The role of the major histocompatibility complex in scent communication

Conclusions

There is incontrovertible evidence that the MHC region is associated with type-specific odours in a number of species. Existing experimental paradigms have demonstrated that animals can discriminate between classes of MHC-associated odour with great acuity. Furthermore, experiments on mate choice and kin selection provide convincing evidence of a functional role for these odours. At the moment, evidence for genuine individual recognition or discrimination, meaning the “learned discrimination among conspecific individuals” (Halpin, 1986; p. 44), is lacking. This is partly because the four main experimental paradigms have not been used explicitly to address the question of individual recognition, but rather to test whether broader subgroups of MHC-associated odours exist and are distinguishable. These types of experiment can certainly be adapted to test individual recognition. For example Gheusi et al., (1997) found evidence for individual discrimination by rats using operant training, although this study did not examine MHC-associated odours specifically. The experimental paradigms described here have been vital in narrowing down the search for potential sources of individual odours, and testing odour discrimination abilities. Ultimately however, they can only allow an assessment of olfactory acuity, rather than whether this acuity is put to any use. In order to demonstrate that MHC-associated odours have an adaptive role in individual recognition, functional tests in a realistic behavioural context are required. Such functional tests exist (e.g. see Rich and Hurst, 1999; Hurst et al., 2001; Lai and Johnston, 2002) and now need to be applied to the unresolved issue of MHC-associated odours and individual recognition.