A brief, chilly spell can change the sex of chickens in the egg, the festival learnt yesterday. The finding, which has been patented, could eventually lead to big changes in the poultry industry, writes Nicholas Schoon.

If the temperature is dropped by a few degrees for three days during the embryological development of a freshly laid egg, some chickens which should hatch out male instead become female.

They have the genes and chromosomes for maleness but they are fully functional females able to lay fertile eggs. If they are then crossed with normal males, the resulting chicks are all male, said Professor Mark Ferguson of the University of Manchester.

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This is what interests the poultry industry.All the female chicks which hatch from the breeding stock which provides the broiler chickens we eat are destroyed after hatching. Only the males put on enough meat, at sufficient speed, to make them economically viable. So there could be large savings if the breeding stock could be made to produce only males.

The chilling technique only changes the sex of 10 per cent of males into females, although if these birds are then crossed with normal males an all-male brood results.

Professor Ferguson believes the cooling can only work its effect on a minority of chicks which are "near the border line" of the male-female spectrum. The cooling is thought to work by slowing down the operation of enzymes involved in sex determination.

The cooling has to be done very precisely. Chickens try to keep their eggs at a steady 37.5C below their body. This is the temperature used in commercial incubators.

Professor Ferguson also believes it is possible, but rather more difficult, to change genetically female chicks into functional males by altering temperature during embryological development which opens up the possibility of only female chicks being hatched out with no males.

That could boost the egg laying industry by ending the need to destroy all the male chicks hatched by the egg laying breeds which provide our egg laying hens. The poultry industry has shown interest in his work, but has yet to come forward with any funding.

Such changes are not unknown in the wild, Professor Ferguson pointed out. The sex ratio of baby crocodiles and alligators is known to be strongly influenced by temperature. If the eggs are kept at 30C the hatchlings are all female and at 33C they are all male.