Asymmetric territorial contests in the European robin: the role of settlement costs

Age, brood fate, and territory quality affect nest-site fidelity in white stork Ciconia ciconia

BACKGROUND

A particular type of site fidelity is faithfulness to the nest site, where birds are not only reoccupying breeding territories but also reusing nests built in previous breeding seasons. Staying faithful to the nest site is believed to be an adaptive strategy, and based on the ability to predict an individual's own breeding success, a hypothesis of "win-stay:loose-switch" was proposed. In this study, we aimed to resolve which factors affect the nest-site fidelity of white stork Ciconia ciconia, species known for reusing nests available in the breeding sites. Basing on ring recoveries from 31 years of studies in Western and Southern Poland, we analysed the impact of intrinsic and extrinsic factors on nest-site fidelity.

RESULTS

We found that increasing age and breeding success (i.e. producing any fledglings or not) increased the probability of reusing the nest, but in the oldest individuals, the probability decreased. In turn, the probability of breeding success increased with age, the increasing number of reproductive events on the particular nest, and the presence on the nest in the previous year. However, the oldest individuals had lower probability of success, as the relationship was curvilinear. The number of fledglings, however, was influenced only by an individual's age. The number of reproductive events on the nest was, in turn, affected by age, with the youngest and oldest individuals using the current nest for the least number of years.

CONCLUSIONS

Our study shows that the decision process of whether to stay faithful to the nest or switch is based on the experience from the previous breeding event, consistently with the "win-stay:loose-switch" hypothesis. Our results also show that site fidelity benefits white storks, as the probability of breeding success increases if the nest is reused. Results also show the senescence effect that lowers breeding success and site fidelity probabilities.

Subjective resource value affects aggressive behavior independently of resource-holding-potential and color morphs in male common wall lizard

Game theory models predict the outcome of a dyadic contest to depend on opponents’ asymmetries in three main traits: resource-holding potential (RHP), resource value (RV) and aggressiveness. Using male common wall lizards Podarcis muralis, a polymorphic species showing three discrete morphs (white, yellow, and red), we investigated how the aggressive behavior varies according to a change in subjective RV and color morphs, while controlling for the asymmetry in RHP (using mirrors). By comparing the aggression of the same individual towards its mirror image in two different arenas (familiar = high subjective RV; novel = low subjective RV), we showed that lizard aggressive behavior was more intense and prolonged in the familiar arena than in the new one, thus supporting the occurrence of a direct relationship between motivation and aggression in this species. We also found the overall aggressiveness to differ from individual to individual, supporting the general hypothesis that aggressiveness is a trait associated with personality. By contrast, no effect of morphs was detected, ruling out the occurrence of morph specific variation in the aggressiveness. Our results highlight that an individual’s motivation and personality might be as important as RHP and RV in the resolution of animal contests.

Pneumococcal quorum sensing drives an asymmetric owner-intruder competitive strategy during carriage via the competence regulon

Competition among microorganisms is a key determinant of successful host colonization and persistence. For Streptococcus pneumoniae, lower than predicted rates of co-colonizing strains suggest a competitive advantage for resident bacteria over newcomers. In light of evolutionary theory, we hypothesized that S. pneumoniae use owner-intruder asymmetries to settle contests, leading to the disproportionate success of the initial resident 'owner', regardless of the genetic identity of the 'intruder'. We investigated the determinants of within-host competitive success utilizing S. pneumoniae colonization of the upper respiratory tract of infant mice. Within 6 h, colonization by the resident inhibited colonization by an isogenic challenger. The competitive advantage of the resident was dependent on quorum sensing via the competence (Com) regulon and downstream choline binding protein D (CbpD) and on the competence-induced bacteriocins A and B (CibAB) implicated in fratricide. CbpD and CibAB are highly conserved across pneumococcal lineages, indicating evolutionary advantages for asymmetric competitive strategies within the species. Mathematical modelling supported a significant role for quorum sensing via the Com regulon in competition, even for strains with different competitive advantages. Our study suggests that asymmetric owner-intruder competitive strategies do not require complex cognition and are used by a major human pathogen to determine 'ownership' of human hosts.

