Male lizards and snakes have to make a choice when they mate: which

penis should they use? They have two. One is connected to their right testis,

and one to their left. Now two biologists from the US have found that some

lizards alternate their penises as each is depleted of sperm. In this way,

they maximise their chance of fertilising a female.

Richard Torkaz and Joseph Slowinski of Miami University studied the

lizard Anolis carolinensis. They found that the penis a lizard used depended

on which they used last time they mated, and how long ago they last used

it (Animal Behaviour, vol 40, p 374).

Torkaz and Slowinski believe that such behaviour can be explained by

‘sperm competition’. Recently, zoologists have realised that competition

for a female’s favour does not simply involve fights among males or displays

of bright plumage; it can continue inside the female’s reproductive tract

as sperms compete to be first to the egg.

In response to such sperm competition, males of some species have developed

sperm that forms into a hard plug, blocking the way for the sperm of any

other males. Males of still other species produce a lot of sperm, so that

these outnumber and swamp the opposition.


However, keeping up a high level of sperm production can be a problem.

If successive copulations follow on too quickly from one another, this can

result in the number of sperm in the ejaculate dropping sharply. By alternating

their penises, lizards of A carolinensis may overcome this problem.

Torkaz and Slowinski found that males alternated the use of their right

and left penises only if one copulation followed less than 24 hours after

another. If the interval between copulations was longer, then they used

the same penis.

The biologists carried out an experiment in which they prevented penis

alternation. They did this by taping over one half of a male’s cloacal vent,

through which the penis protrudes when aroused. This stopped the animal

from using the penis on that side.

The biologists found that males which were forced to re-use the same

penis within 24 hours, transferred fewer sperm in the later copulations.

However, the researchers found no such drop in the numbers of sperm when

successive copulations were as long as 72 hours apart. In this situation,

they say, the sperm duct has time to be restocked with sperm.

According to Torkaz and Slowinski, their research demonstrates that

the lizards adjust their behaviour in response to sperm competition, using

their penises alternately to ensure that maximum sperm transfer is kept

up, even if the gap between copulations is short.