Population density has profound influences on the physiology and behaviour of
many animal species. Social isolation is generally reported to lead to
increased aggressiveness, while grouping lowers it. We evaluated the effects
of varying degrees of isolation and grouping on aggression in a territorial
insect, the Mediterranean field cricket, Gryllus bimaculatus. Substantiating
early observations, we show that dyadic contests between weight-matched, adult
male crickets taken from groups rarely escalate beyond threat displays,
whereas interactions between pairs of previously isolated crickets typically
escalate to physical fights lasting several seconds. No significant
differences were found between 1, 2 and 6-day isolates, or between individuals
grouped for a few hours or lifelong. Unexpectedly, crickets grouped in
immediate proximity within individual mesh cages that precluded fighting while
permitting visual, olfactory and mechanical, antennal contact, were as
aggressive as free isolates. This suggests that reduced aggression of grouped
animals may be an acquired result of fighting. Supporting this notion,
isolated crickets initially engage in vigorous fights when first grouped, but
fighting intensity and duration rapidly decline to the level of life-long
grouped crickets within only 10 min. Furthermore, grouped crickets become as
aggressive as life-long isolates after only 3 hours of isolation, and on the
same time course required for crickets to regain their aggressiveness after
social defeat. We conclude that the reduced aggressiveness of grouped crickets
is a manifestation of the loser effect resulting from social subjugation,
while isolation allows recovery to a state of heightened aggressiveness, which
in crickets can be considered as the default condition. Given the widespread
occurrence of the loser effect in the Animal Kingdom, many effects generally
attributed to social isolation are likely to be a consequence of recovery from
social subjugation