Residency in white-eared hummingbirds (Hylocharis leucotis) and its effect in territorial contest resolution

BACKGROUND

Territory owners usually defeat intruders. One explanation for this observation is the uncorrelated asymmetry hypothesis which argues that contests might be settled by an arbitrary convention such as "owners win." We studied the effect of territorial residency on contest asymmetries in the white-eared hummingbird (Hylocharis leucotis) in a fir forest from central Mexico.

METHODS

Twenty white-eared male adult hummingbird territories were monitored during a winter season, recording the territorial behavior of the resident against intruding hummingbirds. The size and quality of the territory were related to the probability that the resident would allow the use of flowers by the intruder. Various generalized models (logistical models) were generated to describe the probabilities of victory for each individual resident depending on the different combinations of three predictor variables (territory size, territory quality, and intruder identity).

RESULTS

In general, small and low quality territory owners tend to prevent conspecific intruders from foraging at a higher rate, while they frequently fail to exclude heterospecific intruders such as the magnificent hummingbird (Eugenes fulgens) or the green violetear hummingbird (Colibri thalassinus) on any territory size. Our results showed that the identity of the intruder and the size and quality of the territory determined the result of the contests, but not the intensity of defense.

DISCUSSION

Initially, the rule that "the resident always wins" was supported, since no resident was expelled from its territory during the study. Nevertheless, the resident-intruder asymmetries during the course of a day depended on different factors, such as the size and quality of the territory and, mainly, the identity of the intruders. Our results showed that flexibility observed in contest tactics suggests that these tactics are not fixed but are socially plastic instead and they can be adjusted to specific circumstances.

Provenance does matter: links between winter trophic segregation and the migratory origins of European robins

Amongst migratory species, it is common to find individuals from different populations or geographical origins sharing staging or wintering areas. Given their differing life histories, ecological theory would predict that the different groups of individuals should exhibit some level of niche segregation. This has rarely been investigated because of the difficulty in assigning migrating individuals to breeding areas. Here, we start by documenting a broad geographical gradient of hydrogen isotopes (δ ²H) in robin Erithacus rubecula feathers across Europe. We then use δ ²H, as well as wing-tip shape, as surrogates for broad migratory origin of birds wintering in Iberia, to investigate the ecological segregation of populations. Wintering robins of different sexes, ages and body sizes are known to segregate between habitats in Iberia. This has been attributed to the despotic exclusion of inferior competitors from the best patches by dominant individuals. We find no segregation between habitats in relation to δ ²H in feathers, or to wing-tip shape, which suggests that no major asymmetries in competitive ability exist between migrant robins of different origins. Trophic level (inferred from nitrogen isotopes in blood) correlated both with δ ²H in feathers and with wing-tip shape, showing that individuals from different geographic origins display a degree of ecological segregation in shared winter quarters. Isotopic mixing models indicate that wintering birds originating from more northerly populations consume more invertebrates. Our multi-scale study suggests that trophic-niche segregation may result from specializations (arising in the population-specific breeding areas) that are transported by the migrants into the shared wintering grounds.

Avian ecologists and physiologists have different sexual preferences

Seasonal timing is studied by ecologists and physiologists alike and it is now widely recognized that further integration of these fields is needed for a full understanding of phenology. This is especially true in the light of the impact of global climate change on living organisms. In studies of avian reproduction, one obstacle to this integration is that ecologists and physiologists do not allocate their research efforts equally to males and females. The physiological orchestration of breeding stages has been studied almost exclusively in males, while in avian ecology and evolutionary biology females are more often considered. This sex bias has severe implications: sexes differ in the way they use external cues to organize their life cycles, but often cue in on each other's physiology and behavior. The simultaneous investigation of both males and females within single studies is thus essential. In this review, I begin by illustrating the sex-bias in studies and attempt to explain its origin. I then provide a number of examples in which focusing on a single sex would have resulted in misleading conclusions. Finally, I review some classical studies of female reproductive physiology that have promoted and developed research on the "forgotten-sex".

