Spermicide by females: what should males do?

The female reproductive tract can be particularly aggressive towards ejacul ates, often leading to the death of large numbers of sperm. It has been sug gested that males can respond to these actions by investing more in sperm a nd donating larger ejaculates. Such counteractions may lead to arms races, which can have significant implications for the mating system. In a series of simple models we first show that arms races are not necessarily supporte d: in fact, sperm killing may even favour no change or reductions in sperm allocation. Second, we identify a simple mechanistic rule for sperm killing that determines whether an arms race or sperm reduction will be favoured. Which of these responses is favoured by selection depends on whether a cert ain number, or proportion, of sperm are killed. When a specific number is k illed, larger investment in sperm is favoured and when a specific proportio n is killed, no change or lower investment in sperm is favoured. Both of th ese mechanisms are biologically plausible.