Dear Enemies and the Prisoner's Dilemma: Why Should Territorial Neighbors Form Defensive Coalitions?

Game-theoretic arguments are used to derive two new hypothesesto explain why territorial residents so consistently defeatpotential usurpers. Both hypotheses are based on help from established,familiar neighbors. The first hypothesis follows simply fromKrebs' (1982) assertion that the value of a territory to a usurpermust be decremented by the costs of negotiating dear-enemy relationshipswith the remaining neighbors. An implication is that the remainingneighbors will also have to pay these renegotiation costs ifthe usurper succeeds. The first hypothesis is that it may benefita territorial animal to help its established neighbors defendso it can avoid having to renegotiate territorial boundarieswith a new, unfamiliar neighbor. This hypothesis assumes netpositive benefits to helping without requiring reciprocation. The second hypothesis requires reciprocation to compensate forimmediate net costs of helping. An animal should help its neighborsfight off usurpers only if the neighbors will reciprocate. Thishypothesis is based on the prisoner's dilemma game and buildson Axelrod's (1984) work. Cooperative defense (reciprocal help)can be an evolutionarily stable strategy (ESS) if several conditionsare met. One critical condition is that the relationship betweenneighbors is relatively stable. Cooperative defense should helpestablished neighbors retain their territories, and should thereforebe a cause, as well as a consequence of stability. It is suggestedthat the necessary conditions are not very restrictive, thatthey are often met in nature, and that shared defense is observedbut not recognized as such.     

The Costs of Neighbors for a Territorial Dragonfly, Perithemis tenera

Past researchers have often considered neighbors to be beneficial to territorial residents, particularly compared with non‐neighbor conspecific competitors. However, neighbors have the potential to be costly to residents in terms of both defensive costs and lost resources. In this study, we assessed the relative costs of defending a mating territory against neighbors and non‐neighbors for the dragonfly Perithemis tenera, comparing across males with different numbers of contiguous neighbors; we also examined the possibility that the presence of contiguous neighbors might reduce the detection of potential mates. When neighbors were present, residents experienced a greater total number of intrusions by males; this increase in intrusions was due to higher numbers of intrusions by neighbors, as the number of intrusions by non‐neighbor males did not differ. Residents with immediately adjacent neighbors also made more sorties toward neighbors than did residents whose nearest neighbors’ territories were not immediately adjacent. Interestingly, although the number of visits by females did not vary with the presence of neighbors, residents with neighbors made fewer sorties toward females than did residents without neighbors. Our results suggest that defensive costs increased when neighbors were present, that residents with neighbors may have missed opportunities to acquire mates, and thus that living with neighbors can be costly in this species.     

Disruption of Dear Enemy Recognition Among Neighboring Males by Female Leon Springs pupfish, Cyprinodon bovinus

Males defending territories often settle into adjacent areas, sharing a common border that is maintained by a reduced level of aggression known as dear enemy recognition. While social conditions may affect the dear enemy relationship among males, what role females play, if any, is unclear. In a field study of the highly promiscuous Leon Springs pupfish, Cyprinodon bovinus, we asked whether females influenced this relationship of neighbors to their advantage. We observed 16 territorial residents, mapping the precise location of each male's behaviors within its territory. Resident males engaged in less aggression against neighbors compared with intruders, and neighbors intruded less deeply into the residents’ territories than intruders. Despite this locality restriction, neighbors were responsible for as many spawning interruptions as intruders. Females did not spawn randomly in the males’ territories, nor did they spawn near territory centers where aggression was low. Rather, females were more likely to settle and spawn in the outer half of the territories where competition among males was highest. When a neighbor entered a resident's territory to interrupt a spawn, the female was more likely to leave the resident's territory for the neighbor's than to remain. These observations suggest that the female used the intrusion by the neighbor to engage the resident and interrupt the spawn as a measure of this male's quality and that, while neighboring males benefit from the presence of dear enemy recognition, females benefit from its disruption.     

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.     

Defence, intrusion and the evolutionary stability of territoriality

Territorial behaviour can only be adaptive if its costs are outweighed by its benefits. Territorial individuals incur costs by defending their territories against intruders. Usually these intruders are assumed to be non-territorial floaters attempting to take over the whole territory or neighbours trying to extend the borders of their own territory. We instead investigate how costs and benefits of territorial behaviour are affected by neighbours which invade to steal resources on a territory.We show analytically that in the absence of defence intrusion into neighbouring territories always pays and that even if territories are defended intrusion levels can still be high. Using a more detailed simulation model we find that territory defence usually disappears from the population even if owners have a strong advantage over intruders in terms of fighting costs or foraging efficiency. Defence and thus territoriality can only be evolutionarily stable if fighting costs for the intruder relative to the productivity of the territory are very high or if crossing the borders between territories carries additional costs.Our results show that stealing of resources by neighbours can have a considerable effect on the evolutionary stability of territory defence and thus territoriality itself. A more mechanistic model of territorial behaviour is needed to incorporate these kinds of mechanisms into a general theory on the evolution of territoriality.     

