Do big, randy rams run out?
LARGE, promiscuous rams may run out of sperm towards the end of the mating period, according to new research reported in last weeks New Scientist.
Brian Preston, animal behaviourist at Stirling University found the results after studying wild Soay sheep on St Kilda, an island off the west coast of Scotland. But MLC scientist Jenny Anderson disputes the results.
In the trial, Dr Preston observed the behaviour of more than 100 Soay sheep during the mating season. He concluded that the heaviest rams tupped the most ewes, some mating up to 10 times a day due to their ability to fend off smaller males.
But the smaller rams caught up towards the end of the mating season, siring an equal number of lambs to the large ones in the last two weeks. He claims that studies on domestic rams have shown similar behaviour patterns.
Dr Anderson disagrees, saying that most rams can ejaculate four times every 20 minutes throughout the mating season. Despite mating behavioural problems including rams favouring specific breeds, certain ages of sheep, ewes with short wool or being homosexual, there is no evidence to suggest that rams stop producing sperm.
She also questions the validity of the blood test used to DNA type Soay tups and their progeny. "As the sheep are so inbred its difficult to be specific about paternity."
Dr Anderson recommends one ram for every 30 ewes in lowland sheep and one ram for 70 ewes in hill flocks to ensure a tight lambing season.
Including odd rather than even numbers of rams also helps to prevent fighting.