Saving Sperm For A Later Date

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

University of Leeds researcher Dr. Tom Pizzari 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 his 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 raised 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 today in Nature (6 November 2003), also showed that males could distinguish the sexiest chicks – 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.

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”.

Pizzari’s research showed that cocks gradually lose interest if left alone with the same female, thus conserving sperm to spring back into action should a new hen be presented. This is the ‘Coolidge Effect’ – the tendency of a male to lose his sexual appetite unless presented with a new female, and it has been studied for years, taking the name of 1920’s US President Calvin Coolidge.

On a trip to a chicken farm, Coolidge’s wife pointedly remarked on the sexual stamina of the rooster, only to be rebuffed by her husband who explained that the rooster chose a different female for each instalment of his marathon sex life. That sexual familiarity breeds contempt is nothing new, but Pizzari’s work is the first to show that males use the Coolidge effect as part of a wider strategy of prudent sperm investment.