Published online 12 August 1999 | Nature | doi:10.1038/news990812-12


Tadpoles in quarantine

Diseased bullfrog tadpoles infected with the yeast Candida humicola are social pariahs - their fellow tadpoles steer clear of them. Researchers at Yale University, New Haven, Connecticut, have made this discovery, which they suggest could be the first experimental demonstration of ‘pathogen avoidance behaviour’ outside the context of sexual selection.

We know already that members of many species seek out healthy individuals with which to mate. And although it makes sense that animals with the ability to recognize and avoid infected peers in general would have an evolutionary advantage, confirmation of this hypothesis is surprising because it is at odds with the theories of the spread of disease. These so-called ‘epidemiological’ theories are founded on the assumption that the likelihood of infection is equal among the members of a given population in a certain area. Little, therefore, is known about how behaviour can shape disease transmission.

Thus, the existence of behaviours like that of the bullfrog (Rana catesbeiana) tadpole, reported by Joseph Kiesecker and colleagues in Proceedings of the National Academy of Sciences[3 August 1999], could have serious implications for our understanding of the spread of disease. “Where animals are able to recognize and modify their risk of infection,” the group cautions, “we should expect a comparable cascade of impacts on host and pathogen populations.” In other words, the impacts of infectious diseases could be much more complex than we think.

Kiesecker’s group found, for example, that healthy animals spent 75% of their time away from diseased individuals, when forced to share a container with them, whereas ordinarily these tadpoles show no particular preference for one part of the container or another if sharing it with another fit individual. Moreover, this behaviour seems to be mediated by chemical rather than visual cues, as the researchers discovered when they placed the infected tadpoles in cages within the testing arena that exposed the uninfected animals to either chemical cues alone or visual cues alone.

It also emerged that sick animals were indifferent to one another - but this could be because the C. humicola yeast, which gets into tadpoles’ guts when they ingest infected faeces and water, is known to impair their responses to chemical cues.

All of which indicates that it is not a change in the conduct of infected animals that causes them to be excommunicated by their peers, but something in the chemicals that they give off.