Sexually transmitted infections in natural populations: what have we learnt from beetles and beyond?


Ashby B, Jones J, Knell RJ & Hurst GDD (2019) ‘Sexually transmitted infections in natural populations: what have we learnt from beetles and beyond?’ in Wilson K, Fenton A, & Tompkins D. (ed.) Wildlife Disease Ecology: Linking Theory to data and application. Cambridge Univ. Press, Cambridge, UK. pp. 187-222.

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Sexually transmitted infections (STIs) can be found in a wide range of invertebrate and vertebrate hosts. Theory makes extensive predictions with respect to the dynamics and evolutionary ecology of these infections. Their epidemiology is predicted to vary with the mean and variance in the number of mating partners, and in more refined models, contact and social structure. Weak dependence of mating rate on host density leads to the prediction of density-independent dynamics, including the possibility that sterilizing infections could drive their hosts extinct. The impact of infection on the host is predicted to select for mate choice against infected partners, and for reduced mating rates. We examine these predictions against STIs in nature, with a particular focus on studies of beetle-ectoparasitic mite interactions. The Adalia bipunctata (two spot ladybird) – Coccipolipus interaction has given particularly rich insights, with the ease of scoring infection and mating activity in natural populations enabling detailed documentation of dynamics. Laboratory study has further allowed precise estimation of transmission parameters to inform models, and allowed focused analysis of behaviour. The use of replicated population microcosms permits controlled emulation of natural systems. These studies have confirmed the core impact of mating rate on STI dynamics, but revealed unexpected drivers such as food supply (positively driving mating rate) and sex ratio (enhancing spread and producing male-biased prevalence), alongside constraints on spread from host phenology. They have challenged the independence of STI dynamics with respect to host density, but have largely failed to support predictions that STIs drive the evolution of mate choice and mating rate.