Patrolling the area, not ousting intruders, relates to reproductive success for territorial male Leon Springs pupfish, Cyprinodon bovinus

We considered the relationship between the benefits and costs of territorial defense in a wild population of the Leon Springs pupfish, Cyprinodon bovinus. We defined benefit as the number of eggs deposited on an artificial substratum placed within the defender's territory. Costs included two defensive behaviors. First, males frequently "patrolled" their territories, swimming back-and-forth across their area. Second, males chased intruding Pecos gambusia (Gambusia nobilis) as well as small male and female conspecific C. bovinus from their territories. Both of these species prey on the territorial defenders' eggs; additionally, small male C. bovinus will attempt to "steal" spawns from the territorial defender by spawning with females in the territory. Our analyses revealed that only patrol frequency was related to the reproductive benefit of the territory. Neither chases against gambusia nor conspecifics were predicted by egg numbers on the breeding substrata. We speculate that the frequency of patrolling is an indicator of territorial value and note the qualitative differences in chasing behavior against the different species of intruder.

Vocal behaviour reveals asymmetries in neighbour relationships in a semi-colonial raptor, the Eurasian Scops Owl Otus scops

Territorial animals often reduce aggression towards familiar neighbours compared to unfamiliar conspecifics. However, variation in the response to different neighbours is less known. In this work, I examined the territorial behaviour of male scops owls during countersinging interactions with two familiar neighbours and I asked whether vocal behaviour of the focal male reflected dear-enemy relationships. Analysis revealed that the focal male’s vocal frequency was associated with (1) the degree of instability of the territory boundary shared with a neighbour and (2) the motivation to persist in the dyadic interaction with that neighbour. Patterns of movement directed to specific individuals suggest that scops owls do discriminate between neighbours. A case of partial territory takeover was observed that was accompanied by temporal changes in vocal frequency in one of the opponents, confirming that vocal frequency is a flexible, context-dependent feature of the relationship of neighbouring scops owls.

Kinship, dear enemies, and costly combat: The effects of relatedness on territorial overlap and aggression in a cooperative breeder

Many species maintain territories, but the degree of overlap between territories and the level of aggression displayed in territorial conflicts can vary widely, even within species. Greater territorial overlap may occur when neighboring territory holders are close relatives. Animals may also differentiate neighbors from strangers, with more familiar neighbors eliciting less-aggressive responses during territorial conflicts (the "dear enemy" effect). However, research is lacking in how both kinship and overlap affect territorial conflicts, especially in group-living species. Here, we investigate kinship, territorial overlap, and territorial conflict in a habituated wild population of group-living cooperatively breeding birds, the southern pied babbler Turdoides bicolor. We find that close kin neighbors are beneficial. Territories overlap more when neighboring groups are close kin, and these larger overlaps with kin confer larger territories (an effect not seen for overlaps with unrelated groups). Overall, territorial conflict is costly, causing significant decreases in body mass, but conflicts with kin are shorter than those conducted with nonkin. Conflicts with more familiar unrelated neighbors are also shorter, indicating these neighbors are "dear enemies." However, kinship modulates the "dear enemy" effect; even when kin are encountered less frequently, kin elicit less-aggressive responses, similar to the "dear enemy" effect. Kin selection appears to be a main influence on territorial behavior in this species. Groups derive kin-selected benefits from decreased conflicts and maintain larger territories when overlapping with kin, though not when overlapping with nonkin. More generally, it is possible that kinship extends the "dear enemy" effect in animal societies.

Stranger danger: A meta-analysis of the dear enemy hypothesis

The dear enemy hypothesis predicts that territorial individuals will be less aggressive toward known neighbors than to strangers. This hypothesis has been well studied and there is a wealth of data demonstrating its prevalence in some taxa. However, a quantitative synthesis is needed to test the generality of the phenomenon, identify key mechanisms driving the behavior, and guide future research. In this study, we conduct a meta-analysis and, we test the importance of the location of intrusion, the type of experiment conducted (field, laboratory, or neutral arenas), and the sex and breeding status of territory holders, on the occurrence of dear enemy behavior. We also test how various ecological and life history traits, such as territory type, stimulus of intrusion, and taxonomic group, affect the magnitude of dear enemy behavior. We find that this phenomenon is common and that taxonomic class and breeding status are correlated with the expression of dear enemy behaviors. Further, we found that the way authors measure aggression influences the likelihood of identifying dear enemy responses, and thus we discuss potential pitfalls of dear enemy studies. Considering this conclusion, we discuss future lines of inquiry that could more directly examine the mechanisms of the dear enemy phenomenon.

Testing the dear enemy relationship in fiddler crabs: Is there a difference between fighting conspecific and heterospecific opponents?

