting to its local environment.     

Assessment Strategies and the Effects of Fighting Experience on Future Contest Performance in the Green Anole (Anolis carolinensis)

Social experiences can be useful sources of information for animals charged with making fitness‐related decisions. Fighting experience can alter an animal's perception of its fighting ability possibly leading to changes in future contest decisions, which may increase/decrease their probability of winning future contests. Winner and loser effects have been revealed in a wide array of animals, but studies using reptilian models are rare. This study investigated the impact of fighting experience on future contest performance and outcome in the green anole lizard and investigated the assessment strategies used by anoles during contests of different intensities. To determine whether the green anole expresses winner or loser effects, focal animals engaged in a primary contest with a smaller (larger) opponent to gain a winning (losing) experience; opponent size asymmetries were a significant predictor of contest outcome. Focal individuals were isolated for 2 d before being given a secondary contest with a size‐matched, naïve opponent. We found no evidence of winner or loser effects 2 d following a previous contest. Although previous contest outcome did not dictate future contest success, dynamics of the previous contest did. Highly aggressive primary contest losers won a significant proportion of the secondary contests, while less aggressive losers were more apt to lose the secondary contest. Secondary contest success of prior winners was not influenced by earlier contest performance. Further analyses of contest dynamics reveal that individuals may use different assessment strategies depending on the intensity of the contest. Our results demonstrate that future contest success may be driven more by individual performance in a prior contest and less by prior contest outcome.     

Changes in feeding behaviour of the larvae of the damselfly Enallagma cyathigerum in response to stimuli from predators

1. The feeding rates, diet composition and diel periodicity in feeding activity among larvae of the damselfly Enallagma cyathigerum Charp., exposed to chemical, visual and mechanical stimuli from the predators Ranatra linearis (L.), Notonecta glauca L. and Rutilus rutilus (L.) were investigated.
2. In response to chemical cues produced by all the predator species, larvae reduced their rate of feeding significantly (especially on large, sedentary prey).
3. Small larvae reacted more strongly than large ones to the kairomone produced by N. glauca.
4. The fish kairomone induced a reduction in feeding activity during daylight hours only. This reaction was more intense than the reaction to non-chemical cues.
5. Observed predator-induced changes in diet composition, caused directly by reduced activity during feeding, are discussed as an antipredator behaviour of damselfly larvae.     

Information transfer during cricket frog contests

We investigated information transfer during vocal interactions between cricket frogs, Acris crepitans, with a specific focus on information about size and intention. In response to opponents, cricket frogs alter both temporal and spectral (frequency) aspects of their calls. Previous work suggests that males use dominant frequency, which is correlated with size, to provide information about fighting ability, and use temporal call characters, which are independent of size, to provide information about aggressive intent. We tested this hypothesis by investigating the relationship between call characters and contest behaviour. We presented a focal male with a simulated opponent, and categorized his behavioural response as attack, abandon or tolerate. We found that information about opponent size does not appear to influence a male's decision to fight, flee or tolerate an intruder. Whether or not males use the size information that is encoded in call frequency remains unclear. In contrast, changes in call frequency, which are not correlated with size, predicted the outcome of simulated contests, suggesting that male cricket frogs signal information about agonistic intent. Temporal call characters indicated whether or not a resident tolerated an opponent, but they did not predict contest outcome (attack versus abandon). Furthermore, the difference between the temporal call characters of a focal male and the simulated opponent predicted whether the resident tolerated the opponent. We suggest, therefore, that temporal call changes may be a cooperative signal designed to facilitate assessment of size through physical contact in wrestling bouts. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Ontogeny of feeding in two native and one alien fish species from the Murray-Darling Basin, Australia

Investigations into the feeding of the early stages of fishes can provide insights into processes influencing recruitment. In this study, we examined ontogenetic changes in morphology and feeding behaviour of two native Australian freshwater species, Murray cod, Maccullochella peelii peelii, and golden perch, Macquaria ambigua, and the alien species, common carp, Cyprinus carpio. Murray cod free embryos are large and well developed at the onset of feeding, whereas the other two species begin exogenous feeding much younger and are smaller and less-developed. Carp commence exogenous feeding 3 days earlier than golden perch, and show more advanced development of the eyes and ingestive apparatus. We conducted feeding experiments, presenting larvae of the three species with a standardised prey mix (comprising equal numbers of small calanoid copepods, large calanoid copepods, small Daphnia, and large Daphnia). Larvae of most tested ages and species showed a preference for mid-sized prey (300–500 μm wide). This was true even when their gapes substantially exceeded the largest prey offered. Daphnia were consumed more than similar-sized copepods. The results of this study suggest that survival through their larval period will be threatened in all three species if catchable prey <500 μm in width are not available throughout such time. They also suggest that interspecific competition for prey may occur, especially when larvae are very young. The precocious development of structures involved in feeding and the extended transition from endogenous to exogenous feeding of early carp larvae are likely to have contributed to the success of this species since its introduction to Australia.     

