The evolution of respect for property

Although possession is ‘nine‐tenths of the law’, respect for ownership is widespread in the animal kingdom even without third‐party enforcement. Thus, the first individuals to find objects are frequently left unchallenged by potential competitors and tend to win contests when disputes arise. Game theory has shown that respect for ownership (‘Bourgeois’ behaviour) can arise as an arbitrary convention to avoid costly disputes. However, the same theory predicts that a paradoxical respect for lack of ownership (‘anti‐Bourgeois’ behaviour) can evolve under the same conditions and in some cases is the only stable outcome. Despite these predictions, anti‐Bourgeois behaviour is rare in nature, whereas respect for ownership is frequently not absolute. Here, we review extensions of the classic models involving repeated interactions, confusion over roles, strategic coordination of behaviour (‘secret handshakes’), owner–intruder asymmetries and continuous control of fighting investment. Confusion over roles and owner–intruder asymmetries in fighting ability may explain why respect for ownership is often partial. Moreover, although most model extensions facilitate the evolution of Bourgeois‐like behaviour, secret handshakes and continuous control of fighting investment render the alternative anti‐Bourgeois convention unstable. We develop these insights to highlight several key areas for future investigation.     

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.     

Does prior residency interact with loss? A study of male–male contests in the hermit crab Pagurus minutus

Residency is an important predictor of success in contests with ownership asymmetries. Residency often can interact with a winning experience. However, given that some residents lose a contest even when showing an ownership advantage and that the process leading to loss often determines the loser's subsequent success, prior ownership might also interact with a loss. Here, we staged experimental contests between males of the hermit crab Pagurus minutus with a similar-sized weapon (i.e., cheliped) to examine this possibility. Male–male contests in this species occur between a solitary intruder and an owner guarding a mature female. We evaluated (a) whether resource ownership and female value affect the contest outcome and (b) whether the probability of winning after losing differs depending on the initial role of the loser (i.e., owner or intruder) by using precopulatory guarding pairs of P. minutus collected from the field. In the first fighting trial, we found an ownership advantage and increasing owner success as the body size of his partner increased. Although some owners lost the fight, in contrast to our prediction, the frequency of losing in the second fighting trial did not differ between prior owners and prior intruders. Because losers from the first fighting trial of male–male contests have no female regardless of their initial role, this shared solitary status might be related to the lack of difference in success in the second fighting trial. Moreover, unlike in other animals, resident status might not always assure greater fighting ability in P. minutus males because guarding Pagurus males can avoid male–male contests by climbing up objects in the field. Losers in the first trial, therefore, may have been weaker contestants based on traits other than size, regardless of whether they were owners or intruders.     

Effects of female body size and age and male mating status on male combat in Anastatus disparis (Hymenoptera: Eupelmidae)

1. Aggressive behaviour is widely exhibited by animals to acquire important resources and usually shows a dangerous or nondangerous pattern. Dangerous fighting patterns are usually characterized by fights ending with contestants being severely injured or killed. Resource value is an important nonstrategic factor influencing fighting behaviour. Studies of many species addressing nondangerous fighting behaviour have shown that when resource values change, organisms usually adjust their fighting behaviour accordingly. Only a few species show dangerous fighting patterns. Thus, few relevant studies have addressed how variation in resource value affects aggression with a dangerous fighting pattern. 2. Here, an egg parasitoid wasp, Anastatus disparis, which exhibits a dangerous fighting pattern to acquire mating opportunities, was used as an experimental model to study the adjustment of fighting behaviour resulting from a change in resource value. 3. Our results show that the female properties of body size and age affect their objective resource value and that males increased their fighting intensity for relatively large and young females. However, male mating status in A. disparis may not influence the subjective value of mate resources, and fighting intensity did not significantly differ between mated and virgin males. In addition, the number of times a male had previously mated had no significant effect. These results suggest that mating opportunities are important for both virgin and mated males, resulting in neither of them showing any adjustment in fighting for mating opportunities. 4. Generally, A. disparis males with extreme fighting patterns adjust their fighting behaviour according to the variation in resource value, which avoids the meaningless costs of injury and death.     