Territory choice during the breeding tenure of male sedge warblers

A territorial male can shift the location of its territory from year to year in order to increase its quality. The male can base its decision on environmental cues or else on its breeding experiences (when territory shift is caused by breeding failure in previous seasons). We tested these possible mechanisms of territory choice in the sedge warbler (Acrocephalus schoenobaenus), a territorial migrating passerine that occupies wetlands. This species bases its territory choices on an environmental cue: tall wetland vegetation cover. We found that the magnitude of territory quality improvement between seasons (measured as the area of tall wetland vegetation) increased throughout the early stages of a male's breeding career as a result of territory shifts dependent on the earliness of arrival. The distance the territory was shifted between seasons depended negatively on the previous year's territory quality and, less clearly, on the previous year's mating success. On the other hand, previous mating or nesting success had no influence on territory quality improvement between seasons as measured in terms of vegetation. The results imply that tall wetland vegetation is a long-term, effective environmental cue and that a preference for territories in which this type of landcover prevails has evolved into a rigid behavioral mechanism, supplemented by short-term individual experiences of breeding failure.

Prior residency advantage for Atlantic salmon in the wild: effects of habitat quality

Prior residency advantages have been explained by an asymmetry between the ‘owner’ and the ‘intruder’ in fighting ability (resource-holding potential) or motivation (value asymmetry (VA)). Here, we tested for the extent of prior residence effects in individually tagged Atlantic salmon juveniles being released in two bouts (4 days apart) during spring along a natural stream, and recaptured 3 months later. A prior residency advantage was detected both in terms of body growth, energy density and male gonad size. As we controlled for effects of initial body size, which correlates with dominance, these findings are in accordance with the VA hypothesis. The growth advantage of first arrivals also increased with local shelter abundance in the stream, which can be expected if a higher resource value of the habitat results in a higher defence motivation. We also found a prior residence effect on spatial distribution, with the second arrivals within each release site being recaptured further downstream. No effect on apparent survival rates was found. The observed reduced growth and energy density may have fitness consequences for the second arrivals, both in terms of lower winter survival rates and later age at maturity. For mature male parr, both decreased body and gonad growth may give an additional disadvantage through reduced fertilization rates during breeding. These costs may help to explain the tendency for stationary behaviour of stream salmonids, as the potential benefits of moving into less crowded areas would be reduced by the risk of becoming an intruder. Prior residence effects may therefore have influenced the evolution of movement behaviour in these organisms.

Free-ranging dogs assess the quantity of opponents in intergroup conflicts

In conflicts between social groups, the decision of competitors whether to attack/retreat should be based on the assessment of the quantity of individuals in their own and the opposing group. Experimental studies on numerical cognition in animals suggest that they may represent both large and small numbers as noisy mental magnitudes subject to scalar variability, and small numbers (≤4) also as discrete object-files. Consequently, discriminating between large quantities, but not between smaller ones, should become easier as the asymmetry between quantities increases. Here, we tested these hypotheses by recording naturally occurring conflicts in a population of free-ranging dogs, Canis lupus familiaris, living in a suburban environment. The overall probability of at least one pack member approaching opponents aggressively increased with a decreasing ratio of the number of rivals to that of companions. Moreover, the probability that more than half of the pack members withdrew from a conflict increased when this ratio increased. The skill of dogs in correctly assessing relative group size appeared to improve with increasing the asymmetry in size when at least one pack comprised more than four individuals, and appeared affected to a lesser extent by group size asymmetries when dogs had to compare only small numbers. These results provide the first indications that a representation of quantity based on noisy mental magnitudes may be involved in the assessment of opponents in intergroup conflicts and leave open the possibility that an additional, more precise mechanism may operate with small numbers.

Predictors of floater status in a long-lived bird: a cross-sectional and longitudinal test of hypotheses

