ODORS OF PARASITIZED MALES INDUCE AVERSIVE RESPONSES IN FEMALE MICE

The detection and avoidance of parasitized males may be a component of female mate choice. In rodents a reduction in pain sensitivity (analgesia) is induced by exposure to actual or potential danger and threatening stimuli, facilitating the expression of various active (e.g. fleeing) and passive (e.g immobilization) defence responses. Pain, or more appropriately, nociceptive sensitivity (latency of a foot-lifting response to a 50 degrees C thermal surface) was examined in female mice, Mus musculus domesticus, that were exposed to the urine and other odorous secretions of male mice sub-clinically infected for 20 days with the naturally occurring trichostrongylid nematode parasite, Heligmosomoides polygyrus. A 30-min exposure to the odours of the parasitized males induced an endogenous opioid peptide mediated analgesia in the female mice, whereas a 1-min exposure elicited a shorter duration and lower-amplitude analgesia of a non-opioid (serotonergic) nature. Similarly, in an odour preference test, female mice displayed a reduced preference for and aversion to the odours of parasitized males, spending more time in a Y-maze in the vicinity of the odours of an uninfected than an infected male. Together, these results indicate that female mice can discriminate between the odours of parasitized and non-parasitized male mice, and find the odours of parasitized males aversive. It is suggested that the odour-induced analgesic responses and their neuromodulatory correlates are part of an adaptive defence mechanism that facilitates the detection and avoidance of parasitized males by female mice and contributes to female mate choice. (C) 1995 The Association for the Study of Animal Behaviour