Territorial defence in the great tit (Parus major): Do residents always win?

Summary I removed resident pairs of great tits from their territories for short periods and released them after replacement pairs had occupied the spaces. When two pairs are manipulated in this way into occupying the same territory an escalated contest ensures. Contests between residents and replacements are longer, more likely to involve physical fights and longer fights or displays than contests between established neighbours or residents and intruders. The degree of escalation between residents and replacements is an increasing function of replacement time. It rises to a peak and then diminishes again. The probability that the replacement will defeat the former resident is an increasing function of replacement time, reaching 90% after several days. These results are consistent with the hypothesis that territorial residents win in contests against intruders because of an asymmetry in payoff rather than an asymmetry in resource holding potential or an arbitary convention. A possibly asymmetry in payoff in the great tit is the cost of defending the territory against neighbours. The cost is higher for replacements than for residents.     

Predation risk and the territory value of cover: an experimental study

In an experiment on territorial brown trout (Salmo trutta), we addressed the novel hypothesis that protective cover increases the value of a territory in relation to the perceived level of predation risk. We predicted that territory holders should invest more resources defending territories with cover than territories without cover, and that defence should increase as predation risk increases. First, trout were allowed to establish ownership in territories with or without overhead cover. Second, predation risk was manipulated by simulating aerial predator attacks in half of the territories of each type, whereupon the preference for cover was estimated. Third, owners of the four types of territories were staged in dyadic contests against size-matched intruders. Territory owners showed a preference for cover, which increased further after simulated predator attacks. In subsequent contests, conflicts over territories with cover were settled faster than conflicts over territories without cover, which may suggest that the value of cover increases the motivational asymmetry between owner and intruder. Consistent with our hypothesis, owners of territories with cover were much more aggressive if they had been subjected to predator attacks the day before the conflict. These results suggest that territory owners are able to estimate the value of protective cover in response to variation in the level of predation risk in the habitat.Communicated by J. Krause     

Interspecific aggression in Leucorrhinia dragonflies: a frequency-dependent discrimination threshold hypothesis

Summary Male Leucorrhinia dragonflies defend territories from conspecific and heterospecific intruders. Defense against heterospecifics is surprising, as mating and oviposition are the only activities that occur on the territories, and heterospecific males are not expected to pose a reproductive threat. L. frigida and L. intacta males respond aggressively with equal frequency and intensity against intrusions by conspecifics and heterospecifics. In contrast, L. proxima males respond more aggressively against conspecifics. The apparent lack of species discrimination shown by L. frigida and L. intacta males may result because territorial males that assess intruders (as do L. proxima) suffer a tactical disadvantage from hesitating when an intruder flies in. This assessment process may lead to reduced fighting success by the territorial male. Thus there is a tradeoff between assessment cost incurred when the intruder is a conspecific and benefit gained from avoiding conflict when the intruder is a heterospecific. Given this assessment cost, males of species that only rarely encounter conspecific intruders are more likely to evolve species discrimination than males of species that commonly encounter conspecific intruders.     

Age,coloration and dominance in nonbreeding hummingbirds: A test of the asymmetry hypothesis

Summary During the nonbreeding season, adult Anna and black-chinned hummingbirds (Calypte anna and Archilochus alexandri) have lower defense costs and more exclusive territories than juveniles. Adult C. anna are victorious over juveniles in aggressive encounters, and tend to monopolize the most temporally predictable resources.Juveniles are more successful than adults at stealing food from territories (the primary alternative to territoriality), presumably because juveniles are less brightly colored. Juveniles have lighter wing disc loading than adults, and consequently should have lower rates of energy expenditure during flight. Reduced flight expenditures may be more important for juveniles because their foraging strategy requires large amounts of flight time. These results support the contention of the asymmetry hypothesis that dominance can result from a contested resource being more valuable to one contestant than to the other.Among juveniles, defence costs are also negatively correlated with age and coloration; amount of conspicucus coloration is negatively correlated with the number of bill striations, an inverse measure of age.     

