Senescence, or ageing, describes the decline in longevity and fertility suffered by all organisms as they age. According to evolutionary theory senescence is caused by relaxed Darwinian selection acting at old age, and can fuel fierce evolutionary conflict, including conflict between reproductive partners (sexual conflict). The focus my D. Phil., which I recently completed (Nov 2009) at the Department of Zoology of the University of Oxford (UK), has been to investigate how reproductive senescence suffered by males impacts on the reproductive success of their partners, in an avian model system, the fowl, Gallus gallus domesticus.
Our dance depicts the evolutionary sexual battle between ageing roosters and hens. The scene is set in a bar – the arena for sexual selection in humans.
The sexual battle begins with male courtship of females, represented using Jive and Cha Cha Cha. With a single, young male on the scene there is little conflict between the male and female.
However, when an older male arrives, males have to compete for the dominant social position. Male-male competition, portrayed using Paso Doble, is intense because the winner gets to monopolise the female. The old male is able to overcome the younger male and becomes top in the social pecking order (Dean et al., 2010).
The old dominant male can now control access to the female. However, since he suffers from reproductive senescence, he is unable to fertilise all of her eggs (Dean et al., 2010). The Tango shows sexual conflict between the female, attracted by the younger male, and the old dominant male, who is interested in imposing his own reproductive interests at the cost of her reproductive success (Dean et al., 2007). Although she achieves a degree of promiscuity, when old males become socially dominant the result is a cost to female fertility – the battle ends with a lone chick cheeping.
Dean, R., Bonsall, M. B. and Pizzari, T. 2007. Aging and sexual conflict. Science. 316:383-384
Dean, R., Cornwallis, C. K., Lovlie, H., Worley, K., Richardson, D. and Pizzari, T. 2010. Male reproductive senescence causes potential for sexual conflict over mating. Current Biology. 20: 1192-1196