The dear enemy effect or dear enemy recognition is an ethological phenomenon in which two neighbouring territorial animals become less aggressive toward one another once territorial borders are well established.Fisher, J., {1954}. Evolution and bird sociality. In: Evolution As a Process (Huxley, J., Hardy, A. and Ford, E., eds). London, Allen and Unwin. pp. 71-83 As territory owners become accustomed to their neighbours, they expend less time and energy on defensive behaviors directed toward one another. However, aggression toward unfamiliar neighbours remains the same. 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".Bee, M.A., (2003). A test of the "dear enemy effect" in the strawberry dart-poison frog (Dendrobates pumilio). Behavioral Ecology and Sociobiology, 54: 601-610
The dear enemy effect has been observed in a wide range of animals including mammals, birds, reptiles, amphibians, fish and invertebrates. It can be modulated by factors such as the location of the familiar and unfamiliar animal, the season, and the presence of females.
The effect is the converse of the nasty neighbour effect, in which some species are more aggressive towards their neighbours than towards unfamiliar strangers.
Function
The ultimate function of the dear enemy effect is to increase the individual fitness of the animal expressing the behaviour. This increase in fitness is achieved by reducing the time, energy or risk of injury unnecessarily incurred by defending a territory or its resources (e.g., mate, food, space) against a familiar animal with its own territory; the territory-holder already knows about the abilities of the neighbour, and also knows that the neighbour is unlikely to try to take over the territory because it already has one.
Mechanism The interaction between two neighbours can be modelled as an iterated prisoner's dilemma game. In this view, a territory owner that acts non-aggressively towards a neighbour can be thought of as cooperating, while a territory owner that acts aggressively towards its neighbour can be considered to have defected. A necessary condition for the prisoner’s dilemma game to hold is that an aggressive individual should enjoy greater benefits than a non-aggressive individual when each is faced with a non-aggressive opponent. This stipulation is plausible, as an aggressive individual might enlarge their territory or steal food or matings from a non-aggressive individual. When cooperation involves a cost, a possible mechanism for achieving stable co-operation is reciprocal altruism, where pairs of individuals trade bouts of cooperative behaviour with one another. Dear enemy cooperation could be explained by reciprocal altruism if territorial neighbours use conditional strategies such as tit for tat. In the tit-for-tat strategy, a subject will cooperate when its partner (neighbour) cooperates and defect when the partner defects.Hyman, J., (2002). Conditional strategies in territorial defense: do Carolina wrens play tit-for-tat? Behavioral Ecology, 13: 664-669. DOI: 10.1093/beheco/13.5.664
Occurrence
In mammals
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. When the mounds containing the scents were allowed to remain overnight and the beavers' responses measured the following morning, the beavers' responses were stronger to both castoreum and anal gland secretion from a stranger than from a neighbour.Rosell, F. and Bjørkøyli, T. (2002). A test of the dear enemy phenomenon in the Eurasian beaver. Animal Behaviour, 63: 1073–1078
Eurasian badgers (Meles meles) can discriminate between self-, neighbour- and unfamiliar- group faeces near their main sett. Badgers show heightened behavioural responses towards unfamiliar- compared with self-group scents, but there is no difference in response to neighbour- relative to self-group scents. The relative responses towards unfamiliar-group scents are greatest during the breeding seasons, but there is no seasonal differences in the responses to neighbour-group versus self-group scents. In badger populations, levels of aggression between neighbouring territory-holders are likely to be kept relatively low through neighbour recognition. 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.Palphramand1, K.L. and White, P.C.L., (2007). Badgers, Meles meles, discriminate between neighbour, alien and self scent. Animal Behaviour, 74: 429–436
Red squirrels are able to discriminate the odours of familiar neighbours and strangers.Vaché, M., Ferron, J. and Gouat, P., (2001). The ability of red squirrels (Tamiasciurus hudsonicus) to discriminate conspecific olfactory signatures. Canadian Journal of Zoology, 79: 1296-1300 It has been suggested that this discrimination may be used by males to avoid unnecessary chases and fights by becoming known to their neighbours.