Lek structure and territoriality in the chryxus arctic butterfly,Oeneis chryxus (Satyridae)

Summary The territorial and mating systems of the satyrid butterfly Oeneis chryxus were studied for 3 years on individually marked populations in algonquin Provincial Park, Ontario. Males defended territories in a dry open forest clearing by means of chases and spiral flights with conspecifies. Defended areas did not contain food or oviposition sites required by females. Territories were not uniformly distributed over the study area; the highest density of territories was clustered over bare ground covered with sand and small stones rather than areas of vegetation. This pattern of distribution of territories occurred during each year of the study. The patterns of occupancy of territories showed a high degree of consistency from one day to the next, some males defending a given territory for as many as 11 consecutive days. Removal experiments showed that a surplus of non-territorial males did not exist. Vacant territories in the high-density area were absorbed by neighboring males; those in the low-density area remained empty. Male size was not correlated with territory position in the study are. Wing patterning (i.e. size of spots) differed between males occupying territories on sandy/stony versus vegetated areas. The territorial and mating systems of O. chryxus resemble lek polygyny, and are more similar to some vertebrate lek systems than to hilltopping or landmark territoriality.     

Body morphologies shape territorial dominance in the satyrine butterfly <Emphasis Type="Italic">Lethe diana</Emphasis>

Male butterflies compete over mating territories via aerial interactions. It has often been suggested that flight performance, and thus flight-related traits, would play an important role in butterflies’ contest resolution; however, most studies have not demonstrated a clear relationship between these traits and territory ownership in butterflies. Males of the satyrine butterfly Lethe diana compete over territories via linear chases, a behavior that is uncommon among butterflies: rather, contests of most butterfly species consist of circling flights. In the present study, some morphological and physiological traits of L. diana were compared between territory owners and intruders to search for traits correlated with resource holding power (RHP). Territory owners had higher flight-muscle ratio (FMR) and were heavier than intruders, indicating that flight performance plays a key role in their contest resolution. Contest duration was not affected by the owners’ FMR or body mass, and thus I obtained no evidence supporting the hypothesis that contestants assess their relative RHP before making a decision to retreat. I speculate that the fact that these traits had an effect on territorial status in L. diana was a result of the characteristics of the contest behavior of this butterfly. During their contests, males L. diana fly faster in their linear flights than males of most other butterflies, which engage in slower circling flights. Therefore, males of L. diana probably need higher flight performance capabilities in order to win their contests.     

Experimental evidence that bright coloration is not important for territory defense in purple martins

Summary I tested the hypothesis that bright breeding plumage in territorial males acts as a badge of fighting ability or aggressive motivation to intimidate intruders. Territorial male purple martins (Progne subis) whose iridescent blue plumage was lightened to mimic the appearance of subadult males did not suffer an increase in intruder pressure or loss of territory compared with control males. Bright plumage color itself did not deter intruders and was not important for successful territory defense. Furthermore, a bright coloration of owners was not associated with an increased level of aggression toward intruders. Results from parallel studies on this species suggest that bright coloration is important in territory acquisition. The effectiveness of badges of fighting ability and aggressive motivation in territory defense is limited by whether intruders benefit from assessing these traits in owners. Differences in signaling systems between species are due in part to differences in floater tactics and the mode of territory acquisition.     

Food availability,intrusion pressure and territory size: an experimental study of Anna's hummingbirds (Calypte anna)

Theoretical considerations implicate food availability and intrusion pressure as important determinants of territory size, but empirical studies have led to contradictory conclusions about cause-and-effect relationships among these three variables. To investigate this problem, we provided patches of electronically controlled artificial flowers, which were defended by male Calypte anna. Food availability was experimentally manipulated, and intrusion rate and territory size were calculated from behavioral observations of the territory owner. Changes in both food availability and intrusion rate were found to be significantly correlated with changes in territory size under certain conditions. Intrusion rate, which was influenced by food availability, was negatively associated with territory size so long as food availability was high. This association persisted even after possible effects of food availability were controlled statistically. Food availability was negatively correlated with territory size only when intrusion rates were high and after owners had been defending territories for 3 days. As food availability and intrusion rate increased, owners increasingly restricted their defense to the patch itself; partial regressions revealed a significant association for intrusion rate but not food availability. When intrusion rate was low and food availability varied from low to high levels, no relationship was observed between food availability and territory size, apparently because of opposing influences of food abundance on territory size. Correspondence to: P.W. Ewald     

