The dynamics of male-male competition in Cardiocondyla obscurior ants

Abstract

Background: The outcome of male-male competition can be predicted from the relative fighting qualities of the opponents, which often depend on their age. In insects, freshly emerged and still sexually inactive males are morphologically indistinct from older, sexually active males. These young inactive males may thus be easy targets for older males if they cannot conceal themselves from their attacks. The ant Cardiocondyla obscurior is characterised by lethal fighting between wingless ("ergatoid") males. Here, we analyse for how long young males are defenceless after eclosion, and how early adult males can detect the presence of rival males.

Results: We found that old ergatoid males consistently won fights against ergatoid males younger than two days. Old males did not differentiate between different types of unpigmented pupae several days before emergence, but had more frequent contact to ready-to-eclose pupae of female sexuals and winged males than of workers and ergatoid males. In rare cases, old ergatoid males displayed alleviated biting of pigmented ergatoid male pupae shortly before adult eclosion, as well as copulation attempts to dark pupae of female sexuals and winged males. Ergatoid male behaviour may be promoted by a closer similarity of the chemical profile of ready-to-eclose pupae to the profile of adults than that of young pupae several days prior to emergence.

Conclusion: Young ergatoid males of C. obscurior would benefit greatly by hiding their identity from older, resident males, as they are highly vulnerable during the first two days of their adult lives. In contrast to the winged males of the same species, which are able to prevent ergatoid male attacks by chemical female mimicry, young ergatoids do not seem to be able to produce a protective chemical profile. Conflicts in male-male competition between ergatoid males of different age thus seem to be resolved in favour of the older males. This might represent selection at the colony level rather than the individual level.

Figure 1

Fight characteristics depending on the age of ergatoid males of the ant Cardiocondyla obscurior. A) Fights were initiated more frequently by the older male when their opponent had eclosed on the day of fighting (old-d0) or was one day old (old-d1) than when the young male was two days old (old-d2). B) Biting was performed only by the attacking male when the opponent was less than one day old (old-d0), whereas both males attacked when the young male was older (old-d1 and old-d2). C) Fights were won exclusively by the older male when the rival was younger than two days (old-d0 and old-d1), yet in interactions between an old and a two-day old male, both males had similar chances of winning. Some fights resulted in the death of both males. Small italic letters indicate significance groups of the white/grey bars.

Figure 2

Male smearing and worker attacks in male-male fights of the ant Cardiocondyla obscurior. A) The frequency of observed male smearing performed by the later winner of the fight was independent of the age of the two males. Besmearing activity by the later loser increased with the age of the young male. B) Worker attacks towards the later loser of the fight did not vary with the age of the two opponents. The occurrence of worker biting against the later winner of the fight increased with the age of the young male. Bars represent median values and whiskers the 25% and 75% quartiles. Small italic letters indicate significance groups for significant overall differences (black bars in A and white bars in B).

Figure 3

Contact of adult ergatoid males towards pupae of all colony members in the ant Cardiocondyla obscurior. A) Ergatoid males did not preferentially contact any type of pupae when all pupae where white and thus several days prior to emergence. B) Dark, ready-to-eclose pupae of workers and ergatoid males were contacted less frequently than female sexual pupae, as well as winged male pupae. Bars represent mean values and whiskers the standard error of the mean. Small italic letters indicate significance groups for the dark pupae that showed significant overall differences.

Figure 4

Mating and fighting behaviour of ergatoid males towards dark pupae in the ant Cardiocondyla obscurior. Ergatoid males performed copulation attempts towards dark female sexual and winged male pupae, which were close to emergence. Sporadic biting and smearing was observed against ergatoid male pupae and also against winged male pupae.

Figure 5

Chemical analysis of cuticular hydrocarbon profiles of pupae and adults of the ant Cardiocondyla obscurior. One-day old adults of C. obscurior ants were clearly separated by discriminant analysis from the pupae. Ready-to-eclose dark pupae showed a more similar chemical profile to adults than still unpigmented white pupae several days before eclosion.