SMEARING the place you inhabit with faeces is, among people, an act of desperation rarely seen outside the confines of a prison. Some cockroaches, though, do it all the time.

Rebeca Rosengaus of Northeastern University, in Boston, thinks she knows why. As she and her colleagues discovered in a study just published in Naturwissenschaften, wood-cockroach faeces protect the insects from a parasitic fungus. This finding, she thinks, may also explain the existence of one of the world’s most successful groups of animals: termites.

Wood cockroaches nest in decaying tree trunks, in crevices they plaster with their faeces. These are also home to fungi that parasitise their insect neighbours. Past observations have hinted that cockroach faeces protect against such parasitism. Dr Rosengaus decided to test the idea.

She and her colleagues collected cockroach faeces, mashed them up with water, mixed in fungal spores and smeared the mixture onto slides covered in nutrient-rich agar. As a control, they repeated the process without the faeces.

The fungus germinated on all the control slides. On those smeared with the faecal mix it germinated on only a third. Faeces thus stop fungal germination and presumably protect cockroaches. And what works for individuals might work even better for a huge colony, in which individuals benefit from the pooled excreta of thousands of neighbours. Which is where the termite connection comes in.

Termites, a specialised group of social cockroaches, are one of entomology’s puzzles. Ants, bees and wasps—the other insects that form colonies in which sterile workers raise the offspring of a queen—have unusual genetics. Their workers are female, and these genetics explain the propensity of females to help their mothers reproduce, instead of breeding themselves.

Termites, though, do not have this unusual genetic pattern and the sterile workers come from both sexes. So the question is, what made them become colonial?

Dr Rosengaus speculates that the impetus was faeces-pooling, and she has found that at least one living termite species thought to resemble the earliest members of the group has anti-fungal faeces too. Many a householder has no doubt issued a lavatorial expletive on discovering termite damage to his house. If Dr Rosengaus is correct, such outbursts may be more appropriate than anyone had realised.