Mating system shift in a pronghorn population

Summary At the National Bison Range (western Montana, USA), pronghorn (Antilocapra american) maintained a territorial mating system from as early as 1965 through 1978. Preliminary observations in 1981 suggested that the mating system had changed. Data from the ruts, 1982–1984 revealed a progressive decay in territoriality. In this paper, data from the territorial years 1969–1978 are contrasted with data from the decay years 1982–1984, with results as follow: 1. In 1982–84, fewer territories were defended than in 1969–78. This was attributable to a smaller proportion of males defending territories in 1982–84, not to a smaller number of males. 2. In 1982–84, most territory owners either abandoned their territories early in ruts, or lost control of females on them following frequent, persistent intrusions by non-territorial males. Abandonment and loss of control did not occur in 1969–78. 3. In 1982–84, territorial males that maintained control of females on their territories did so by shrinking their zones of defense to small areas around female groups. 4. In 1982–84, following the disruption or severe disturbance of all territories, many females left territories, and mated elsewhere, with non-territorial males. In 1969–78, most females remained on territories throughout rut, and mated with territory owners. —The mating system change followed catastrophic winter mortality, 1978–79, that removed 75% of the males, including all males older than 5 years, and all male fawns, from the population. In 1982–84, the number of males present was not different from the number of males in 1969–1978, but the frequency distribution of male ages was strongly shifted toward younger ages. The small number of older males, 1982–84, likely resulted in smaller proportions of males initially defending territories, and in less effective territory defense. When females then clustered on the few territories where defense was at first successful, they attracted large numbers of non-territorial males. The resulting high rates of raids on these territories, coupled with reduced defense radii by territorial males, allowed females only 12% reclining time (summer percentage was 39%). This increased energy cost, plus an apparently greater risk of injury on weakly defended territories, appeared to prompt many females to seek calmer matings elsewhere. Also, if female pronghorn practiced mate selection based upon horn or body size, they may have reduced their efforts to remain on territories in 1982–84. Males from 1969–78 were larger than males from 1982–84, and showed greater variance in horn size. At least two conditions appear to influence the tendency of males to be territorial. First, males must be at least three years old before they attempt to defend a territory. Second, the declining proportion of males defending territories, 1982–84, that coincided with an increasing number of males three years and older, suggests that males also decide whether or not to attempt territory defense based upon the frequency of territorial defense in the population.     

Influence of territoriality on adult density in two rockfishes of the genus Sebastes

Nonterritorial Sebastes carnatus and S. chrysomelas existed, along with territorial individuals, at 3 tagging sites off southern California, USA, which were monitored for nearly 1 yr. To test the hypothesis that territoriality affected adult density in these species, territorial fish were removed and the subsequent utilization of vacated territories by other fish was monitored. Intrusion of other fish into vacated territories increased significantly in 90% of the removals. Other fish colonized both the feeding and sheltering parts of the vacated territories, indicating that the previous owners had successfully defended both parts of their territories. Many of the colonizers had already possessed territories; they expanded their territories or moved into presumably better havitat. Several previously-nonterritorial fish also moved into vacated areas, and at least some of them appeared to establish territories. These fish, then, had previously been capable of establishing territories, but were prevented from doing so by resident territory holders. Thus territoriality, rather than such other factors as predation or low recruitment, limited the number of territorial fish at each site. However, territorial fish did not inhibit the settlement of larval recruits, and the relative mortality rates of older territorial vs nonterritorial fish were not determined. Thus the question of whether territoriality was a major factor controlling total density remains unresolved.     