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.     

Ecotypic variation in the asymmetric Hawk-Dove game: When is Bourgeois an evolutionarily stable strategy?

Summary This paper develops a mathematical model of an iterated, asymmetric Hawk-Dove game with the novel feature that not only are successive pairs of interactants — in the roles of owner and intruder contesting a site — drawn randomly from the population, but also the behaviour adopted at one interaction affects the role of a contestant in the next. Under the assumption that a site is essential for reproduction, the evolutionarily stable strategy (ESS) of the population is found to depend on the probability, w, that the game will continue for at least a further period (which is inversely related to predation risk), and five other parameters; two of them are measures of site scarcity, two are measures of fighting costs, and the last is a measure of resource holding potential (RHP). Among the four strategies — Hawk (H), Dove (D), Bourgeois (B) and anti-Bourgeois (X) — only D is incapable of being an ESS; and regions of parameter space are found in which the ESS can be only H, or only X, or only B; or either H or X; or either X or B; or either H or B; or any of the three. The scarcer the sites or the lower the costs of fighting, or the lower the value of w, the more likely it is that H is an ESS; the more abundant the sites or the higher the costs of fighting, or the higher the value of w, the more likely it is that X or B is an ESS. The different ESSs are interpreted as different ecotypes. The analysis suggests how a non-fighting population could evolve from a fighting population under decreasing risk of predation. If there were no RHP, or if RHP were low, then the ESS in the non-fighting population would be X; only if RHP were sufficiently high would the ESS be B, and the scarcer the sites, the higher the RHP would have to be. These conclusions support the thesis that if long-term territories are essential for reproduction and sites are scarce, then ownership is ruled out not only as an uncorrelated asymmetry for settling disputes in favour of owner, but also as a correlated asymmetry.     

Fighting costs stabilize aggressive behavior in intersexual conflicts

We analyze the evolution of aggressive behavior in intersexual conflicts, with a special reference to mate guarding behavior in crustaceans. An analysis of a discrete-strategy game shows that an ESS with only one of the sexes being aggressive prevail if fighting costs or fitness values of winning are asymmetric. Non-aggressiveness of both sexes is stable if fighting behavior is very costly for females and if the cost is at least partly paid independent of the strategy of the opponent. Most interestingly, the solutions of both sexes being aggressive prevails only if both sexes have some probability of winning, and if fighting costs are small. Second, we solve for the expected levels of aggressiveness in a game with continuous strategies. The form of the fighting cost function largely determines the stability of the solution. When fighting cost increases linearly with aggressiveness, mutual aggressiveness fluctuates cyclically instead of stabilizing at an ESS. However, if there is an asymmetry in fitness payoffs, a solution with only the sex having most to lose being aggressive alone is possible. With quadratically increasing fighting costs an ES combination of mutual aggressiveness may exist. It is predicted that fights between the sexes should be hardest when payoffs are symmetric, and that an overt behavioral conflict will always take place as long as there is a fitness loss to each of the sexes if losing the conflict and both sexes have a chance to win. We discuss the models in the context of fights preceding precopulatory guarding, but the models offer a general frame for analyzing any intersexual conflict. This revised version was published online in July 2006 with corrections to the Cover Date.     

Common Loons can differentiate yodels of neighboring and non‐neighboring conspecifics