How costly is the honest signaling of need?

ESS models of biological signaling have shown that costly signals can provide honest information. In the context of parent-offspring conflict over the allocation of resources by parents to their young, the theory explains costly offspring solicitation behavior as an accurate signal of offspring need to parents who cannot assess offspring condition directly. In this paper, we provide a simple but general characterization of the honest signaling of need in models of parent-offspring conflict: the offspring's signaling cost is proportional to the parent's fitness loss from satisfying the offspring's resource requirement. The factor of proportionality is given by a measure of the extent of parent-offspring conflict that depends only on coefficients of relatedness. These results hold for interbrood conflict with uniparental investment even if the relationship between offspring condition and resource requirement is not monotonic, and extend to cases of biparental care, uncertainty concerning the parent's condition, and intra-brood conflict. Copyright 1999 Academic Press.     

Ownership-dependent mating tactics of minor males of the beetle <Emphasis Type="Italic">Librodor japonicus</Emphasis> (Nitidulidae) with intra-sexual dimorphism of mandibles

Intra-sexual dimorphism is found in the weapons of many male beetles. Different behavioral tactics to access females between major and minor males, which adopt fighting and alternative tactics, respectively, are thought to maintain the male dimorphism. In these species major males have enlarged weapons that they use in fights with rival males. Minor males also have small weapons in some of these species, and it is unclear why these males possess weapons. We examined the hypothesis that minor males might adopt a fighting tactic when their status was relatively high in comparison with that of other males (e.g., ownership of a territory). We observed the behavioral tactics of major and minor males of the beetle Librodor japonicus, whose males have a dimorphism of their mandibles. Major males fought for resources, whereas minor males adopted two status-dependent tactics, fighting and sneaking, to access females, depending on their ownership of a sap site. We suggest that ownership status-dependent mating tactics in minor males may maintain the intra-sexual dimorphism in this beetle.     

Landmarks in territory partitioning: a strategically stable convention?

A convention is a rule based on arbitrary cues that allows quick resolution of potentially protracted disputes. A familiar example is the Bourgeois strategy, in which the second of two animals to discover a resource yields it to the first, even though it may be stronger than its opponent. Here we develop a game-theoretic model to show that neighbors with imperfect information about one another's fighting abilities can be favored to accept a landmark as the designator of a territory boundary, even when the resulting territory is smaller than the one that would have been won through fighting. Thus, the use of landmarks or other mutually obvious solutions can serve as a convention for territory partitioning. For a distribution of fighting ability with low variance and high skew, there is a remarkably high probability that an animal will accept a smaller territory than it would have won through fighting. The analysis provides a possible explanation for the observed use of landmarks as boundary markers by territorial animals in a variety of taxa, including birds, fish, insects, and mammals. The analysis also suggests why territory boundaries are stable, once established, despite changes in characteristics of the residents or the environment.     

The evolution of aggression: can selection generate variability?

Three models--the war of attrition, the size game and the badges of dominance game--are described, in which natural selection can maintain genetic variability for aggression. The models differ in whether or not the traits that settle contests are costly in contexts other than fighting, and also in whether signals are used. It is concluded that contests will be settled by non-costly traits only if the value of the contested resource is small relative to the cost of fighting, and that 'honest' signalling of aggressiveness is stable only if individuals giving signals that are inconsistent with their behaviour suffer costs. The literature on 'badges of dominance' in birds is reviewed. New data on great tits, greenfinches and corn buntings show that there is plumage variability within age and sex that sometimes serves to settle contests, and that, in the first two species but not the third, the badges are uncorrelated with size, and settle contests only over trivial resources.     

Forehead Patch Size Predicts the Outcome of Male–Male Competition in the Pied Flycatcher

Males of many animal species express ornaments that affect their reproduction opportunities through male–male competition or female mate choice. Such ornaments can, for example, inform conspecifics about the fighting ability, condition or territory ownership of the bearer. Pied flycatcher (Ficedula hypoleuca) males have a conspicuous white forehead patch that varies greatly in size. We examined whether the white forehead patch is an intrasexually selected trait in a Finnish population. We artificially manipulated forehead patch size to represent two naturally occurring extremes and competed males against each other in the presence of a female. Males with a large forehead patch were more aggressive than males with a small patch, whereas the original patch size of the male had no influence on aggression. Neither manipulated nor original patch size influenced resource dominance (over female or nest box). These results indicate that forehead patch signals fighting ability of the bearer in the pied flycatcher. The next step is to find out what kind of costs may maintain the honesty of this signal.     