The effect of prey consumption on territorial defense by harriers: differential responses to neighbors versus floaters

Summary Food consumption may reduce fighting intensity of territory owners by decreasing resource value (additional food cannot be consumed) and/or increasing fighting costs (food in the digestive tract may raise injury risks). A territorial harrier's (Circus cyaneus, adult females) decision to reduce its level of aggression should depend upon whether or not the intruder was a competitor for individual prey items (as are smaller male floaters) or for the territory proper (as are female floaters and especially female neighbors). Accordingly, following meals, aggressive intensity of owners was strongly reduced towards male floaters (more were ignored), slightly reduced towards female floaters (more were called at than chased), and remained unchanged towards neighbors (virtually all were chased). Hence, alterations in aggressive behavior of territory owners following food consumption may depend upon the type of intruder and the resource under contest (a food item or a territory).     

Female-biased immigration and male peace-keeping in groups of the shell-dwelling cichlid fish Neolamprologus multifasciatus

The small cichlid fish Neolamprologus multifasciatus is endemic to Lake Tanganyika and is unique among fish because it lives in complex social groups with several reproductive males and females, all of which participate in defending their territory against neighbors and intruders. Individuals use empty snail shells for breeding and shelter. Previous parentage analysis using microsatellites suggested occasional exchange of individuals between groups. In field experiments, we found that females showed a higher tendency than males to migrate into territories already occupied by a resident pair. The phenomenon and causes of female-biased immigration were further investigated in aquarium experiments: Nine of 15 females, but only 2 of 15 equally sized males, settled in territories of established pairs. Territorial males exhibited more aggression toward strange males (potential reproductive competitors) than toward strange females (potential additional mates); their females were more aggressive toward strange females (probably competitors for shells) than toward strange males. Apparently, a conflict exists between the sexes regarding the immigration of additional females. This conflict seems to be the selective basis for observed active male interference in aggressive disputes between females in the territory. Interfemale tolerance in a group was greater in the male’s presence than in his absence. Received: 1 February 2000 / Received in revised form: 2 May 2000 / Accepted: 5 May 2000     

Territory defence in tropical birds: are females as aggressive as males?

Much of our knowledge concerning the functions of territorial behaviour and how territories are defended by individuals comes from research on birds. The vast majority of this work has focused on temperate zone breeding territoriality in which territories are defended most obviously by males. Our understanding of the female role in territory defence is limited because they are less conspicuous and much harder to observe. We studied sex roles in territory maintenance and defence in a duetting, resident neotropical passerine, the white-bellied antbird (Myrmeciza longipes). This species maintains territories and pair bonds year round and both sexes sing and actively participate in territory defence. We performed a series of playback experiments throughout the dry (non-breeding) and wet (breeding) seasons. We exposed territorial pairs to three types of stimuli including: (1) single sex, male only songs, (2) single sex, female only songs, and (3) both sex songs/duets. Contrary to findings for most other tropical species, individuals defended their territories with equal levels of aggression regardless of stimuli. Furthermore, sex roles were very different, with males responding more aggressively than females to all stimuli throughout both seasons. Both males and females consistently responded more aggressively to territorial intrusions during the dry season than during the wet season, likely because food abundance is low in the dry season and territory value is high. Our analysis of duetting behaviour suggests that duets do not serve a significant role in mate guarding, or territory defence.     

Habitat preference increases territorial defence in brown trout (Salmo trutta)

The striking ability of territory owners to repel intruders has generated a number of theoretical explanations as well as experimental studies in many animal species. However, effects of individual habitat preferences on territorial defence have rarely been studied. From the territory value hypothesis, we predicted that owners of preferred habitats should invest more resources in defence than owners of non-preferred habitats. We tested this prediction with young territorial brown trout in a two-stage experiment. First, trout were allowed to choose individually between gravel and a uniform bright substrate. As expected, they showed a significant (79%) preference for gravel. However, there was considerable variation between individuals in substrate preference, with a few fish preferring the bright substrate. Half of the tested fish were then transferred to a gravel substrate and the rest to a bright substrate, manipulating habitat type in relation to preference. Territory owners were then staged against size-matched intruders whereupon contest aggression was observed and the winner of each contest determined. Overall, owners won most of the contests. Satisfied owners won 86% and owners of less preferred territories, 74% of the contests. Furthermore, more satisfied owners attacked sooner and were more aggressive relative to the intruders. We conclude that brown trout show individual variation in habitat preference, which appears to be linked with their investment in territorial defence. These results suggest that understanding and modelling of animal contests could benefit from considering how territorial defence is influenced by individual habitat preference and specialisation. Received: 25 February 2000 / Revised: 22 May 2000 / Accepted: 25 June 2000     