ABSTRACT Although previous studies have identified elements of the yodel calls of male Common Loons (Gavia immer) that might be important for neighbor/non‐neighbor discrimination, no one to date has determined whether loons can distinguish between the yodels of neighbors and non‐neighbors. Our objectives were to determine if Common Loons respond differently to playback recordings of yodels of neighbor and non‐neighbors and, if so, if elements of the introductory phrase or the repeat phrases are important in such differentiation. We studied loons occupying single‐lake territories in Oneida County, Wisconsin, in 2001 (N= 20 pairs) and 2007 (N= 16 pairs). Playback experiments revealed no significant difference in number of different types of vocalizations (yodels, tremolos, and wails) loons gave in response to neighbor and non‐neighbor yodels. However, loons gave significantly more tremolos in response to yodels lower in peak frequency than those of resident male (P= 0.01), indicating they were more threatened by such calls. In addition, loons gave significantly more tremolos (P < 0.01) and yodels (P < 0.01) in response to the lower frequency yodels of non‐neighbors than neighbors. Because previous studies have revealed that males with greater resource‐holding ability produce lower frequency yodels, our results suggest that the response of Common Loons to unfamiliar yodels depends on perceived condition‐dependent fighting ability. When we used playbacks containing a non‐neighbor's introductory phrase and a neighbor's repeat syllables, we found that loons uttered more tremolos (P= 0.01) and yodels (P= 0.01), suggesting that the introductory phrase is more important than the repeat phrases for neighbor/non‐neighbor discrimination. Thus, the yodels of male Common Loons appear to provide conspecifics with information about their status (neighbor or non‐neighbor) as well as their condition and aggressive motivation.     

Investigating the relationship between neighbor root biomass and belowground competition: field evidence for symmetric competition belowground

Little is known about how small-scale variation in neighbor biomass can influence the strength of root competition experienced by an individual plant. In this study, modified root exclusion tubes were used to vary the accessibility of the soil space surrounding Amaranthus retroflexus target plants to the neighboring plants. A gradient of root accessibility was created by drilling varying numbers of holes into standard root exclusion tubes, made of 15 cm diameter PVC pipe. Belowground competitive intensity, defined as biomass reduction due to root interactions alone, relative to growth in the absence of neighbors, was then measured along the resulting gradient of neighbor root densities. At low neighbor root abundances the strength of belowground competition was proportional to neighbor root biomass, consistent with prior evidence that belowground competition is symmetric. If belowground competition were asymmetric, neighbor roots should have had little effect on target plants when they are rare relative to those of the target plant. At higher neighbor root abundances, belowground competitive intensity should increase rapidly. The strong relationship found between neighbor root biomass and belowground competitive intensity suggests relatively small variations in root biomass could lead to large variations in belowground competition. Reduced belowground competition in areas with low root biomass could have important implications for the establishment and growth of poor belowground competitors, suggesting a mechanism by which species coexistence may occur despite extremely intense root competition.     

The relative importance of prey availability and intruder pressure in feeding territory size regulation by harriers,Circus cyaneus

Summary The relative importance of prey availability and intruder pressure in the regulation of harrier (Circus cyaneus) territory size was investigated over two years using analytical methods chosen to permit comparison with Myers et al. (1979) study of sanderlings (Calidris alba). Relationships between territory area and two variables, prey type (mice; large, medium, and small birds) and intruder type (conspecific neighbors, conspecific floaters, and heterospecific floaters), and the consistency of these relationships between years, also were examined. Individual harrier territory areas were highly variable, ranging from 7.8 to 1249 ha in 1984/1985, and 3.9 to 71.3 ha in 1985/1986. Of the prey variables, only mouse availability was significantly inversely correlated with territory area in both years, and slopes resulting from correlations between the logarithms of these two variables did not differ significantly from — 1, the expected result if harriers were adjusting mouse availabilities. The abundance of mice in conjunction with their greater ease of capture relative to birds made them functionally more available, and hence harriers' primary prey. This may explain why mice, rather than birds, were apparently the defended resource. Of the intruder variables, neighbor variables were most strongly inversely cortelatd with territory area. Partial correlation analyses to determine the relative importance of intruder pressure and prey availability in regulating territory size revealed that in 1984/1985, mouse availability and intruder pressure were relatively independent and each explained some variation in territory area, whereas in 1985/1986, mouse availability, rather than intruder pressure, significantly explained all variation in territory area. Possible explanations for why territory sizes of harriers appear to be regulated more closely by food density, whereas territory sizes of sanderlings appear to be regulated more closely by intruder pressure, are based upon differences in 1) neighbor effects, 2) environmental ronmental variability, and 3) accuracy of resource assessment by intruders. The variation in intruder rates and prey availabilities observed between years suggests the need to conduct studies over several years in order to assess accurately the relative importance of these variables in territory-size regulation.     

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.     