Male contest investment changes with male body size but not female quality in the spider Nephila clavipes

Animals use rules to adjust their level of investment in a contest. We evaluate male strategies during contests over females in the golden orb-web spider Nephila clavipes. We tested whether male behaviour changes with female value, and found that contests were similar in intensity and outcome whether the female was a juvenile or adult, virgin or non-virgin, or whether one male had invested sperm in the female. We found evidence that males use a self-assessment strategy when deciding to withdraw from a contest. Loser body size and contestant size difference were correlated with a higher frequency of contest escalation, and fights involving two large males were more likely to escalate than a fight in which one male was small. A multiple regression showed that loser body size had a stronger effect on contest escalation than contestant body size difference. More importantly, the size of the winning male had no effect on contest escalation, a key prediction of a self-assessment strategy. In N. clavipes, body size is the primary factor that determines the outcome of male contests, and males do not appear to assess their opponent or the quality of the resource when deciding to withdraw from the fight.     

Predator–prey interactions between larval damselflies and mining larvae of Glyptotendipes gripekoveni (Chironomidae): reduction in feeding activity as an induced defence

1. The feeding methods and intensity of predation by larvae of the damselfly Erythromma najas on leaf‐mining larvae of the chironomid Glyptotendipes gripekoveni were examined in artificial habitats differing in complexity. The experiments assessed the influence of chemical stimuli from the predator, light and the concentration of suspended food on the feeding activity of G. gripekoveni inside and outside of the mine.
2.  Erythromma najas preyed upon G. gripekoveni as the latter grazed outside mines. The intensity of this predation decreased significantly at night in a habitat offering alternative prey.
3. When the food concentration for the chironomid was high, it significantly reduced both filtering activity and activity outside mines in response to the kairomone produced by E. najas . Feeding activity did not change when food was scarce.
4. The induced reduction in filter‐feeding and deposit‐feeding activity probably reduced predator success by reducing the probability of long‐distance detection of a mine and location of the chironomid's hole.
5. The predator can detect and catch mining prey in either the light (visually) or dark (mechanically). This may explain the lack of diel periodicity in the chemically induced differences in prey activity.
6. Reduced feeding activity of mining larvae in the chemically simulated presence of a larval damselfly can be explained as an induced antipredator behaviour, illustrating the trade‐off between feeding demands and predation risk in a poorly known link of the littoral foodweb.     

Agonistic interactions between nymphs of Lycorma delicatula (Hemiptera: Fulgoridae)

The nymphs of Lycorma delicatula typically aggregate on tree branches for feeding. We studied agonistic interactions between nymphs in the field and investigated the effects of prior residence and body size on contest outcome. Resident nymphs maintained positions on tree branches and engaged in feeding, whereas intruding nymphs moved along tree branches. When an intruder approached, the resident typically raised its front legs as a threat posture. The intruder would either move away or palpate the resident, and the resident often lowered its body when palpated. The intruder would then attempt to shake the resident back and forth, with the number of attempts varying from one to several. The resident either maintained its position or fled. After a contest, the winner was determined to be the one occupying the contested position on the branch. The resident nymphs won 84.2% of the agonistic encounters, and the nymphs with larger body size won 63.2% of the agonistic encounters. Logistic regression analysis revealed that prior residence was a significant factor for the contest outcome, but body size was not. We discuss the hypotheses for the dominance of the residents in agonistic interactions between nymphs of L. delicatula.     

Startle durations reveal visual assessment abilities during contests between convict cichlids

In animal contests, the widespread ability of contestants to assess their opponents’ resource holding potential (RHP) relative to their own, termed mutual assessment, has recently been questioned. It is possible that each contestant may only have information about its own abilities or state, incurring costs up to a particular threshold then giving up, termed self-assessment. We used a technique that provides a measure of fight motivation to discriminate between different assessment models during aggressive encounters between male convict cichlids, Amatitlania nigrofasciata. A novel stimulus was applied to cause a startle response in one contestant of an aggressively interacting, size mis-matched pair, whereby the animal temporarily stops fighting. The time taken to resume the contest has been verified to provide a measure of the motivation to fight, from which it is possible to infer if any visual information concerning opponent asymmetries has been gathered. The data showed support for two differing types of assessment. There was some support for self-assessment, with startle duration being negatively related to own size, and, in a later trial series some support for an opponent only assessment strategy, with startle duration being positively related to opponent size. These results are consistent with individuals learning to use visual information about opponents when deprived of other sensory cues. Evidence within a trial supporting visual mutual assessment was lacking and possible reasons for this are discussed.     