In birds
Audio playback studies are often used to test the dear enemy effect in birds. 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),Lovell, S.F. (2004). Neighbor-stranger discrimination by song in a suboscine bird, the alder flycatcher, Empidonax alnorum. Behavioral Ecology, 15: 799-804 DOI: 10.1093/beheco/arh082 male blue grouse,Falls, J.B. and McNicholl, M.K., (1979). Neighbor-stranger discrimination by song in male blue grouse. Canadian Journal of Zoology, 57: 457-462 European robin (Erithacus rubecula),Brindley, E.L., (1991). Response of European robins to playback of song: neighbor recognition and overlapping. Animal Behaviour, 41: 503-512 and male banded wren (Thryothorus pleurostictus).Mollesf, L.E. and Vehrencamp, S.L., (2001). Neighbour recognition by resident males in the banded wren, Thryothorus pleurostictus, a tropical songbird with high song type sharing. Animal Behaviour, 61: 119–127
Neighbouring male song sparrows (Melodia melospiza) differ individually in their aggressiveness. Increased aggression by residents towards intruders indicates that residents not only respond to intrinsic aggressiveness of their neighbours, but also to short-term changes in aggression levels.Akçay, C. et al., (2009). Good neighbour, bad neighbour: song sparrows retaliate against aggressive rivals. Animal Behaviour, 78: 97–102
Although neighbour–stranger discrimination has been reported in many passerine birds, it has seldom been investigated in territorial non-passerine species. The nocturnal raptor, the little owl (Athene noctua), hoots to defend its territory. Male little owls respond less to their neighbour's hoots played back from the usual location. However, responses to playback of a neighbour from an unusual location are similar to responses to playback of a stranger's hoots from either location.Hardouin, L.A., Tabel, P. and Bretagnolle, V., (2006). Neighbour–stranger discrimination in the little owl, Athene noctua. Animal Behaviour, 72: 105–112
During the breeding season of the skylark (Alauda arvensis), particular common sequences of syllables (phrases) are produced by all males established in the same location (neighbours), whereas males of different locations (strangers) share only few syllables. 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.Briefer, E., Aubin, T., Lehongre, K. and Rybak, F., (2008). How to identify dear enemies: the group signature in the complex song of the skylark Alauda arvensis. Journal of Experimental Biology, 211: 317-326. DOI: 10.1242/jeb.013359 Studies have shown that the dear enemy effect changes during the breeding season of the skylark. Playbacks of neighbour and stranger songs at three periods of the breeding season show that neighbours are dear enemies in the middle of the season, when territories are stable, but not at the beginning of the breeding season, during settlement and pair formation, nor at the end, when bird density increases due to the presence of young birds becoming independent. In song sparrows, where neighbours are most often the sires of extra-pair offspring, males will alter their aggression toward neighbouring males with their female's fertility status. When presented with simulated stranger and neighbour intruders during their female's pre-fertile and post-fertile periods, males displayed the dear enemy effect. 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. Thus, the dear enemy relationship is not a fixed pattern but a flexible one likely to evolve with social and ecological circumstances.Briefer, E., Rybak, F. and Aubin, T., (2008). When to be a dear enemy: flexible acoustic relationships of neighbouring skylarks, Alauda arvensis. Animal Behaviour, 76: 1319–1325
In reptiles
Males of a territorial lizard, the tawny dragon (Ctenophorus decresii), reduced their aggression levels in repeat interactions with familiar rivals and increased their aggression levels towards unfamiliar males. The time taken for interactions to be settled was also lower towards familiar than unfamiliar males.Osborne, L., (2005). Rival recognition in the territorial tawny dragon (Ctenophorus decresii). Acta Ethologica, 8: 45-50
Another territorial lizard, the common collared lizard (Crotaphytus collaris), can individually recognize neighbours and will increase aggression towards them as the threat to territorial ownership increases. Resident males treat familiar neighbours that had been moved to the opposite boundary to the shared boundary as equally aggressive as strangers. However, residents responded more aggressively towards strangers than towards neighbours on natural territories and also in neutral arena encounters.Husakf, J.F. and Fox, S.F., (2003). Adult male collared lizards, Crotaphytus collaris, increase aggression towards displaced neighbours. Animal Behaviour, 65: 391–396
In the brown anole lizard (Anolis sagrei), dyads of males behave differently depending on whether the lizards are prior neighbours, with prior neighbours exhibiting less bobbing relative to nodding forms of headbob displays than non-neighbours.McMann, S. and Paterson, A.V., (2012). Display behavior of resident brown anoles (Anolis sagrei) during close encounters with neighbors and nonneighbors. Herpetological Conservation and Biology, 7(1): 27−37
The Iberian wall lizard (Podarcis hispanicus) has been observed to be less aggressive towards conspecifics. It also exhibits headbob activity similar to that of the brown anole.
In amphibians