Sociality favours the adjustment to ecological niches and facilitates the spread of a species. Group living is therefore widespread, from insects to mammals, but differs in the degree of cooperation and altruism. In subsocial species (e.g. giant water bugs, several spiders or fish), the parents stay with their offspring for some time until the latter start to reproduce independently. In cooperatively breeding birds (e.g. Southern Ground-Hornbill) or mammals (e.g. mongooses), sociality reaches a higher level with older siblings or other group members helping to raise offspring.
The highest form of social cooperation is eusociality, which is rare in mammals (only naked-mole rats), but abundant in two insect orders: Hymenoptera (e.g. bees, wasps, ants) and Blattodea (termites). The intense cooperation between individuals in these species might be the cause of the evolutionary and ecological success of eusocial insects, which can make up to 75% of a habitat’s animal biomass.
The organisation of eusocial insect colonies
In general, colonies of eusocial Hymenoptera consist of overlapping generations of related, female individuals, whereas males are often only produced seasonally for mating. Most females, the “workers”, refrain from reproduction or are completely sterile. They care for other colony members and the brood, supply them with food, and maintain or defend the nest. In many species these tasks are distributed by age: the younger workers conduct work within the nest, while the older ones take over the risky tasks outside the nest. The reproductive queens do not participate in such non-reproductive tasks but are responsible for the production of new offspring. They often differ from the workers in body size and morphology.
The fitness of insect societies mainly depends on the efficiency of interactions between reproductives and helpers, as the reproductive task is not exchangeable in many species. This makes ants a suitable model organism to study the interrelations between individual and group characteristics.
Unpredictable social or environmental changes, like, e.g., destruction of the nest, might afford adaptions regarding the task allocations. Ants of our study species Cardiocondyla obscurior found new nests via budding, which means that a few individuals – e.g. workers, together with brood and queen(s) – leave the maternal nest to initiate a new colony. In this case, workers might have to react flexibly to new conditions and possibly have to switch from outer to inner nest tasks and vice versa. Although studies in bees and ants have confirmed a flexible task allocation, its effect on the performance and survival of individuals has rarely been studied. In our study we wanted to test whether a forced task shift has an effect on colony growth and fitness.
Our experimental set-up
In our study, we created new colonies that consisted of only young (freshly eclosed) or only old (12 weeks old) workers, together with either a single young or old queen. This setup forced a part of the young workers to leave the nest precociously to forage, while several old workers had to revert from foraging to brood care.
We then examined the effect of queen and worker age on colony growth and task performance by estimating the number of produced brood and offspring and its quality (weight) during a complete colony lifecycle.
Our experiment showed that colonies perform much better when queens and workers are of similar age. Task performance of a worker did not mainly depend on its chronological age but on group composition.
Surprisingly, lifespan appears to be a very plastic trait in ants and is strongly affected by colony demography. The event of colony founding seems to reset the age of the workers and indicates that chronological and physiological age are not necessarily linked. Twelve-week old workers that were exposed to the condition of a newly founded nest with a new queen and without brood outlived workers that eclosed in the same situation by several weeks. A similar effect of social environment was also observed in queens.
Why observe ageing in ants?
Aging and senescence are ubiquitous phenomena throughout multicellular life and have fascinated humanity for centuries. Although many studies have focussed on the mechanisms underlying aging, its proximate and ultimate causes are still not completely understood.
The ant Cardiocondyla obscurior provides an ideal model organism for ageing in social insects. It appears that the lifespan of workers is more affected by colony stage and demography than by chronological age and that workers do not suffer strongly from an age-related decline of performance.
Furthermore, queens of this species not only outlive their conspecific sterile nestmates (workers) by weeks, despite sharing the same genome, but in contrast to other animals they also do not suffer from reduced survival after an increased investment in reproduction.
Negative senescence and the effects on physiological age still raise many questions. This is why our team at the University of Regensburg, together with colleagues in a DFG-funded research unit (“Sociality and the reversal of the fecundity-longevity trade-off,” FOR 2281), focuses on reproduction and ageing in social insect queens.