Altruistic Ants

Life Story describes an epic struggle for survival in a weaver ant colony. Hunting parties take down prey many times larger than themselves. A jumping spider deceives the ants, sneaking past colony guards. Raiders from an adversarial colony attack, attempting to “expand their empire.” The battle concludes with the image of a single warrior ant perched atop the end of a twig, surveying a land littered with the bodies of ant warriors. How does this complex society of coordination, communication, and self-sacrifice work to the adaptive benefit of Weaver ants?

Weaver ants live in large colonies of up to a half a million individuals. Colonies can even cover several trees (3). They exhibit many cooperative social behaviors, one of the most conspicuous being nest construction with larval silk. The workers take over the cocoon spinning activity of the larvae to build a nest (3, 8).

Complex communication systems are an important adaption for these ants. Weaver ants communicate with a variety of multimodal signals, including vibrations, stridulating, visual displays, motor displays (5), and utilization of a complex pheromone system (1, 5,6), as seen in Life Story when a scout ant alerts the nest of intruders (2). Weaver ants are a highly territorial species and compete with other Weaver ant colonies (1). Researchers, particularly Bert Holldobler, have described fascinating ritualized tournaments between colonies involving signaling displays, recon trips by scouts, assessment of colony strength, and deadly attacks (5). As Life Story describes, the ants immediately recognize colony mates from foe (2). They accomplish this primarily through both learned and innate recognition of a colony odor (3, 5, 6, 7). Such kin recognition is vital as ants must properly direct altruistic and aggressive behavior (7). Interestingly, spiders, such as the jumping spider, have managed to adapt a strategy to mimic ant pheromones and fly under the radar (1).

Why are behaviors like silk contribution by larvae and colony workers’ self-sacrifice in battle selected for? These cooperative and altruistic acts increase the indirect fitness of the ants as they protect large numbers of their kin, effectively ensuring their shared genetic information is passed to the next generation. Weaver ants have a haplodiploid mode of sex determination (3), meaning males develop from an unfertilized egg (they are haploid) and females develop from a fertilized egg (they are diploid). Most Weaver ant colonies consist of one queen who mates usually one but up to five times (1, 9). Therefore, most of the workers in the colony are closely related, sharing a great proportion of their genetic information. Because of this, most ants can forgo reproduction and take care of their fellow workers (mostly their sisters) and still have high fitness over their lifetimes. Thus, self-sacrifice for the good of the colony is well worthwhile. As David Attenborough says, “A single grub is an insignificant loss.”

by Claire Dillenbeck

Life Story, Season 1, Episode 3: Home, starting at approximately 0:34:00

References

1. Crozier R, Newey P, Schluns E, Robson S. 2010. A masterpiece of evolution – Oecophylla weaver ants (Hymenoptera: Formicidae) 13: 57-71
2. Gunton, M. (2014). Life Story. BBC Natural History Unit, The Open University, Discovery Channel, France Télévisions.
3. Holldobler B, Wilson EO. Weaver Ants: Social Establishment and Maintenance of Territory. Science 195: 900-902
4. Holldobler B, Wilson, E. 1983. The Evolution of Communal Nest-Weaving in Ants. American Scientist 71: 490-409
5. Holldobler B. 1998. Multimodal Signals in Ant Communication. Journal of Comparative Physiology 184: 129-141
6. Jackson DE, Ratnieks RLW. 2006. Communication in Ants. Current Biology 16; R570-R574
7. Newey P. Not one odour but two: a new model for nestmate recognition. Journal of Theoretical Biology 270, 7-12
8. Santos JC, Del-Claro k. 2009. Ecology and behaviour of the weaver Ant Camponotus (Myrmobrachys) Journal of Natural History. 43: 1423-1435
9. Scluns EA et al. 2008. Breeding system colony and population structure in the weaver ant Oecophylla smaragdina. Molecular Ecology