December 6, 2021

I, Science

The science magazine of Imperial College

How do organisms like ants evolve, where many are sterile and help others to breed?

Ants and Aphids_BinuKSWhen the great evolutionary biologist and geneticist JBS Haldane was asked: ‘Would you give your life to save your brother?’ he replied ‘No, but I would to save two brothers or eight cousins.’


Haldane wasn’t being perverse; he was making a mathematical statement about evolutionary biology. All organisms are driven in one way or another to reproduce their genes, whether this is via the preservation of genes in their own offspring, or in the form of another related individual. So, genetically speaking, saving eight cousins or two brothers is the equivalent to preserving your own genetic make-up. This has been mathematically enshrined in ‘Hamilton’s Rule‘.

Understanding the genetic relatedness between organisms enables us to understand some of their behaviour and allows us to in part understand how and why the subject of this blog – eusociality – exists. This behaviour is most commonly found within the insect order Hymenoptera, which includes the bees, ants and wasps.


So what is eusociality and why do I think it’s massively massively cool? Eusocial means ‘truly social’ and the traditionally accepted criteria for eusosocial organisms includes:

  • individuals cooperating in the care of their young
  • mother and offspring living in the same nest at the same time
  • a division of labour, with sterile individuals who carry out all the work to care for the nest for the apparent benefit of a reproducing Queen.

Gazing, for example, at the intensely complex termite mound, or the highly organised ants nest, or the busy hive of bees, I can’t help but wonder why they’ve evolved to live as they do.


Hymenoptera are fascinating because their inheritance occurs via a system known as haplodiploidy. Whereas most sexual organisms receive half their genetic material from a mother and half from a father, this is true only for Hymenopteran females. If male, the insect simply gains a set of chromosomes from its mother. This not only means that Hymenopteran mothers can produce sons without even having sex, but also that the males are haploid (one set of chromosomes from Mum), whereas females are diploid (a set of chromosomes from each of Mum and Dad). This sets up a genetic system that has fascinating consequences for behaviour.

Super-sisters: a complicated family life

Hymenopteran females are (like us) related to each of their parents by a ½. However, since they only on average share 1 in 4 genes with their brothers (since males are haploid), the females are only related to their brother by a ¼. A Hymenopteran Queen usually mates once, keeps hold of the sperm, finds herself a nice spot to nestle down in and starts pumping out eggs. The Queens store the sperm they received in their special sperm sack (the spermatheca) and keep it safe to produce offspring for their entire lifespan. This means that Hymenopteran sisters are twins on their Father’s side, making Hymenopteran sisters related to one another by ¾: they are super-sisters.

What effect does this have on the behaviour of Hymenopteran insects? This fabulous genetic quirk means that for many Hymenopteran insects it is more reproductively successful if she stays at home and farms her mother for sisters rather than leave the nest and produce her own children.

A genetic battle, with sisters in control?

Hymenopteran colonies, such as that of the ant, are organised societies in which each individual has a specific role that defines the cast of the individual. There is a Queen; a grotesquely enormous, swollen thing that can resemble a giant maggot. Arguably as much a slave as a Queen, she squeezes out hundreds of babies a day. The workers harvest the eggs and take care of the larvae, ensuring they are well fed with enough space to grow. Then there are soldiers; aggressive large individuals that defend the nest.

Beneath this organised surface, there is a battle for genetic supremacy taking place.

The super-sisters are often genetically most successful when they propagate more sisters. Interestingly, the super-sisters have been known to bite the heads off of their larval brothers (to whom they are only related by ¼) in order to free up more resources for sisters. The female ants work tirelessly to propagate their own genes not via having their own offspring, but by manipulating their mother to produce more reproductive sisters.

Why else?

But there are many examples of eusocial species which employ the normal diploid mating system – for example, termites with their extraordinary mounds containing elaborate ventilation systems and the naked mole rat, bizarre looking creatures that live underground.

So what could’ve caused eusociality to evolve in these organisms? Essentially, Hamilton’s rule suggests that eusociality evolves when it becomes more advantageous to stay at home and help the mother reproduce than it does to have your own young. There could be many reasons for this, such as a low probably of survival outside of the nest, or the need for many individuals to care for vulnerable infants.

Lessons for humanity?

Learning about eusociality causes me look at my own species, Homo sapiens. Unlike many other organisms, humans have the apparent ability to predict the outcomes of their actions and to imagine what others may be thinking. But zoom out: look at our species as one whole giant organism. Then, does it look like we are making logical decisions, made with foresight and empathy?

As our population sweeps past the 7 billion mark, we are transforming our planet. Like a single super-organism, we systematically consume finite resources. Do we appear more like a mindless colony, bound by our genetics and ecology?

Unlike ants, humans have the fantastic ability to imagine – and therefore change – our future. It is my hope that with time, every single human is able to make better life choices based not only on their own desires, but also based on a devotion to a future which has retained a diverse and prosperous biosphere.

Image: Ants and aphids by Binu KS