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It will come as little surprise to the female of the species � males are less likely to say �not tonight dear�. A study found that even when they are in excruciating pain, males are interested in sex. Females, however, prefer to take to their sick bed. The researchers studied mice but said their finding could explain why the very human saying �not tonight dear, I have a headache� is normally attributed to female half of a couple. In one of the first studies of its kind, the Canadian researchers looked at how pain affected mice�s desire for sex. They first checked which males and females were interested in each other and then put them on opposite sides of a partitioned cage. The dividing wall contained holes too small for the males to squeeze through but just big enough for the females to make their way to the male area � and escape back to their own lair when necessary. The results were �very striking�, with the females spending less time on the male side when in pain. Even something such as a sore cheek dramatically depressed their desire. In contrast, males given free access to females behaved the same whether in pain or not. Even the injection of a chemical to their genitals barely affected their libido, the Journal of Neuroscience reports. Canadian researcher Jeffery Mogil said it has long been known that women� s sexual desire is more �context dependent�, or affected by factors such as self-confidence and relationship fears, than male libido. However, it wasn�t known if this had its roots in biology or in upbringing. The finding in mice �strongly suggests� there is a biological underpinning. Dr Mogil, of McGill University in Montreal, said it isn�t known if a women�s libido is more affected by pain. But if it is, it could be nature�s way of stopping them from getting pregnant and taking on the responsibility of raising a child while they are ill. Men, in contrast, have no such worries. Dr Mogil, who has previously shown that redheads are better at coping with pain than blondes or brunettes, said: �For males, once conception is over, their work is done.� It is hoped the research, published in the Journal of Neuroscience, will lead to a better understanding of why migraine sufferers, people with arthritis and others in chronic pain, often lose interest in sex.