Reduction of aggressiveness toward familiar neighbors, when compared to aggressiveness toward unfamiliar strangers, can decrease the costs of territory defense. This phenomenon is known as the "dear enemy effect". Individuals may shift their aggressiveness toward neighbors or strangers from the same or different species, depending on the relative threat associated with different opponents. Therefore, a reduced level of aggressiveness between heterospecific neighbors is expected in relation to conspecific intruders, since the latter compete not only for territory, but also for mates. Herein we investigated the occurrence of the dear enemy effect in territorial fights between conspecific pairs of Leptuca leptodactyla and heterospecific pairs of L. leptodactyla versus Leptuca uruguayensis. Across both conspecific and heterospecific fights, medium- and high-intensity fight components were more used in resident-stranger than in resident-neighbor fights. Thus, residents showed a dear enemy response, regardless of opponent species. Moreover, conspecific fights induced a greater number of low- and medium-intensity fight components than did fights between heterospecifics, both neighbors and strangers. Finally, conspecific resident-stranger fights took longer than heterospecific resident-stranger fights. Our results indicate that fiddler crabs adjust their territorial response according to the species and resident status of intruders, consistent with the risks posed by different intruder types.

A dear enemy relationship in a territorial cichlid: evidence for the threat-level hypothesis

Despite competing for resources such as space, food and mates, many territorial animals are less aggressive towards neighbours who rarely go beyond their territorial boundaries. This so-called dear enemy phenomenon is advantageous in territorial defence, but it has not been well studied in fish. In this work, we tested the ‘correct–incorrect boundary paradigm’ of the dear enemy phenomenon using the territorial cichlid fishNeolamprologus pulcher, which exhibits dear enemy relationships. When the fish was placed in a small experimental tank, in which fish established its territory, it was initially very aggressive against a neighbouring fish in an adjacent tank, but the aggression level decreased rapidly (within 4 days). When the tank containing the neighbour was shifted to the opposite side, the focal fish was more aggressive than the day before, but it exhibited less aggression than it did against a stranger placed on the shifted side. This lower level of aggression suggested that the focal fish did not regard the shifted neighbour as a stranger. Our observations provide support for the threat-level hypothesis, according to which territory owners will modulate aggression intensity based on the threat level.

Dear Enemies Elicit Lower Androgen Responses to Territorial Challenges than Unfamiliar Intruders in a Cichlid Fish

In many territorial species androgen hormones are known to increase in response to territorial intrusions as a way to adjust the expression of androgen-dependent behaviour to social challenges. The dear enemy effect has also been described in territorial species and posits that resident individuals show a more aggressive response to intrusions by strangers than by other territorial neighbours. Therefore, we hypothesized that the dear enemy effect may also modulate the androgen response to a territorial intrusion. Here we tested this hypothesis in male cichlid fish (Mozambique tilapia, Oreochromis mossambicus) using a paradigm of four repeated territorial intrusions, either by the same neighbour or by four different unfamiliar intruders. Neighbour intruders elicited lower aggression and a weaker androgen response than strangers on the first intrusion of the experiment. With repeated intrusions, the agonistic behaviour of the resident males against familiar intruders was similar to that displayed towards strangers. By the fourth intrusion the androgen response was significantly reduced and there was no longer a difference between the responses to the two types of intruders. These results suggest that the dear enemy effect modulates the androgen response to territorial intrusions and that repeated intrusions lead to a habituation of the androgen response.

Mating patterns of female Leon Springs pupfish Cyprinodon bovinus

In a field study of Leon Springs pupfish Cyprinodon bovinus, two questions about female promiscuity were investigated. First, were females selective in the males with whom they spawned or were they unselective, spawning randomly among males? Second, how promiscuous were the females, i.e. with how many males did they spawn? If simply spawning with many males maximized a female's reproductive success, then females might be expected to spawn randomly with as many males as possible. Alternatively, if females were selective but engaged in multiple mating, they would limit their spawning to preferred males. In the only wild population of this endangered fish, breeding males defend closely associated territories in the shallow margins of a single desert pool. No territories were observed elsewhere in the pool. Therefore, all territorial males were present simultaneously and females could survey all of them, depositing any number of eggs with one, a few or many males. Rather than spawning randomly, females surveyed many males first, visited relatively few males and ultimately spawned with a small fraction of those available males. With increasing numbers of spawns, however, females increased the number of different mates with whom they spawned. Thus, females showed a bet-hedging tactic of having a narrow mate preference while also laying eggs in the territories of other males, possibly to reduce egg predation and to avoid inbreeding.

