Neighborhood bully: no difference in territorial response toward neighbors or strangers in marmots

Territorial animals are expected to adjust their response to intruders according to the perceived threat level. One of the factors that drives threat level is the identity of the intruder. The dear enemy phenomenon theory postulates that individuals should respond with lower intensity to neighbors, already possessing a territory, than to strangers that may fight to evict them. In social species, the hierarchical status of the intruder might also mediate this response. Such behavioral adjustments presuppose a capacity to discriminate between individuals posing different threat levels. Here, we tested the behavioral response of Alpine marmots to territorial intrusions in a wild population. We compared both dominant females’ and males’ responses to scents from neighbor and stranger dominant males (dear enemy phenomenon) and to dominant and subordinate stranger males (social status-specific response). In addition, we tested for any covariance between male scents and social status. We showed that female and male dominant marmots do not adjust the intensity of their behavioral responses to whether the intruder’s territory is bordering or not (neighbors or strangers) or to the intruder’s social status, even though dominant and subordinate males are thought to pose different threats and social status is encoded in scents. Thus, we did not find support for the dear enemy phenomenon and conclude instead that, in dominant Alpine marmots, no intruder should enter a foreign territory. Research taking a more holistic approach of the evolution and maintenance of territoriality is required to understand the flexibility of responses to intruders in group-living species.

Dear Enemy Phenomenon in the Ant Ectatomma brunneum (Formicidae: Ectatomminae): Chemical Signals Mediate Intraspecific Agressive Interactions

The integrity of ant colonies depends exclusively on social relationships between their individuals, especially the ability of communication between group members, which is mainly mediated through chemical signals. Another important feature of ant behavior is territory defense, since they need to gather large amounts of food to feed their larvae, males and breeding females. Thereby, ants might display behavioral strategies to defend their territories from intruders. Here we investigated whether Ectatomma brunneum displays the Dear Enemy Phenomenon, what is the relationship between Cuticular Hydrocarbon composition and levels of aggression during their intraspecific interactions and which compounds and/or classes of compounds might be the most important to modulate the level of aggression. To test our hypothesis, we evaluated the levels of aggression through behavioral observations during interactions between 23 pairs of colonies nested in two distinct sites at varied distances. Then, we analyzed the cuticular chemical profile of the individuals involved in the interactions, and compared these results with the levels of aggression displayed between colonies tested. The results allow us to confirm our hypothesis that the DEP occurs in E. brunneum. The higher tolerance between closer colonies can be explained due to their kinship level in addition to sharing the same microhabitats. The results also showed there are significant differences in CHCs profiles, especially between colonies nested at relatively greater distances, and it is likely that differences in content of some branched alkanes are the most important to establish these differences and, therefore, the levels of aggression during the interactions.

Individual variation in the dear enemy phenomenon via territorial vocalizations in red squirrels

Territoriality arises when the benefits of resources exceed the costs of defending them. The dear enemy phenomenon, where familiar territorial neighbours refrain from intruding on one another and mutually reduce their defensive efforts, allows for reduction of these costs but requires discrimination between conspecifics. We hypothesized that territorial vocalizations in red squirrels (Tamiasciurus hudsonicus) are used for this discrimination. We performed a speaker replacement experiment where red squirrels (n = 41) were temporarily removed from their territories and replaced with a speaker broadcasting their own call, an unfamiliar call, or silence. Contrary to our prediction, there were no differences in overall intrusion risk among our three playbacks, but the identity of intruders did vary. Existing variation in familiarity within territorial neighbourhoods should be considered, rather than the binary classification of familiar or stranger, when studying dear enemy effects. We also discuss the variable importance of silence in acoustic territorial populations.

Individual variation in the dear enemy phenomenon via territorial vocalizations in red squirrels

Territoriality arises when the benefits of resources exceed the costs of defending them. The dear enemy phenomenon, where familiar territorial neighbours refrain from intruding on one another and mutually reduce their defensive efforts, allows for reduction of these costs but requires discrimination between conspecifics. We hypothesized that territorial vocalizations in red squirrels (Tamiasciurus hudsonicus) are used for this discrimination. We performed a speaker replacement experiment where red squirrels () were temporarily removed from their territories and replaced with a speaker broadcasting their own call, an unfamiliar call, or silence. Contrary to our prediction, there were no differences in overall intrusion risk among our three playbacks, but the identity of intruders did vary. Existing variation in familiarity within territorial neighbourhoods should be considered, rather than the binary classification of familiar or stranger, when studying dear enemy effects. We also discuss the variable importance of silence in acoustic territorial populations.

Aggression towards unfamiliar intruders by male lizards Eurolophosaurus nanuzae depends on contestant’s body traits: a test of the dear enemy effect

A reduction of territory owners’ aggression towards their neighbours in relation to the intrusion of strangers characterises the dear enemy phenomenon. Supposedly, the disparity in aggression levels of territory owners is due to a higher threat imposed by strangers compared to the threat imposed by neighbours. To evaluate the occurrence of the phenomenon in males of the small-sized lizard Eurolophosaurus nanuzae we performed a field manipulative study. We considered three models to run intrusions in males’ territories: neighbour, tailed stranger (unfamiliar) and tailless stranger intruders. Our results lend support to the presence of dear enemy for this species as residents acted more aggressively towards strangers than to neighbours. In addition, the information we provide supports the relative threat hypothesis as territory owners were more aggressive towards tailed stranger intruders than to tailless stranger intruders. In this sense, tail condition can represent a trait that signals the ‘resource holding power’ (RHP) of a lizard. Therefore, we show that beyond neighbourhood recognition, residents are able to evaluate the potential threat of stranger intruders in general, thereby extending the evolutionary gains of the dear enemy by saving energy even in the context they are expected to acts with higher costs.

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.

Absence of kin discrimination in the hatchlings of a lizard, Calotes versicolor (Agamidae)

Laboratory born Calotes versicolor hatchlings were reared for 15 days in pure or mixed sibships or singly in isolation and then tested for their kin discrimination ability. As C. versicolor orients itself visually, visual displays like push-ups, gular extensions, and distance between the test individuals were used as indicators of arousal/aggression towards the opponents (familiar vs. familiar/unfamiliar sib or non-sib). The hatchlings exhibited aggressiveness towards unfamiliar sibs as well as non-sibs in the form of push-ups and gular extensions; they also remained at a longer distance from each other. In contrast, they did not exhibit such behaviors against familiar individuals regardless of their relatedness, and stayed closer to each other. In tests involving once familiar individuals, sib, or non-sib reared together for 15 d from hatching and then separated for 7 or 30 d, the lizards showed aggressiveness towards each other following 30 d separation. A greater number of push-ups and gular extensions were exhibited by the test individuals after 30 d separation compared to those separated for 7 d. Thus, in this non-social lizard, there is no kin discrimination. An early dispersal of hatchlings in this species may have led to a loss of kin discrimination. However, familiar individuals are recognized as long as they continue to remain familiar thereby suggesting a ‘dear enemy phenomenon’.