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.
the messy but effective experiment, the researchers
collected sperm from mating fowl, using an
avian ‘femidom’ to catch the male’s
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”.
further information, see the press