Differential attack by a cichlid fish on resident and non-resident fish of another cichlid species

Abstract

An algivorous cichlid, Variabilichromis moorii( Vm), defends permanent territories in Lake Tanganyika, Africa. A zoobenthivorous cichlid, Neolamprologus mustax( Nm), spends 60% of daylight hours foraging in Vmterritories, from which other zoobenthivorous fishes are chased out and consequently which are much richer in prey animals than areas outside of Vmterritories. We conducted a field experiment to examine whether Nmresidents and non-residents received different degrees of attacks from Vm. Nmfish were caught in their territories, released at a point distant from these territories, and followed to observe interactions with Vmfish. The frequency of attacks received by the displaced Nmfish was greater than attacks received by Nmresidents, indicating that Nmresidents had easier access to Vmterritories than non-residents did. A possible mechanism for this is reduced aggression of Vmtowards Nmresidents, as a result of the ‘dear enemy’ effect that has been reported in territorial contests between rivals. An alternative mechanism is that tolerance towards Nmdiffers among Vmfish and Nmresidents selectively visit more tolerant Vmfish due to previous experience while non-residents randomly approach both tolerant and hostile Vmfish. The ability of Vmto discriminate between Nmresidents and non-residents is essential to the former mechanism but not to the latter. To more specifically examine which mechanism works in the Vm– Nmcommensal system, we will need to follow individually identified Vmfish interacting with Nmresidents and non-residents.

How to identify dear enemies: the group signature in the complex song of the skylark Alauda arvensis

Song geographic variation and Neighbour–Stranger (N–S)discrimination have been intensively but separately studied in bird species,especially in those with small- to medium-sized repertoires. Here, we establish a link between the two phenomena by showing that dialect features are used for N–S recognition in a territorial species with a large repertoire, the skylark Alauda arvensis. In this species, during the breeding season, many pairs settle in stable and adjoining territories gathered in locations spaced by a few kilometres. In a first step, songs produced by males established in different locations were recorded, analyzed and compared to identify possible microgeographic variation at the syntax level. Particular common sequences of syllables (phrases) were found in the songs of all males established in the same location (neighbours), whereas males of different locations (strangers) shared only few syllables and no sequences. In a second step, playback experiments were conducted and provided evidence for N–S discrimination consistent with the dear-enemy effect,i.e. reduced aggression from territorial birds towards neighbours than towards strangers. In addition, a similar response was observed when a `chimeric'signal (shared phrases of the location artificially inserted in the song of a stranger) and a neighbour song were broadcast, indicating that shared sequences were recognized and identified as markers of the group identity. We thus show experimentally that the shared phrases found in the songs of neighbouring birds constitute a group signature used by birds for N–S discrimination, and serve as a basis for the dear-enemy effect.

'Nasty neighbours' rather than 'dear enemies' in a social carnivore

Territorial animals typically respond less aggressively to neighbours than to strangers. This 'dear enemy effect' has been explained by differing familiarity or by different threat levels posed by neighbours and strangers. In most species, both the familiarity and the threat-level hypotheses predict a stronger response to strangers than to neighbours. In contrast, the threat-level hypothesis predicts a stronger response to neighbours than to strangers in species with intense competition between neighbours and with residents outnumbering strangers, as commonly found in social mammals such as the banded mongoose (Mungos mungo). The familiarity hypothesis predicts reduced aggression towards neighbours also in these species. We exposed free-living banded mongoose groups to translocated scent marks of neighbouring groups and strangers. Groups vocalized more and inspected more samples in response to olfactory cues of the neighbours than to the strangers. Our results support the threat-level hypothesis and contradict the familiarity hypothesis. We suggest that increased aggression towards neighbours is more common in social species with intense competition between neighbours, as opposed to reduced aggression towards neighbours typical for most solitary species.

Changing tactics: dominance, territoriality, and the responses of "primary" males to competition from conditional breeders in the variegated pupfish (Cyprinodon variegatus)

Using variegated pupfish, we examined the flexibility in "primary" male tactics when dealing with the aggressive costs of competition. Analogous to conditional mating strategies, we expected primary males to exhibit one of two interrelated tactics (i.e. dominance or territoriality) in response to different numbers of competitors. In the field, competitors influenced aggression. Primary males defended territories; residents facing more intruders engaged in more chases and obtained fewer spawns per female. In the laboratory, primary males showed dominance at low density, controlling most of the aquarium. With increasing competitor numbers, primary males reduced the area controlled and defended territories. Territories occurred with intermediate to high competitor numbers and only under male-biased sex ratios. During these interactions, aggression was highest and competitors were too many for dominant males to suppress the assertions made by each subordinate to increase its rank. Relinquished control of the entire domain enabled a previously subordinate male to establish a territory in the undefended portion of the aquarium. Reduction in defended area related to a reduction in territorial males' spawning success relative to dominants. These results suggested that primary males, like conditional breeders, would adopt the tactic that enabled them to spawn despite the constraints of competition.