Hamilton's rule and kin competition: the Kipsigis case

Evolutionary studies of human behavior have emphasized the importance of kin selection in explaining social institutions and fitness outcomes. Our relatives can nevertheless be competitors as well as sources of altruism. This is particularly likely when there is local competition over resources, where conflict can lead to strife among nondispersing relatives, reducing or even negating the effects of relatedness on promoting altruism. Here, I present demographic data on a land-limited human population, utilizing large within-population variation in land ownership to determine the interactions between local resource competition and the benefits of kin in enhancing child survival, a key component of fitness in this population. As predicted, wealth affects the extent of kin altruism, in that paternal relatives (specifically father's brothers) appear to buffer young children from mortality much more effectively in rich than in poor households. This interaction effect is interpreted as evidence that the extent of nepotism among humans depends critically on resource availability. Further unanticipated evidence that maternal kin play a role in buffering children from mortality in situations where paternal kin control few resources speaks to the important role that specific local circumstance plays in shaping kin contributions to child welfare.     

Mortal combat: an apparent intraspecific killing by a male Black‐capped Chickadee

ABSTRACT Much theory about communication in aggressive contexts hinges on the assumption that injury or death is a potential outcome of fighting. However, published accounts of intraspecific killings during competitive interactions between songbirds are extremely rare. Here, we report an observation where one adult Black‐capped Chickadee (Poecile atricapillus) appeared to kill another. The two opponents were high‐ranking males that had been in the same winter flock, consistent with the prediction that mortal combat is most likely between evenly matched individuals. The interaction occurred when birds were defending territories and had begun nesting activities. A female that had previously been paired with the victim was also present during the interaction. Our observations suggest that the male chickadees were fighting for access to territorial resources or to a female, and indicate that aggressive interactions between songbirds can be costly and potentially fatal.     

Sexual selection constrained by life history in a butterfly

Trait evolution via sexual selection has traditionally been viewed as isolated from life-history constraints. Recent theoretical treatments, however, predict that costly sexually selected characters should be mediated by the same allocational trade-offs that apply to more conventional aspects of reproductive investment. Participation in risky competitive behaviours, for example male-male combat, should therefore increase as the opportunity for future reproduction declines. However, the demonstration of such trade-offs has proven to be elusive due to concomitant age-based variation in the physical determinants of fighting 'ability'. Here, I exploit the unique nature of a butterfly contest system to provide compelling evidence for lifetime partitioning of risky and aggressive sexually selected behaviours. I show that male Hypolimnas bolina become more willing to persist in contests over mating territories, and more generally accepting of injury risks, as they age. Contest persistence in this species is not mediated simply by physical condition, and I experimentally isolate the effect of ageing per se from resource ownership and previous contest experience. These results demonstrate how sexually selected behaviours can be ultimately mediated by a shifting trade-off between contemporary reproductive effort and future opportunities.     

The importance of food supply in high‐productivity ecosystems: Short‐term experimental tests with small rodents

Food availability is considered to be a primary factor affecting animal populations, yet few experimental tests have been performed to evaluate its actual importance in species‐rich ecosystems such as rainforests. It has been suggested that in such systems certain plant species may act as “keystone” resources for animals, but the importance of presumed keystone resources for populations has not been quantified experimentally. Using complementary seed removal and seed‐addition experiments, we determined how the supply of a presumed keystone resource, seeds of Araucaria angustifolia, affects short‐term demography of their main consumer group (small rodents) in a biodiversity hotspot, the Brazilian Atlantic Forest. We hypothesized that (i) the harvest of A. angustifolia seeds by human populations has negative impacts on rodents, and (ii) these seeds are a limiting resource for rodent populations. To test these hypotheses, we monitored populations of two species of numerically dominant rodents (Delomys dorsalis and Akodon montensis) within replicated control‐experimental plots. Manipulations of seed supply over 2 years had little effect on population size, body condition, survival, or reproduction of the two rodents, suggesting that, in the short‐term (within one generation), their populations are not food limited in Araucaria forests. Despite apparently having all the characteristics of a keystone resource, as currently defined in the literature, the seeds of A. angustifolia had limited influence on the short‐term demography of their main consumer group. In situations where purported keystone resources are seasonally abundant, their actual importance may be lower than generally assumed, and these resources then may have only localized and temporary effects on consumer populations.     

