- Factors regulating signal composition and task allocation in colonies of the ant Myrmicaria eumenoides (2004)
- The integrity of social insect colonies is facilitated by nestmate recognition. A defining characteristic of social insect colonies is their elaborate division of labour system. Cuticular hydrocarbons (CHCs) have long been implicated in nestmate recognition. In the African ant Myrmicaria eumenoides they also appear to vary with task. The factors regulating the task allocation (i.e. division of labour), as well as the CHC profiles (i.e. signalling) are still not well understood. Both external and internal (endocrine) factors have both been postulated in the past. Most of the evidence to-date suggests that juvenile hormones (JHs) and ecdysteroids are the prime endocrine control candidates. The aim of this project is to investigate how signalling and task allocation are dynamically governed in M. eumenoides colonies. Firstly, the CHC profiles of individual workers in relation to their task (behavioural development phase) over their life span are assessed. Secondly, the influences of the two endocrinological factors (JHs, ecdysteroids) hypothesised to play a role in task allocation, as well as signalling, are investigated. 1) The behavioural transition from callow to brood-tender, and on to forager was shown to be continuous and to change in a synchronous manner in all individual M. eumenoides focus workers. In addition, all workers were shown to go through the “reserve” phase (characterised by a high degree of inactivity) before changing to foraging activities. 2) The CHC profiles of all individual workers were shown to change parallel to the behavioural development transition noted above. Overall, the CHC profile change is only quantitative, and characterised by a decrease in short-chain, and a simultaneous increase in long-chain components, as well as an increase in saturated, and a decrease in unsaturated CHCs. 3) Direct behavioural evidence, and the strong correlation of signalling with task suggests that in M. eumenoides the CHC profiles inform task decisions. Only five components were shown to contribute more then 90% to the variance between brood-tender-type and forager-type CHC profiles. This suggests that perhaps “task signalling” requires only a few CHCs. These may be of a saturated and/or unsaturated nature, but must be of a high or medium high relative concentration. 4) The levels of ecdysone and 20-hydroxyecdysone (found to be the main free ecdysteroids in workers), as well as the sum of the total free ecdysteroids in workers, are only high after eclosion, and then fall to a low, steady level within two weeks. Thus no correlation was found between the above and signalling, or task. 5) Interestingly, JH I was found (only) in the haemolymph of female M. eumenoides alates: this is the first time this JH homologue has been identified in the Hymenoptera. It is hypothesised that JH I plays a role in either flight or migratory behaviour, and is thus absent from worker haemolymph. 6) Foragers were demonstrated to have significantly higher JH III titres than brood-tenders. Exogenous JH III and JH analogue application however did not result in an observable change in, or acceleration of, the behavioural development of workers. This may however be due to the much reduced longevity of the applied workers, and the hence decreased observational period. 7) The changes from a brood-tender-type to a forager-type CHC profile are accelerated in a concentration-dependent manner by the application of JH III, and JH analogue. This appears to take place independent of task changes, resulting in e.g. brood-tending workers that display clear forager-type CHC profiles. Thus these results demonstrate for the first time that signalling in Hymenoptera is strongly influenced by JH. Several possible pathways for the action of JH III are discussed. 8) Finally, it is suggested that workers assess the colony needs using CHCs as simple, localised cues. The CHCs would thus play a key role in efficiently regulating a colony’s behaviour. Several promising approaches were also detailed with respect to further work on this topic.