The Reporter
Issue 493, 27 October 2003
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Every sperm is sacred - to chickens

Tom Pizzari and chickensThe Monty Python song was right: every sperm is sacred – if you’re living in the promiscuous world of chickens that is. Leeds scientists studying the evolution of reproductive behaviour have shown that cockerels use sophisticated strategies to maximise reproductive return from limited sperm reserves.

Dr Tom Pizzari (left) from biology said: “When females are promiscuous, several males inseminate the same female, and their ejaculates compete inside the hen. This is ‘sperm competition’, which males can win by inseminating more than their rivals. It means that male reproductive success becomes limited by sperm production, so males must use sperm wisely.”

Pizzari, and colleagues at the universities of Sheffield and Stockholm, found that males allocate sperm according to their own position in the pecking order and the number of competing suitors. When a male is alone with a female, he uses a minimum of sperm, aware that sperm competition is not likely to leave him cuckolded. When rivals appear on the scene, dominant males raise their sperm investment to increase the chances of fertilizing at least some of the hen’s eggs. In contrast, low ranking males reduced their sperm contribution if more than one other male was present, presumably saving it for a better opportunity.

The research, published in Nature, also showed that males could distinguish the sexiest birds – those with the most voluptuous combs which lay the biggest, most yolk-rich eggs – and cocks responded to hen comb size with bigger sperm investments, hoping to get the best mother for their young.

In the messy but effective experiment, the researchers collected sperm from mating fowl, using an avian ‘femidom’ to catch the male’s investment package.

Dr Pizzari said: “Collecting sperm is unpleasant, but it’s a real breakthrough in the study of reproductive behaviour. Now we can quantify sperm investment, not just guess from the birds’ behaviour”.

For further information, see the press release


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