Evolutionary routes to stable ownership

Ownership can evolve in potentially any species. Drawing on insights from across disciplines, we distinguish between possession and ownership and present species‐neutral criteria for ownership, defined as respect for possession. We use a variant of the tug‐of‐war evolutionary game to demonstrate how ownership can evolve in the form of a new, biologically realistic strategy, Restraint With Retaliation (RWR). In our game, resource holding potential (RHP) is assumed to be equal between interactants, and resource holding asymmetry determines whether ownership is adaptive. RWR will be evolutionarily stable when the ratio of resource holdings between interactants is relatively low, but not when this ratio is sufficiently high. We offer RWR as one evolutionary route to ownership among many, and discuss how ownership unites previously described behavioural phenomena across taxa. We propose that some but not all mechanisms of territory formation and maintenance can be considered ownership, and show that territories are not the only resources that can be owned. We argue that ownership can be a powerful cooperative solution to tragedies of the commons and problems of collective action throughout the biological world. We advance recent scholarship that has begun to investigate the biological importance of ownership, and we call for a comprehensive account of its evolutionary logic and taxonomic distribution. We propose that ownership should be considered a fundamental, unifying biological phenomenon.     

Resource value and the context dependence of receiver behaviour

Many animals use signals of fighting ability to minimize the costs of competition. Theory predicts that signals must be costly to remain reliable indicators of their bearer's abilities, but many signals of fighting ability lack obvious developmental costs. Instead, receivers are thought to maintain signal accuracy by behaving aggressively towards individuals with inaccurate signals (i.e. social costs). Models predict that the evolutionary stability of social cost signals depends on receivers trusting signals in certain contexts and testing signal accuracy in other contexts. Here, I use the signals of agonistic ability in Polistes dominulus wasps to provide the first experimental evidence that receiver responses to social cost signals are context dependent. During contests over low-value resources, wasps trust signals; they avoid patches of food guarded by rivals with elaborate signals. As the value of the resource increases, wasps become more likely to test signal accuracy. In fact, receivers challenge guards regardless of their signal phenotype when the resource is sufficiently valuable. Context-dependent receiver responses are likely to be an important behavioural mechanism underlying the evolution of social costs, as context-dependent responses allow receivers to minimize the costs of conflict while also ensuring signal accuracy.     

Alternatives to Lotka-Volterra competition: Models of intermediate complexity

A family of one-level differential-equation competition models in which two populations are limited by the energy flowing into the system generates the following results. For competitors on the same and only resource: 1) Purely exploitative competition leads to exclusion; which species wins depends on relative abilities to appropriate and extract energy from the resource, and the relative death and maintenance rates. 2) If conspecific interference (e.g., deaths or energy loss from fighting, cannibalism, or display) is sufficiently high relative to abilities to exploit the common resource, competition for the same resource can lead to coexistence. 3) If heterospecific interference is sufficiently high relative to abilities to exploit the common resource, competition for the same resource can lead to a priority effect, in which the outcome depends on initial population sizes. 4) Depending on whether situation (2) or (3) prevails, an increase in the amount of the common resource can convert an outcome in which one species always wins into one giving coexistence (2) or a priority effect (3). 5) If species are similar to one another in their abilities to appropriate and extract energy from the common resource and show reciprocity in intererence costs, competition can have multiple outcomes; either one species wins or the species coexist, depending on initial values.For competition on the same resource, but with each species monopolizing an exclusive resource as well: 1) Purely exploitative competition always leads to a unique point coexistence. 2) If interference is added to the system described in (1), two points of coexistence, separated by a saddle (an “unstable equilibrium”) are possible. This is favored by a) a small yield from the exclusive resources relative to the common one; and b) strong interspecific relative to intraspecific interference.     

