Abstract
BackgroundDiscriminating threatening individuals from non-threatening ones allow territory owners to modulate their territorial responses according to the threat posed by each intruder. This ability reduces costs associated with territorial defence. Reduced aggression towards familiar adjacent neighbours, termed the dear-enemy effect, has been shown in numerous species. An important question that has never been investigated is whether territory owners perceive distant neighbours established in the same group as strangers because of their unfamiliarity, or as dear-enemies because of their group membership.Methodology/Principal FindingsTo investigate this question, we played back to male skylarks (Alauda arvensis) songs of adjacent neighbours, distant neighbours established a few territories away in the same microdialect area and strangers. Additionally, we carried out a propagation experiment to investigate how far skylark songs are propagated in their natural habitat and we estimated repertoire similarity between adjacent neighbours, distant neighbours and strangers. We show that skylarks, in the field, respond less aggressively to songs of their distant and likely unfamiliar neighbours, as shown by the propagation experiment, compared to stranger songs. The song analysis revealed that individuals share a high amount of syllables and sequences with both their adjacent and distant neighbours, but only few syllables and no sequences with strangers.ConclusionsThe observed reduction of aggression between distant neighbours thus probably results from their familiarity with the vocal group signature shared by all members of the neighbourhood. Therefore, in skylarks, dear-enemy-like relationships can be established between unfamiliar individuals who share a common acoustic code.
Highlights
The ability to discriminate potential territory and/or mate usurpers from non-threatening individuals enables territory owners to modulate their territorial responses according to the threat posed by each intruder
The observed reduction of aggression between distant neighbours probably results from their familiarity with the vocal group signature shared by all members of the neighbourhood
In skylarks, dear-enemy-like relationships can be established between unfamiliar individuals who share a common acoustic code
Summary
The ability to discriminate potential territory and/or mate usurpers from non-threatening individuals enables territory owners to modulate their territorial responses according to the threat posed by each intruder. We investigated vocal distant neighbour recognition in a songbird with a large repertoire (average repertoire size per male = 340 syllables [8]) and a complex song, the skylark Alauda arvensis, in its natural environment To study this process, we considered for the first time the three following related problems: 1) how territory owners respond to songs of their adjacent neighbours, of their distant neighbours and of strangers; 2) how far skylark songs are propagated in their natural habitat and 3) what is the degree of repertoire sharing between individuals. Discriminating threatening individuals from non-threatening ones allow territory owners to modulate their territorial responses according to the threat posed by each intruder This ability reduces costs associated with territorial defence. An important question that has never been investigated is whether territory owners perceive distant neighbours established in the same group as strangers because of their unfamiliarity, or as dear-enemies because of their group membership
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