Why Do Helpers Help? :Lessons from the Eusocial Insects

By Azita Chellappoo

Eusocial insects are some of the most successful animals on the planet. They include ants, bees, wasps and termites, and can form massive colonies with intricate social structures and systems of labour division to rival human societies.Yet at first glance, eusocial insect societies appear to be a fly in the ointment of Darwin’s theory of natural selection. If individuals are selected to maximise their reproductive success, how can you explain eusocial insect workers, which are often sterile? Kin selection has solved this puzzle. Individuals can maximise their overall fitness by foregoing their direct fitness (their own reproduction) for an increase in their indirect fitness (the reproductive success of their kin discounted by their relatedness). If their indirect fitness exceeds the reproductive success they could achieve on their own, they will help their kin instead of reproducing themselves. Kin selection has resulted in most individuals in eusocial insect societies devoting their energy to working for the good of the colony. However, the question of in what circumstances this becomes selected for, and how this is maintained is still a hot topic. Additionally, the creation of these eusocial societies creates new sources of conflict, and there is much we still don’t know about how this conflict manifests and how it is resolved.

Image

Leafcutter Ants

Eusocial insect societies are not all alike. They range from primitive societies, such as the allodapine bees, which can be as small as four individuals living inside a twig, to highly derived societies, such as the honeybee, which can have colonies of up to 60,000 worker bees and strict castes with morphological specialisations. Studying primitive societies can be helpful in shedding light on how eusociality evolved, as it is likely that at least some of the challenges they face are those that were faced in the initial transition to eusociality (which has happened independently multiple times).

Primitive societies have dominance hierarchies, rather than strict castes. Often all individuals can reproduce, but most don’t. In this situation we would expect conflict over breeding: in highly derived societies, individuals are constrained from birth to be either workers or queens, thus resolving any potential conflict. In some societies this conflict never arises due to relatedness: in a society with a singly mated female breeder who is the mother of the nonbreeders, nonbreeders are equally related to her offspring (their siblings) as they are to their own. There is therefore no benefit to overthrowing the breeder. In addition, other nonbreeders would prefer to rear the current breeders offspring (their siblings) than the offspring of a nonbreeder who overthrew the breeder (their nieces or nephews), and therefore would try to stop the overthrow1.

In other primitive societies we do see actual conflict. When the relatedness between the breeding and nonbreeding individuals is less straight forward, due to factors such as multiple mating (which decreases the relatedness between sisters), there are more benefits to replacing the breeder. For example, when the numbers of queens are low in some polygynous epiponine wasps, more females mate than are needed2. In this case, coercion is an effective form of conflict resolution, where the excess is reduced via queen-queen competition or elimination by workers.

Given the potential for conflict over breeding, why would individuals forego breeding to help in the first place? One reason is the “insurance” advantage to helping. When adult mortality is high, and rearing a brood takes a long time, if one individual dies the surviving members can rear the brood to maturity. This preserves some or all of the helpers’ investment. Facultatively eusocial species are useful to study, as under what conditions they become eusocial may shed light on the initial transition. In many such species we see high adult mortality: for example, the rate of lone foundress failure before the emergence of adult offspring was found to be 38-100% among 19 species of polistine wasps3.

A key underlying assumption of the insurance explanation is that when a helper dies, the rest will continue to rear the brood that would have been reared if the helper was there (thereby preserving their investment). One study set out to test this claim in a tropical hover wasp4. They removed two helpers from some multi-female nests, with some unaltered multi-female and lone-female nests as controls. They found that significantly more large larvae were reared to maturity by the altered nests than by the controls of equivalent (post-removal) size, and this was in fact the same as controls of pre-removal size. The large larvae represented 93% of the overall brood weight, so most of the helpers’ investment was preserved. The authors suspect the mechanism to be a combination of increased worker recruitment and the sacrifice of the least valuable brood (feeding them to the larger larvae!).

Image

 Tropical Paper Wasps

Eusocial insects provide a useful model to learn about aspects of kin selection, as well as other areas (such as ageing – finding out why termite queens live for decades and termite workers live for days could unlock the secret to living longer). However, these complex and beautiful societies are deserving of understanding in themselves: the more we learn about the weird and wonderful organisms that populate the world around us and the processes that shaped them, the more awe and wonder we have at the “endless forms most beautiful and most wonderful [that] have been, and are being, evolved” – Charles Darwin.

[1] Hart, A. G., & Ratnieks, F. L. W. (2005). Crossing the taxonomic divide: conflict and its resolution in societies of reproductively totipotent individuals.Journal of evolutionary biology, 18(2), 383-395.

[2] Ratnieks, F. L., Foster, K. R., & Wenseleers, T. (2006). Conflict resolution in insect societies. Annu. Rev. Entomol., 51, 581-608.

[3] Queller, D. C. (1996). The origin and maintenance of eusociality: the advantage of extended parental care. Natural history and evolution of paper-wasps, 218-234.

[4] Field, J., Shreeves, G., Sumner, S., & Casiraghi, M. (2000). Insurance-based advantage to helpers in a tropical hover wasp. Nature, 404(6780), 869-871.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s