Drivers of global variation in land ownership

Land ownership shapes natural resource management and social–ecological resilience, but the factors determining ownership norms in human societies remain unclear. Here we conduct a global empirical test of long‐standing theories from ecology, economics and anthropology regarding potential drivers of land ownership and territoriality. Prior theory suggests that resource defensibility, subsistence strategies, population pressure, political complexity and cultural transmission mechanisms may all influence land ownership. We applied multi‐model inference procedures based on logistic regression to cultural and environmental data from 102 societies, 71 with some form of land ownership and 31 with no land ownership. We found an increased probability of land ownership in mountainous environments, where patchy resources may be more cost effective to defend via ownership. We also uncovered support for the role of population pressure, with a greater probability of land ownership in societies living at higher population densities. Our results also show more land ownership when neighboring societies also practiced ownership. We found less support for variables associated with subsistence strategies and political complexity.     

Examining the potential for resource-dependent female reproductive fluid-sperm interactive effects in a livebearing fish

Sexually selected traits can be costly to produce and maintain. The amount of resources available to an individual is therefore expected to influence investment in costly sexual traits. While resource-dependent expression of sexually selected traits has traditionally been examined in males, resource limitation can also influence how sexual selection operates in females. Female reproductive fluids are thought to be costly to produce and may play an important role in shaping the outcome of postcopulatory sexual selection by influencing sperm performance. However, we know surprisingly little about whether and how female reproductive fluids are influenced by resource limitation. Here, we examine if resource restriction influences female reproductive fluid-sperm interactive effects in the pygmy halfbeak (Dermogenys collettei), a small internally fertilizing freshwater fish where females store sperm. After experimentally altering female diets (high vs. restricted diets), we compared how female reproductive fluids influence two key metrics of sperm quality: sperm viability and velocity. While female reproductive fluids enhanced sperm viability and velocity, we found no evidence that female diet influenced the interactive effect between female reproductive fluids and sperm viability or velocity. Our findings build on the growing evidence that female reproductive fluids influence sperm performance and call for further attention to be devoted to understanding how resource quantity and quality influence how female reproductive fluids affect sperm performance.     

Territorial Marking in the Desert Ant Cataglyphis niger: Does It Pay to Play Bourgeois?

Here we report on territorial behavior in the desert ant Cataglyphis niger. We show that, similar to the bourgeois strategy, contests are settled quickly through nonescalating fights, with the owner of the territory usually being placed in the winning role. The chemical cue that advertises ownership is shown to come from pheromones secreted by the cloacal gland. Differences in size did not influence the outcome of the experimental contests, but resource value did. The observed results fit in well with the biology of Cataglyphis, where, due to the harsh semidesert environment, fighting costs (C) are expected to be high relative to the value of the contested resources (V). A situation where V < C is exactly that in which bourgeois behavior is expected to be seen.     

Analysis of a summary network of co-infection in humans reveals that parasites interact most via shared resources

Simultaneous infection by multiple parasite species (viruses, bacteria, helminths, protozoa or fungi) is commonplace. Most reports show co-infected humans to have worse health than those with single infections. However, we have little understanding of how co-infecting parasites interact within human hosts. We used data from over 300 published studies to construct a network that offers the first broad indications of how groups of co-infecting parasites tend to interact. The network had three levels comprising parasites, the resources they consume and the immune responses they elicit, connected by potential, observed and experimentally proved links. Pairs of parasite species had most potential to interact indirectly through shared resources, rather than through immune responses or other parasites. In addition, the network comprised 10 tightly knit groups, eight of which were associated with particular body parts, and seven of which were dominated by parasite–resource links. Reported co-infection in humans is therefore structured by physical location within the body, with bottom-up, resource-mediated processes most often influencing how, where and which co-infecting parasites interact. The many indirect interactions show how treating an infection could affect other infections in co-infected patients, but the compartmentalized structure of the network will limit how far these indirect effects are likely to spread.