Fighting in Males of the Autumn Spider,Metellina segmentata: Effects of Relative Body Size,Prior Residency and Female Value on Contest Outcome and Duration

Males of the autumn spider, Metellina segmentata (***Araneac: Metidae), compete for access to mates by guarding the orb webs of mature females. We investigated the influences of relative male righting ability and resource value on fighting behaviour by staging interactions in the field on webs occupied by females. In these contests, the larger male nearly always defeated its opponent when it was at least 10% greater in size. For smaller size asymmetries between opponents, the male previously resident on the female's web usually won the contest. Contest duration decreased exponentially with increasing size asymmetry between opponents, as predicted if each male assessed its relative size and adjusted its fighting strategy according to its likelihood of winning. Contest duration was also greater when the prior resident was the lighter opponent, or when size and residency asymmetries favoured different opponents as winners. Prior residents fought longer over larger, more fecund females, indicating an adjustment of fighting effort according to assessments of resource value. In contrast, intruders did not increase their fighting effort over larger females, suggesting an inability to assess female size quickly and accurately. Assessment appears to reduce the costs of settling conflicts, but imperfect information can result in inaccurate assessments and unexpected outcomes. Assessment strategies are used by other types of spiders to resolve contests, but this appears to be the first evidence for such strategies among orb-web-building spiders.     

Neuromolecular correlates of cooperation and conflict during territory defense in a cichlid fish

Cooperative behavior is widespread among animals, yet the neural mechanisms have not been studied in detail. We examined cooperative territory defense behavior and associated neural activity in candidate forebrain regions in the cichlid fish, Astatotilapia burtoni. We find that a territorial male neighbor will engage in territory defense dependent on the perceived threat of the intruder. The resident male, on the other hand, engages in defense based on the size and behavior of his partner, the neighbor. In the neighbor, we find that an index of engagement correlates with neural activity in the putative homolog of the mammalian basolateral amygdala and in the preoptic area, as well as in preoptic dopaminergic neurons. In the resident, neighbor behavior is correlated with neural activity in the homolog of the mammalian hippocampus. Overall, we find distinct neural activity patterns between the neighbor and the resident, suggesting that an individual perceives and processes an intruder challenge differently during cooperative territory defense depending on its own behavioral role.     

The Effects of Isolation on the Sensitization and Habituation of Aggression in the Threespine Stickleback (Gasterosteus aculeatus)

Our previous research showed that territorial threespine sticklebacks are more aggressive toward a male conspecific placed in their territory if they have been housed adjacent to a gravid female rather than a male or a nongravid female. This study replicated the condition of a territorial male with an adjacently housed male and compared the results with isolated territorial males. This permitted us to contrast explanations for the increased aggression when the neighbor was a gravid female. If the social context determines the level of aggression, then males with no neighbors (isolates) might be less aggressive because there would be no unique reproductive resource to protect nor any neighbor to protect resources from. Alternatively, if aggression is higher in the isolated group it might be because there was no male neighbor to redirect aggression toward. We found that isolated males were more aggressive toward an intruded male stimulus than males with a male neighbor. The study also provided evidence that sensitization produced by the appearance of an intruding male energizes other aggressive behavior, but not those related to feeding, nor does it cause increase in general activation as measured by increased locomotion.     

Mechanisms and fitness effects of interspecific information use between migrant and resident birds

Interactions with potential competitors are an important componentof habitat quality. Due to the costs of coexistence with competitors,a breeding habitat selection strategy that avoids competitorsis expected to be favored. However, many migratory birds appearto gain benefits from an attraction to the presence of residentbirds, even though residents are assumed to be competitivelydominant. Thus far the mechanisms of this habitat selectionprocess, heterospecific attraction, are unknown, and the consequencesfor resident birds of migrant attraction remain untested. Throughheterospecific attraction, migrants may gain benefits if thedensity or territory location of residents positively reflectshabitat quality, and/or they gain benefits through increasedfrequency of social interactions with residents in foragingor predator detection. In this experiment, we examined the reciprocaleffects of spatial proximity on fitness-related traits in migrantpied flycatcher (Ficedula hypoleuca) and resident great tit(Parus major) by experimentally forcing them to breed eitheralone or in close proximity to each other. Surprisingly, greattits bore all the costs of coexistence while flycatchers wereunaffected, even gaining slight benefits. In concert with anearlier study, these results suggest that flycatchers use titsas information about good-quality nest-site locations whilebenefits from social interactions with tits are possible butless important. We suggest that utilizing interspecific socialinformation may be a common phenomenon between species sharingsimilar resource needs. Our results imply that the effects ofinterspecific information use can be asymmetric and may thereforehave implications for the patterns and consequences of speciescoexistence.     

