Contest Escalation Research Articles

Nest platforms increase aggressive behavior in common loons

Artificial manipulations of habitat, such as those that incorporate adding nesting boxes or platforms for birds, often enhance the breeding success of threatened animals. However, such alterations are likely to have unintended behavioral and ecological effects that might negatively impact the target species or others in its community. We conducted a controlled study to investigate the effect of artificial nesting platforms on aggressive behavior and reproductive success of male common loons (Gavia immer). Males residing on territories to which platforms were added produced longer territorial "yodels" (reflecting willingness to escalate a contest), experienced increased levels of confrontation and aggression with territorial intruders, and experienced increased rates of territorial displacement by intruders. Surprisingly, males of treatment territories also experienced lower productivity. Therefore, in addition to providing novel empirical support of sequential assessment models of animal contests that predict contest escalation with increasing resource quality, this study is one of a few to show that tools used to mitigate habitat loss can negatively impact reproductive fitness in a threatened species.

Twilight fighting in the evening brown butterfly, Melanitis leda (L.) (Nymphalidae): age and residency effects

Butterflies are conspicuous among animals that fight for mating opportunities because it is not clear how contest costs could accrue. Nevertheless, the bulk of research in this group suggests that contests are settled on the basis of asymmetries in fighting ability (the superior competitor hypothesis). There is also a consistent effect on contest escalation due to apparent 'confusion' over residency, which could result from resident butterflies playing different tactics than non-residents, or from a causal link between residency and fighting ability. I addressed these hypotheses by studying contest behavior in the crepuscular species Melanitis leda (L.) (Nymphalidae) over a 5-month period in tropical Australia. Males competed via conspicuous two-stage maneuvers of a form unique among butterflies. Prior residents won 77% of all contests. Non-resident males arrived and perched in occupied sites until challenged by incumbent residents, and the aggressiveness of these interlopers increased as a function of the time before they were detected. Contest winners also tended to be younger than losers, and contest escalation was negatively related to both the age of the losing male and the magnitude of the between-combatant age asymmetry. These results are consistent with the superior competitor hypothesis based upon age as a determinant (or correlate) of fighting ability, but also suggest residency has a unique influence on contest behavior. Further research is clearly required to unravel the effects of age, residency and other potential biophysical determinants of fighting ability in this species.

Gender contests in a simultaneous hermaphrodite snail: a size-advantage model for behaviour

Adaptive gender expression in simultaneous hermaphrodites will reflect the relative costs and benefits of each sexual function. The net gain through either or both functions may vary with body size. Shell length measurements of Physa gyrinaand P. heterostropha in the laboratory and P. heterostropha in the field revealed that successful mating generally involved smaller sperm donors and larger recipients. In contrast, attempts by larger snails to donate sperm were contested by smaller snails. These data suggest substantial plasticity in gender expression in accordance with a game-theoretic extension of Ghiselin’s (1969, Q. Rev. Biol., 44,189–208, 1974; The Economy of Nature and the Evolution of Sex) size-advantage model. Specifically, the larger of two interacting snails is more fecund, so the smaller snail has more to gain by donating sperm. This asymmetry creates a contest in which the smaller snail can afford to escalate the contest more and still profit, so long as escalation costs are not greater for the smaller snail. In the present study, the usual form of contest escalation was a visually obvious shell-swinging display, which attracts predators. Thus, the smaller of two contestants can use the risk of predation as a weapon in contest escalations (i.e. a game of chicken), rather than using a purely physical contest that might favour the larger contestant.

Confusion over residency and the escalation of damselfly territorial disputes

Territorial contests among male Calopteryx maculata (Odonata) damselflies normally involve one or a few short (4–10s) pursuits of an intruder by the resident. However, in a 3-year sample of 2005 bouts, 18·5% involved escalated, spiralling aerial chases that were significantly longer, some lasting several hours. These escalations usually occurred when two males shared the same territory but were rare in contests with intruders or with neighbouring territorial males. Escalations were most probably due to situations in which two males accidentally became residents in the same territory. This removed the ‘residency’ asymmetry that appears to settle most territorial disputes in these damselflies. Using moving territories (oviposition sites), the ‘confusion over residency’ hypothesis for escalation was tested experimentally. Bouts between males on adjacent, equally sized sites were escalated significantly in duration and intensity when their territories were merged. These experiments also showed that two alternative hypotheses, the ‘male aggressiveness’ and ‘valuable resource’ hypothesis, were not important general explanations of the escalation of contests.