(E)-Methylgeranate, a chemical signal of juvenile hormone titre and its role in the partner recognition system of burying beetles

In recent years, studies have shown that animals can communicate their physiological state or condition by means of chemical signals. However, as the chemicals involved in the condition-dependent signals have rarely been identified, evolutionary mechanisms that ensure their reliability are not well understood. We identified a volatile chemical signal that may serve as a reliable indicator of hormonal state in burying beetles, and is involved in their partner recognition system. Burying beetles reproducing on carcasses are able to distinguish between their breeding partners and infanticidal conspecifics. This discrimination depends on breeding status, which is positively linked to juvenile hormone III (JH III) titre. Breeding Nicrophorus vespilloides beetles, in contrast to nonbreeding ones, emitted considerable amounts of (E)-methylgeranate. The amount of emitted (E)-methylgeranate was positively correlated with juvenile hormone titres known from other burying beetle studies. Moreover, our behavioural experiments showed that dummies treated with methylgeranate induced tolerant behaviour, whereas control dummies were treated aggressively. The fact that (E)-methylgeranate and JH III share a conspicuous structural similarity and the same biosynthetic pathway may explain how the reliability of the signal for JH titre is ensured. We discuss the implications of our results in the light of theoretical work on the evolution of chemical communication, particularly on the origin of chemical signals.