Effectiveness of territorial polygyny and alternative mating strategies in northern fur seals, <Emphasis Type="Italic">Callorhinus ursinus</Emphasis>

We conducted a 6-year longitudinal behavioral and genetic investigation of a highly polygynous pinniped, the northern fur seal (Callorhinus ursinus), to determine the contribution of terrestrial polygyny to male fertilization success and to assess the occurrence of alternative mating strategies. Genetic samples from 37 adult males, 50 adult females, and 85 pups were collected and genotyped using five polymorphic microsatellite loci. Pup paternity was assigned using Cervus 2.0 at 99% confidence level. Paternity of 83 pups (98%) was assigned to terrestrial males who held territories or stayed temporarily in the study area during the breeding season when fertilization occurred. For 56 pups of which attendance records of their mothers were available, paternity of 45 pups (80%) was assigned to the associate males in whose territory their mothers stayed during the perioestrus period. In addition to defending breeding territories, territorial males have often been observed attempting to forcibly abduct adult females from adjacent territories (female stealing): We observed a total of 95 such cases, in which the stealers had significantly fewer females than the territorial males from whose territories they stole females. Our results indicate that terrestrial resource-defense polygyny is the major mating system in this species and that nonassociated paternity occurs mostly as a result of alternative mating strategies of less successful males. Male northern fur seals thus appear to adopt conditional alternative strategies that depend on their current social status to maximize their life-time reproductive success.     

Social influences on territorial signaling in male greater sac-winged bats

Acoustic territorial displays are common among birds but comparatively rare among mammals. An exceptionally vocal mammal well-known for its elaborate territorial displays is the polygynous greater sac-winged bat, Saccopteryx bilineata. Male S. bilineata are often philopatric and establish small territories in their birth colony in which females can roost during the day. During territorial defense, males produce complex territorial songs that are learned through vocal imitation. Territorial songs are mainly produced at dawn and dusk. We studied social influences on male vocal activity and the occurrence of vocal signatures in territorial songs of 27 male S. bilineata from 12 different-sized colonies in Panama. Males produced significantly more territorial songs when they had more territorial neighbors or when they had females roosting in their territories, indicating that male vocal activity rises with increasing male–male competition. Territorial songs are multisyllabic vocalizations with low-frequency buzz syllables being most prominent. We found statistical evidence for a pronounced individual signature encoded in the buzz syllables of territorial songs that could facilitate individual recognition among rival neighbors. Additionally, we found a vocal group signature in territorial songs, suggesting that young males may learn territorial songs from more than one tutor male. Resident male S. bilineata appear to cooperatively defend their colony against male intruders, making a group signature in territorial songs potentially advantageous.     

Interference asymmetries among age-sex classes of rufous hummingbirds during migratory stopovers

Summary Three age-sex classes of rufous hummingbirds (Selasphorus rufus) overlap temporally and defend feeding territories during migratory stopovers in the Sierra Nevada of California. We demonstrate that these classes differ in their ability to secure and maintain high-quality feeding territories for refueling, and that these differences result in differences in resource use. Data on acquisition of territories, territory characteristics, and responses of territory owners to intruders suggest that several mechanisms are involved in determining dominance, involving sex- and age-related differences in wing disc loading, coloration, and experience. We discuss the implications of these results for understanding intraspecific variation in migration strategies. Correspondence to: F.L. Carpenter     

Defending young biparentally: female risk-taking with and without a male in the burying beetle, <Emphasis Type="Italic">Nicrophorus pustulatus</Emphasis>

