Most species of ant live their complex lives the honest, hardworking way: the ‘workers’ do the work, and the ‘sexuals’ mate and disperse, spreading the queen’s genes far and wide by founding more colonies. But some species have found devious ways to avoid doing any work at all.

In a Plagiolepis pygmaea colony, four or five queens produce eggs capable of development into different castes (types of ant with different jobs in life). Caste development depends on how much food is given to the larvae as they grow. If workers feed a larva less than a certain amount, or ‘caste threshold’, it becomes a fellow worker. Above the threshold, the larva will become a sexual. When conditions are less favourable for dispersal, host workers stop producing sexuals; they destroy any that develop, and only feed up to the lower caste threshold.

A successful colony produces lots of sexuals, but to do this, workers are needed for feeding and caring duties. Importantly, they don’t have to be the colony’s own workers. Plagiolepis xene, a specialist parasitic ant species, hijacks the workforce of P. pygmaea… and they are so good at getting others to do the work that, over the years, they have lost their own worker caste, and adopted a lifestyle called inquilinism.

A P.xene queen infiltrates the host nest, settles down somewhere, and starts to churn out her own eggs. Oblivious workers collect them, and the larvae are fed and treated the same as any other developing young. But how do they get away with it?

S. Aron from the Universite Libre de Bruxelles and colleagues measured the sizes of ants in parasitized nests, and found that the hijacker sexuals are indistinguishable from the host workers. P. xene larvae have a different caste threshold to their host - the same amount of food needed by a host worker will make a parasite sexual. When the time of year comes to switch to workers only, hijacker sexuals are still made.

The researchers suggest that although the parasite sexuals are smaller and weaker, the size reduction allows inquilines to exploit an important loophole.

Background

Other parasite ants:

  • Slavemakers raid other colonies, kidnap worker eggs, and take them back to their own nest. There, they use them to do all the work - raising fellow slaves as well as slavemakers, who can then go out to locate more recruits. Often, slavemakers can’t function without their home help.
  • Temporary parasites take advantage of another nest for a while, before being ousted. The parasite queen lays her eggs to be looked after by the resident workforce. Eventually, the hosts cotton on to the freeloaders - but not before the queen has produced enough sexuals to do it all again somewhere else.

Proceedings of the Royal Society of London B, vol. 266, p173-7