Prenatal 'sex change' for a leaner ewe.

Prenatal "sex change' for a leaner ewe

Pregnant sheep given five 200-milligram injections of testosteroneover an eight-week period delivered offspring that appeared to be solely male. In fact, many were chromosomally female. Not only did treatment with the male sex hormone cause the female lambs to develop what appeared to be male genitalia, but it also caused profound changes in the way they metabolized their food. And these metabolic changes interested researchers at the Agriculture Department's Meat Animal Research Center in Clay Center, Neb.

Explains John Klindt, who heads the four-year-old projectthere, "In ruminant animals such as sheep, males are heavier, grow faster, and produce more lean muscle tissue [meat].' After treating some 600 pregnancies, he and his co-workers have now established that prenatal testosterone will cause female lambs to gain at least 10 percent more weight daily, after birth, than untreated female lambs. While this weight gain is 10 percent less than occurs in the normal male lamb, Klindt notes it's still an important improvement. "And in our best study,' he adds, "we got a 30 percent per day weight increase--which roughly comes to about an extra pound per week on 16 percent less feed.' Treated females were also 13 percent leaner than their untreated female counterparts--or about as lean as the normal male sheep. Details of the work appear in the June PROCEEDINGS OF THE SOCIETY FOR EXPERIMENTAL BIOLOGY AND MEDICINE.

Klindt's studies show that the testosterone, which has athree-day half-life in sheep, does not carry over into the meat. Moreover, the treatment appears to have no measurable effect on the males treated in utero, nor lasting effects on the pregnant ewes. As to what it does to female fetuses, that remains a mystery: Klindt and others had always assumed the increased weight gain in treated females resulted from some early testosterone-fostered changes that altered both the eventual production of growth hormone and the hormone prolactin. In fact, Klindt's new data show no change in either hormone among treated females.

Klindt's lab is working to reduce the number of injectionsnecessary to achieve the desired gains in weight and leanness. His most recent data show that treatment can be successfully started later in gestation than tried originally; doing so prevents obvious changes to the female's genitalia.