Experimental analysis of territory size in a population of the fire ant Solenopsis invicta

Alternative models of territoriality are based on contrastingassumptions about the behavioral processes determining territorysize. In a series of controlled field experiments on the fireant Solenopsis invicta, I tested whether territory size is affectedby the availability of food, as predicted by most economic models,and whether territory size is affected by fighting ability,as predicted by models of competition among neighbors. Abundantfood was offered for 30–35 days to selected colonies eitherimmediately next to the nest (experiment 1) or at peripheralsites near the territory boundary (experiment 2). These foodsupplements had no detectable effect on territory size. Furthermore,food placed near the periphery of the territory did not significantlyalter local boundary positions. During both experiments, largecolonies lost more territory than did small colonies, reflectingtemporary declines in worker number due to the seasonal productionof reproductives. Such losses by large colonies during the summermonths create opportunities for newly founded colonies to expandterritories. In a third experiment, colonies from which workerswere removed lost significantly more territory than did unmanipulatedcontrols. These results show that territory areas in S. invictaare strongly affected by the relative fighting ability of neighboringcolonies but provide no evidence that colonies adjust territoryarea in response to short-term changes in the availability offood.     

Male song sparrows have elevated testosterone in response to neighbors versus strangers

Upon hearing a conspecific signal, animals must assess their relationship with the signaller and respond appropriately. Territorial animals usually respond more aggressively to strangers than neighbors in a phenomenon known as the “dear enemy effect”. This phenomenon likely evolved because strangers represent a threat to an animal's territory tenure and parentage, whereas neighbors only represent a threat to an animal's parentage because they already possess a territory (providing territory boundaries are established and stable). Although the dear enemy effect has been widely documented using behavioral response variables, little research has been conducted on the physiological responses of animals to neighbors versus strangers. We sought to investigate whether the dear enemy effect is observed physiologically by exposing territorial male song sparrows (Melospiza melodia) to playback simulating a neighbor or a stranger, and then collecting blood samples to measure plasma testosterone levels. We predicted that song sparrows would exhibit increased testosterone levels after exposure to stranger playback compared to neighbor playback, due to the role testosterone plays in regulating aggression. Contrary to our prediction, we found that song sparrows had higher testosterone levels after exposure to neighbor playback compared to stranger playback. We discuss several explanations for our result, notably that corticosterone may regulate the dear enemy effect in male song sparrows and this may inhibit plasma testosterone. Future studies will benefit from examining corticosterone in addition to testosterone, to better understand the hormonal underpinnings of the dear enemy effect.     

A test for repertoire matching in eastern song sparrows

Repertoire matching occurs when one songbird replies to another with a song type that the two birds share. Repertoire matching has previously been demonstrated to occur at well above chance levels in a western population of song sparrows, where it is hypothesized to serve as a low level threat in a hierarchy of aggressive signals. Here we test for repertoire matching in an eastern population of song sparrows. Previous work indicates that this eastern population differs from the western one in having lower levels of song sharing between neighboring males and in showing no association between song sharing and territory tenure. Here we confirm that males in this eastern population on average share few whole songs with their neighbors. The eastern males are familiar with their neighbors’ repertoires, as evidenced by a stronger singing response to stranger song than to neighbor song. Males in the eastern population did not repertoire match: when played an unshared song type from a specific neighbor, they did not reply with a song type shared with that neighbor more often than expected by chance or more often than in response to playback of a control song (an unshared stranger song). The results thus demonstrate a qualitative difference in vocal signaling strategies between two populations of the same species.     

Can transitive inference evolve in animals playing the hawk-dove game?

What should an individual do if there are no reliable cues to the strength of a competitor when fighting with it for resources? We herein examine the evolutionarily stable strategy (ESS) in the hawk-dove game, if the opponent's resource-holding potential (RHP) can only indirectly be inferred from the outcome of past interactions in the population. The strategies we examined include the classical mixed strategy in which no information on past games is utilized, the 'imprinting' strategy in which a player increases/decreases its aggressiveness if it wins/loses a game, the 'immediate inference' strategy in which a player can infer the strength of those opponents it fought before, and the 'transitive inference' strategy in which a player can infer the strength of a new opponent through a third party with which both players have fought before. Invasibility analysis for each pair of strategies revealed that (i) the transitive-inference strategy can always invade the mixed strategy and the imprinting strategy, and itself refuses invasion by these strategies; (ii) the largest advantage for transitive inference is achieved when the number of games played per individual in one generation is small and when the cost of losing an escalated game is large; (iii) the immediate inference, rather than the transitive inference, can be an ESS if the cost of fighting is small; (iv) a strong linear ranking is established in the population of transitive-inference strategists, though it does not perfectly correlate to the ranking by actual RHPs. We found that the advantage of the transitive inference is not in its ability to correct a misassessment (it is actually the worst in doing so), but in the ability of quickly lining up either incorrect or correct assessments to form a linear dominance hierarchy.