…that I couldn’t fit in the last post. Here goes:

1) Fire ants, despite their name, are waterproof.

If you read my last post you might have watched a video showing fire ants using their bodies to create a living raft, allowing the whole colony to survive a flood. So it won’t be too surprising to learn that fire ants are waterproof. But they aren’t just waterproof, they’re water-repellent. In fact they’re not just water-repellent, but a paper by Mlot et al shows that they’re actively water-repellent – linking their bodies together in a configuration which increases the water-repellent properties of the raft as a whole. This cooperative behaviour increases raft stability making it virtually impossible to sink a fire ant raft. Unless you put the raft in soapy water (in which case they sink – verified experimentally by Mlot et al), rafts can stay afloat for months on end!

Fire ant rafts are so water repellent that even the ants on the bottom of the raft do not drown. The water actually bends around their interlocked bodies, forming a sort of bubble around them. Even ants which break through the water tension of this bubble are able to trap pockets of air around their bodies which stop them from drowning. This trapped air is actually essential to making ants buoyant, and without it they sink. Amazing!

From Mlot et al’s paper.

2) Ant song

It’s been known for a long time that ants communicate via chemical pheromones, but many species also communicate by making (quite frankly, adorable) sounds. Check out this heart wrenching fire ant distress call. The sounds are made by rubbing two parts of their abdomen together in a process that people in the know apparently call ‘stridulation’. Dr Robert Hickling has more recordings of ants stridulations on his website – I think I’ve found a new ringtone!

Sound can be used to communicate specific information over shorter ranges. Leaf cutter ants use it to tell each other how nutritious a particular leaf is, helping other workers decide which leaves to take first. Ants ‘chirp’ when they are panicked or alarmed, such as if they become buried and need someone to help unearth them.

Until recently it was thought that only adult ants produce sound, but recent research by Casacci et al shows that pupae in some species use sound to communicate too. Pupal “stridulation” sounds were observed to cause changes to adult worker behaviour, such as moving toward the pupae, adopting the ‘guard’ posture and digging nearby. Casacci also found that stridulating pupae were preferentially evacuated first if the nest was disturbed. It seems that growing pupae make demands of the adults to improve their survival.

Sound is so central to the day-to-day running of a colony, that some ant parasites have evolved to exploit it. The parasitic caterpillar stage of the ‘cuckoo’ Masculinae butterfly first uses chemical signals to trick Myrmica ants into thinking it is part of the colony. It then mimics the sounds the queen makes, thus convincing worker ants to feed it.


3) Ant policing

Platythyrea punctata is a species of ant in which the workers can lay eggs – producing genetically identical workers without the need for a male. Genetic recombination is an extremely rare in this species, and since there is usually just one egg-laying worker in each colony, the whole colony is typically made up of clones.

The colony actively police the reproductive monopoly of the established egg-layer by attacking any other workers which start laying eggs and removing or eating their eggs. This behaviour is a little confusing, because if the whole colony is made up of clones there should be no competition between individual workers for reproduction. Furthermore, policing the colony uses energy – attacking the colony’s workforce or destroying eggs is a waste of resources. There must be an evolutionary advantage to the behaviour which offsets this cost.

Two possibilities exist. There may be a colony-wide advantage to having only one reproducing individual, by allowing regulation of the colony population level). Alternatively there may be an advantage at the individual level, for the individuals which are able to become the established egg-layer.

Elizabeth Brunner and her lab conducted an experiment to try to find out more. Platythrea punctata colonies were split into two groups and the groups were isolated from one another. This causes a new reproductive worker to emerge in the group which doesn’t already contain one. Later, the two groups were reunited, causing the reproductive workers (both of them) to be attacked. Finally, both reproductive workers were removed leaving the colony orphaned, until a new reproductive worker emerges.

By observing which workers became the new reproductive worker in the final phase, and how aggressively each worker reacted to the egg-layers, Brunner found that the ants that most likely to attack surplus reproductive workers were the ones which are most likely to become the next reproductive worker themselves. This suggests an individual level advantage with competition between workers. The colonies sometimes adopt new workers from other nests, so perhaps being the worker with a monopoly on reproduction infers an advantage on one sub-section of the colony. More research is probably needed, but either way I can’t help thinking there’s a dystopian Disney film in here somewhere.

Thanks antweb.org!
This is a very dead looking Platythrea punctata. Maybe it is the latest victim of draconian ant policing? Thanks antweb.org!

I promise to stop writing about ants now.

Cover picture is of a Platythyrea punctata taken from the wonderful antweb.

References and links

Cool ant documentary “Planet Ant – Life inside the colony”. Watch it here: https://www.youtube.com/watch?v=8n0SkIGARuo

  1. Mlot, N. J., Tovey, C. A., & Hu, D. L. (2011). Fire ants self-assemble into waterproof rafts to survive floods. Proceedings of the National Academy of Sciences, 108(19), 7669-7673. Available here.
  2. Casacci, L. P., Thomas, J. A., Sala, M., Treanor, D., Bonelli, S., Balletto, E., & Schönrogge, K. (2013). Ant pupae employ acoustics to communicate social status in their colony’s hierarchy. Current Biology, 23(4), 323-327.
  3. Barbero, F., Bonelli, S., Thomas, J. A., Balletto, E., & Schönrogge, K. (2009). Acoustical mimicry in a predatory social parasite of ants. The Journal of experimental biology, 212(24), 4084-4090.
  4. Brunner, E., Kellner, K., & Heinze, J. (2009). Policing and dominance behaviour in the parthenogenetic ant Platythyrea punctata. Animal Behaviour, 78(6), 1427-1431.