A positive correlation between the parental effort of a male and female should promote stable biparental care. Risk-taking (as assessed by injuries) against infanticidal intruders by Nicrophorus pustulatus females was expected to be low when females had a low probability of successful defense of the young. I tested the hypothesis that when the presence of a male partner increased the probability of successful defense from low to moderate that female risk-taking would increase. Single females and pairs with first instar larvae were confronted by potentially infanticidal male and female conspecific intruders. Male intruders routinely took over nests from unpaired females (30 of 36 trials). Unpaired females and male intruders were injured infrequently, indicating less intense fights despite the high probability of infanticide. A resident female defending against a male intruder was injured more often when paired than unpaired, suggesting greater risk-taking. A male parent that delays desertion, therefore, receives fitness benefits not only from his own defense of the young, but from greater female defense against male intruders as well. It is hypothesized that the threat of infanticidal takeovers by males promotes extended biparental care in burying beetles. When the intruder was female, on the other hand, a female parent on her own had a moderate probability of successfully defending the brood (22 of 36 trials). The presence of a male partner against female intruders almost guaranteed successful defense (35 of 36 trials) and female intruders did not appear to contest pairs vigorously. Against female intruders the presence of a male partner did not significantly change injury rates of the defending female.     

How reliable are behavioral cues for assessment of male mating status in polyterritorial wood warblers,Phylloscopus sibilatrix?

It has been suggested that polyterritoriality in birds, i.e. mated males defending two spatially separated territories, deceives females into accepting polygyny against their best interest. Mated males singing on secondary territories should make it difficult for new arriving females to assess the mating status of unmated and mated males advertising to females. However, there have been objections to the deception hypothesis, suggesting that differences in territorial behavior between males of different mating status may make correct assessment possible. Polyterritorial males frequently leave their secondary territory to visit the female in the primary territory, thereby revealing their mating status to females in search of mates. The aim of this study is to investigate how reliable behavioral cues are in assessing the mating status of males in polyterritorial wood warblers. Our study shows that singing activity of mated males on secondary territories and unmated males was similar in the morning, while there was a significant difference in the afternoon. Unmated males spent, on average, more time on their territories than mated males spent on their secondary territories. However, there was considerable variation in both unmated and mated males in the frequency in which they left their territories. For instance, unmated males left their territories quite frequently early in the season compared to later. Hence, females must be able to interpret singing behavior and a male leaving his territory differently depending on time of the season and time of the day in order to correctly assess the mating status of advertising males. Estimating the increase in probability of finding an unmated male by using behavioral cues showed that females could increase the probability of finding an unmated male compared to random choice with respect to mating status. The uncertainty was, however, still considerable. The most important factor affecting the probability of finding an unmated male was the proportion of mated males on secondary territories compared to unmated males. Our study suggests that presence on a territory is not a reliable cue for the assessment of male mating status in polyterritorial wood warblers.     

The effect of familiarity with a neighborhood on territory acquisition

Summary Several authors have suggested that familiarity with the neighborhood around a territory might improve the ability of contestants to compete successfully for that territory. In this study, juvenile Anolis aeneus lizards residing in clearings were allowed to observe, but not enter, artificial homesites for 9 to 12 days. Then the homesites were made accessible to both previous residents and new arrivals to the clearing.The settlement success of new arrivals onto the homesites was dramatically lower than that of comparably-sized previous residents. For both new arrivals and previous residents, the period from first entry to first received attack was an excellent predictor of settlement success, but whereas most previous residents spent many hours on the homesites prior to being attacked by other juveniles, almost all new arrivals were, attacked soon after entering the homesites. Many new arrivals made tactical errors (e.g. joined larger lizards already on the homesites, and then were promptly chased), whereas previous residents almost never joined lizards larger than themselves. In addition, new arrivals which were alone on the homesites were more quickly attacked by neighboring individuals than were previous residents under comparable circumstances. These results suggest that information about a territorial habitat and its inhabitants is an important determinant of territory acquisition success in this species.