Evolutionarily stable strategies for a finite population and a variable contest size

This paper presents a generalization of Maynard Smith's concept of an evolutionarily stable strategy (ESS) to cover the cases of a finite population and a variable contest size. Both equilibrium and stability conditions are analysed. The standard Maynard Smith ESS with an infinite population and a contest size of two (pairwise contests) is shown to be a special case of this generalized ESS. An important implication of the generalized ESS is that in finite populations the behaviour of an ESS player is "spiteful", in the sense that an ESS player acts not only to increase his payoff but also to decrease the payoffs of his competitors. The degree of this "spiteful" behaviour is shown to increase with a decrease in the population size, and so is most likely to be observed in small populations. The paper concludes with an extended example: a symmetric two-pure-strategies two-player game for a finite population. It is shown that a mixed strategy ESS is globally stable against invasion by any one type of mutant strategist. The condition for the start of simultaneous invasion by two types of mutant is also given.     

Lack of preference for low-predation-risk habitats in larval damselflies explained by costs of intraspecific interactions

Many studies indicate prey organisms select microhabitats with high structural complexity as a way of reducing risk of predation. We used laboratory experiments to show that damselfly larvae, Ischnura verticalis, suffer higher predation rates from pumpkinseed sunfish in low-density vegetation. However, larvae do not preferentially occupy microhabitats with high vegetation density in either the presence or absence of sunfish; when given a choice, the number of larvae per stem of vegetation was equal across all densities of vegetation. That larvae do not congregate in dense vegetation may reflect costs of aggressive interactions. Results from laboratory experiments indicated larval interactions increase conspicuous behaviours (most notably swimming) and consequently increase fish predation. A subsequent experiment indicated that frequency of larval interactions increases with increased vegetation density when number of larvae/stem is constant. Thus, larval microhabitat selection may reflect a trade-off between reduced risk of predation in areas of high vegetation density, caused by reduced fish foraging ability, and increased aggressive larval interactions, due to decreased proximity of larvae. Copyright 2000 The Association for the Study of Animal Behaviour.     

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.     

Behavioural and life history effects of predator diet cues during ontogeny in damselfly larvae

A central issue in predator–prey interactions is how predator associated chemical cues affect the behaviour and life history of prey. In this study, we investigated how growth and behaviour during ontogeny of a damselfly larva (Coenagrion hastulatum) in high and low food environments was affected by the diet of a predator (Aeshna juncea). We reared larvae in three different predator treatments; no predator, predator feeding on conspecifics and predator feeding on heterospecifics. We found that, independent of food availability, larvae displayed the strongest anti-predator behaviours where predators consumed prey conspecifics. Interestingly, the effect of predator diet on prey activity was only present early in ontogeny, whereas late in ontogeny no difference in prey activity between treatments could be found. In contrast, the significant effect of predator diet on prey spatial distribution was unaffected by time. Larval size was affected by both food availability and predator diet. Larvae reared in the high food treatment grew larger than larvae in the low food treatment. Mean larval size was smallest in the treatment where predators consumed prey conspecifics, intermediate where predators consumed heterospecifics and largest in the treatment without predators. The difference in mean larval size between treatments is probably an effect of reduced larval feeding, due to behavioural responses to chemical cues associated with predator diet. Our study suggests that anti-predator responses can be specific for certain stages in ontogeny. This finding shows the importance of considering where in its ontogeny a study organism is before results are interpreted and generalisations are made. Furthermore, this finding accentuates the importance of long-term studies and may have implications for how results generated by short-term studies can be used.     

Competitive displacement alters top-down effects on carbon dioxide concentrations in a freshwater ecosystem

Climate change and invasive species have the potential to alter species diversity, creating novel species interactions. Interspecific competition and facilitation between predators may either enhance or dampen trophic cascades, ultimately influencing total predator effects on communities and biogeochemical cycling of ecosystems. However, previous studies have only investigated the effects of a single predator species on CO₂ flux of aquatic ecosystems. In this study, we measured and compared the individual and joint effects of predatory damselfly larvae and diving beetles on total prey biomass, leaf litter processing, and dissolved CO₂ concentrations of experimental bromeliad ecosystems. Damselfly larvae created strong trophic cascades that reduced CO₂ concentrations by ~46 % relative to no-predator treatments. Conversely, the effects of diving beetles on prey biomass, leaf litter processing, and dissolved CO₂ were not statistically different to no-predator treatments. Relative to multiplicative null models, the presence of damselfly larvae and diving beetles together resulted in antagonistic relations that eliminated trophic cascades and top-down influences on CO₂ concentrations. Furthermore, we showed that the antagonistic interactions between predators occurred due to a tactile response that culminated in competitive displacement of damselfly larvae. Our results demonstrate that predator identity and predator–predator interactions can influence CO₂ concentrations of an aquatic ecosystem. We suggest that predator effects on CO₂ fluxes may depend on the particular predator species removed or added to the ecosystem and their interactions with other predators.