Long Live the Queen – Ants Respond Innovatively to Rising Waters

Social insects have developed weird and wonderful strategies to overcome daily struggles. One important group of these social insects are ants. Found throughout the world, ants have developed a myriad of fascinating behaviours that continue to promote their success in most ecosystems. Chemical trailing, formic acid biting, fungus gardening, leaf cutting, stem bridging, the list goes on and on. A new study published in PLoS One describes a fascinating behaviour of ants that is particularly topical to the Oxford audience – flood evasion.

Led by Jessica Purcell – the study examined a species by the name of Formica selysi, commonly found within flood plains throughout Europe. When these ants are exposed to rising water levels, a collective behaviour occurs where ants physically link together tarsus-tarsus, or mandibles-tarsus connections. This ant-conglomerate floats on top of the rising waters, and functions like a raft. Previous studies have indicated that ant brood (larvae and pupae) are positioned at the raft base, which led previous authors to hypothesize that brood were more buoyant. Purcell and her team tested this hypothesis, while also asking questions about raft architecture and determining whether flood-exposure paid a cost to the brood.

Ants coming together to form a raft. Photo from publication; taken by D. Galvez.

Purcell’s team developed an elegant experiment which examined the functional geometry of the rafts. By manipulating original population composition in a laboratory setting, ants were exposed to flooding conditions and filmed. They found that workers are distributed throughout the raft, while queens remained ‘high and dry’ in the centre position, at the highest tier available. Meanwhile, the brood were placed by workers at the raft base. These raft placements were constant between treatments.

The second experiment looked at the ability of worker ants to survive flooding conditions. This was completed by placing ants within a water filled tube, inverted within a larger column of water. Surprisingly, the ants were very successful in recovering with 79% survival after eight hours of submergence. Although survival was high, it took an average of more than an hour for submerged workers to begin freely moving.

Brood were experimentally allocated to either 3-hours of rafting conditions or a control treatment of ambient lab conditions. The ants were then moved into separate containers, and provided ample food and water. Neither treatment included a queen, so the final number of brood eclosing could be measured through a count of adults. After the colonies had run their course, the total number of ants were counted. No significant differences were found between eclosion rates of brood subjected to flooding conditions, compared to the control group. This suggests low cost paid by brood in rafting, despite unfavourable placement at the bottom of the raft.

Purcell and her team also completed an experiment to better understand differences in the buoyancy of colony members. Larvae, pupae and workers were removed from each of the field colonies. The workers and brood were then placed into solutions with increasing concentrations of detergent. The addition of detergent reduces surface tension, making ants more likely to sink as detergent concentration increased. This sink-or-float experiment supported that brood were much more buoyant than adult workers, further reinforcing their importance at the raft base.

By provisioning brood along the raft base – workers (who are impressively scrappy to begin with), can reduce or avoid the cost of a slow recovery. By giving the queen a first class ticket, her likelihood of being harmed during the flood is mitigated. As the brood are not affected by the flooding, life can continue on happily providing the ants float ashore in proximity to a suitable new colonisation site. The faster the workers are able to begin constructing a new home, the more likely the colony will survive. These findings support that rafting greatly improves the fitness of a colony during flooding events.

Those who have a plan and react quickly are likely have a leg-up in event of a catastrophe. This is certainly the case with the resilient F. selsyi, further supporting cooperation as a useful strategy in prevailing through life’s challenges.

References:
Purcell, J., Avril, A., Jaffuel, G., Bates, S., & Chapuisat, M. 2014. Ant Brood Function as Life Preservers during Floods. PLoS ONE.