1. Few studies have been capable of monitoring the nonterritorial sector of a population because of the typically secretive behaviour of floating individuals, despite the existing consensus over the demographic importance of floating. Furthermore, there is almost no information on floating behaviour for migratory species. 2. The factors that determine whether an individual will be a floater or a territory owner have been framed into five, non-mutually exclusive hypotheses: (i) territory holders are morphologically superior to floaters (resource-holding potential hypothesis); (ii) age confers skills and fighting motivation which lead to social dominance and territory ownership (age hypothesis); (iii) occupancy time of a site determines asymmetries in its knowledge, familiarity and value for potential contenders (site-dominance hypothesis); (iv) contenders use an arbitrary rule to settle contests leading to pre-defined cut-off points for a biologically meaningful trait (e.g. age, body size) separating floaters from territory holders (arbitrary convention hypothesis); and (v) floaters set up a 'war of attrition' at arbitrarily chosen territories (arbitrary attrition hypothesis). 3. We tested these hypotheses using long-term data on a long-lived, migratory raptor, the black kite Milvus migrans Boddaert. 4. Floating status was best explained by the concerted action of mechanisms consistent with the age and site-dominance hypotheses. 5. In both cross-sectional and longitudinal analyses, acquisition of a territory was determined by a complex interaction between age and early arrival from migration, suggesting: (i) a progressive incorporation of early arriving individuals in the territorial contingent of the population, and (ii) the existence of an alternative restraint strategy of delayed territoriality mediated by long-term acquisition of social dominance. 6. Such results suggested that territory acquisition was mediated by the establishment of site dominance through pre-emption and, secondarily, despotism. In this population, age and arrival date aligned individuals along a demographic continuum ranging from successful breeders monopolizing high-quality resources to floaters with no resources, consistent with the notion of floating as an extreme form of breeding failure.

Aggressive monopolization of mobile carers by young of a cooperative breeder

Competition between young of the same brood or litter is of particular interest in the fields of behavioural and evolutionary ecology, because the competing individuals are likely to be closely related, where evolutionary theory predicts a greater degree of cooperation. Studies of cooperative breeding species typically concentrate on who contributes care to rearing young, and assume a passive role of the young. Relatively, little attention has been devoted to considering how intralitter competition between young affects the distribution of care in cooperative breeders. In banded mongoose (Mungos mungo) groups, the majority of pups each form a stable exclusive one-to-one association with an adult group member (its 'escort') that is its principal care provider. This paper presents experimental evidence that each pup aggressively defends access to its escort, preventing other pups approaching, and therefore monopolizes the care provided by its escort. Each pup travels with the group and follows its escort, around which its exclusion zone is fixed: a form of mobile territoriality. This represents a novel system of care of young in a mammal species, but has general implications for the study of the distribution of care of young, particularly in cooperative breeding species. Parents and helpers may provide biased care to young, not due to preference but due to the competitive actions of the young within the brood or litter.

From Hawks and Doves to Self‐Consistent Games of Territorial Behavior

Explaining the "prior-residence effect" (automatic owner status of individuals who arrived first in an area) was one of the very first applications of game theory in animal behavior. These models, however, predict paradoxical solutions where intruders always win, with no satisfactory explanation for the absence of such cases in nature. We propose a solution based on new developments in evolutionary game theory. A self-consistent model with feedbacks between individual behavior and population dynamics produces qualitatively different frequency-dependent selection on intruders (floaters) than on territory owners. Starting with an ancestral population with no respect for ownership, the most likely evolutionary end point is complete or partial respect. Conventional rules of conflict resolution thus can rely on "uncorrelated asymmetries" without differences in resource-holding power or territory value, although they will be strengthened by such differences. We also review the empirical literature on animal contests, testing whether asymmetries in resource-holding power are required to explain the observations. Despite much empirical effort, results remain inconclusive, because experiments are often unable to distinguish between the motivation of individuals to fight and the behavioral outcome of a contest. To help arrive at conclusive answers, we suggest a standardized empirical approach to quantify prior-residence effects.

The property 'instinct'

Evolutionary theory and empirical studies suggest that many animals, including humans, have a genetic predisposition to acquire and retain property. This is hardly surprising because survival is closely bound up with the acquisition of things: food, shelter, tools and territory. But the root of these general urges may also run to quite specific and detailed rules about property acquisition, retention and disposition. The great variation in property-related behaviours across species may mask some important commonalities grounded in adaptive utility. Experiments and observations in the field and laboratory suggest that the legal rules of temporal priority and possession are grounded in what were evolutionarily stable strategies in the ancestral environment. Moreover, the preferences that humans exhibit in disposing of their property on their deaths, both by dispositions made in wills and by the laws of intestacy, tend to advance reproductive success as a result of inclusive fitness pay-offs.