Dear enemy effect

[2] Some authors have suggested the dear enemy effect is territory residents displaying lower levels of aggression toward familiar neighbours compared to unfamiliar individuals who are non-territorial "floaters".

[3][4] The dear enemy effect has been observed in a wide range of animals including mammals, birds, reptiles, amphibians, fish and invertebrates.

Dear enemy cooperation could be explained by reciprocal altruism if territorial neighbours use conditional strategies such as tit for tat.

[6] Territorial Eurasian beavers (Castor fiber) presented with a two-way choice sniffed both castoreum and anal gland secretion from a stranger longer than from a neighbour.

Furthermore, beavers responded aggressively—standing on the mound on their hind feet, pawing, overmarking, or a combination of these—longer to castoreum, but not to anal gland secretion, from a stranger than from a neighbour.

However, increased levels of aggression will be shown towards dispersing or itinerant (alien) badgers, especially during periods such as the breeding season when the potential threats to the long-term fitness of territory owners are greatest.

These studies have demonstrated several bird species respond more aggressively to played back songs of strangers than to songs of neighbours; such species include the alder flycatcher (Empidonax alnorum),[10] male blue grouse,[11] European robin (Erithacus rubecula),[12] and male banded wren (Thryothorus pleurostictus).

Playback experiments provided evidence for neighbour–stranger discrimination consistent with the dear enemy effect, indicating that shared sequences were recognised and identified as markers of the group identity.

When presented with simulated stranger and neighbour intruders during their female's pre-fertile and post-fertile periods, males displayed the dear enemy effect.

[17] However, when presented with simulated stranger and neighbour intruders during their female's fertile period, males exhibited an equal response to both stimuli, likely in order to protect their paternity.

Males of the territorial breeding agile frog (Rana dalmatina), have a large variability in call characteristics and are able to discriminate between neighbouring and unfamiliar conspecifics.

[22] Terrestrial red-backed salamanders, Plethodon cinereus, defend territories under rocks and logs on the forest floor in the eastern United States.

[24] Some researchers have staged three-way contests between male convict cichlids (Cichlasoma nigrofasciatum) to examine the dear enemy effect.

[25] Individual recognition of noises produced males of the bicolor damselfish (Pomacentrus partitus) have been demonstrated in the field.

In laboratory experiments, the frequency and severity of agonistic interactions among workers from different colonies increases with the distance between their nests; this has been reported for Leptothorax nylanderi[27] and Pheidole ants.

[30] Male sand fiddler crabs (Uca pugilator) defend territories that consist of a breeding burrow and a display area where they wave their claw to attract females.

This is opposite to the dear enemy phenomenon and suggests that neighbouring females pose a greater threat than strangers in this species.