Instinct vs learning in the initiation of a species-specific caterpillar-ant mutualism

Ants are formidable predators, yet a number of species engage in mutualistic associations with nectar-producing hemipteran and lepidopteran insects.  The mechanism by which mutualistic associations are initiated has received little attention and studies have been restricted to facultatively myrmecophilous insects that associate with a broad range of ants.  Results indicate that tending is initiated via associative learning as ants are rewarded with honeydew from their trophobiotic partner. A minority of myrmecophiles, however, have obligate, species-specific associations with ants. Consequently, ants may instinctively recognize these partners as mutualists. To test this hypothesis, we compared tending and aggressive responses of one attendant and three non-attendant ant species toward caterpillars of an obligately ant-associated butterfly, Jalmenus evagoras (Lycaenidae).  We recorded interactions in the laboratory for 10 min at 0, 2 and 24 h after introducing naïve ants to a final instar caterpillar.  I. gracilis, which naturally tends J. evagoras in the wild, spent more time in contact with caterpillars than did non-attendant ant species, whereas I. purpureus and Froggatella kirbii, which do not naturally tend J. evagoras but do associate with other Jalmenus species, spent more time tending J. evagoras than did I. rubriceps, a species not known to associate with lycaenids in the wild.  No ant species significantly increased its tending duration of caterpillars over the 24 h period, but I. gracilis and I. rubriceps decreased their aggression toward caterpillars over time.  We conclude that, relative to non-attendant ant species, I. gracilis shows some instinctive tending of J. evagoras caterpillars and that associative learning may account for decreased aggression toward these mutualistic partners, but does not result in increased attendance.