Our study shows that ergatoid males benefit from attacking emerging ergatoid rivals as soon as possible after eclosion, even if the latter do not yet engage in sexual activities. Older males consistently won fights against males of an age of one day or less, whereas they only survived 43% of the fights against two-day old opponents (Figure ). Very young males hardly showed any fighting behaviour themselves (Figure ). Their cuticula is not yet fully sclerotized and does not provide any protection against bites by older individuals (SC and AS, unpublished observation). Furthermore, the softness of their mandibles does not allow freshly emerged males to attack others. In contrast, two-day old ergatoid males were similarly active as older males and equally likely to initiate or win a fight. Encounters among older males led to pronounced fighting, in which occasionally both males were killed. By attacking freshly emerged ergatoid males, adult ergatoids therefore remove future competitors at the lowest possible risk. Old males that are unable to quickly detect an emerging rival male have a
50% chance of being killed and replaced by a younger male.
Earlier eclosing males appear to have a competitive advantage over those that eclose later also in other species with fatal fighting and local mate competition (LMC), e.g.
, parasitoid wasps [6
]. In addition, differences in body size and previous fighting experience may affect the outcome of fights in parasitoids [25
] and several insects without LMC [26
]. However, these two factors appear to be much less important in fights between ergatoid males of C. obscurior
than the timing of emergence (AS and SC, unpublished data).
Obviously, the advantage of early emergence results in a conflict of interest over the timing of the detectability of ergatoid males. On the one hand, it pays for an adult ergatoid to recognize and remove competitors as early as possible, as eliminating pupae is presumably even safer than killing freshly emerged ergatoids. On the other hand, young ergatoid males would benefit by concealing their identity until their cuticula is sclerotized and they have a reasonable chance of surviving a confrontation with an older male. Ergatoid males almost incessantly patrol through the nest and examine the brood pile for freshly eclosed ergatoids and young female sexuals [30
]. Our behavioural analysis revealed that ergatoid males did not differentiate between the different types of unpigmented pupae several days prior to emergence, but showed increased contact to close-to emergence (dark) female sexual and winged male pupae (Figure ). Also, whereas ergatoid males only showed antennation (i.e.
]) behaviour towards young, unpigmented pupae several days before emergence, they were occasionally seen attempting to mate with female sexual and winged male pupae and attacking ergatoid male and also winged male pupae (Figure ). Attacks against pupae were weak compared to encounters between adults and more intense behavioural observations will be required to determine how frequently these interactions occur and whether workers may kill besmeared ergatoid male pupae. Nevertheless, it appears that the conflict of interest has been decided in favour of the adult ergatoids.
The observed differences in ergatoid male behaviour towards unpigmented early pupae and dark pupae shortly before emergence may be based on their chemical signatures. We found that the cuticular hydrocarbon profiles of dark, ready-to-eclose pupae were more similar to those of adults than the profiles of white pupae (Figure ). The chemical profile of dark pupae might therefore allow ergatoid males to determine the destiny of the emerging ant with at least some accuracy. Still, differences in the chemical profiles between ant types are likely not large enough to allow faultless determination. The costs of misidentifying and inadvertently killing a female sexual or worker pupa might explain why ergatoid males of C. obscurior did not kill ergatoid male pupae. This may be additionally pronounced by the fact that the chemical signal of pupae was also quantitatively less intense than that of adults: for GC analyses, we had to pool five pupae to obtain similar quantities of cuticular hydrocarbons as from single adults.
It has been shown previously that winged males of C. obscurior
perform a chemical female mimicry that protects them against attacks of ergatoid males in the early days after emergence before they leave the colony [18
]. Moreover, they adjust the timing of dispersal from the nest both to the availability of mating partners in the nest and the presence of ergatoid fighter males [33
]. This might suggest that winged males may become a target for ergatoid male aggression once their chemical similarity to female sexuals fades [33
]. In the present study, we found some indication that ergatoid males can already be lured to attempt copulation with dark winged male pupae prior to emergence. Whereas adult C. obscurior
ants are characterised by a large set of 44 cuticular hydrocarbons [19
], only 11 peaks overlapped between the pupal and adult profiles and were thus analysed in the current study. A comparison between the one-day old adults of the current study revealed that these shared 11 peaks are not identical with those peaks thought to be responsible for the chemical female mimicry [18