Deer littermates have different dads. (Paternity).

Genetic analysis of white-tailed deer in a Michigan reserve has turned up what may be the first documentation of more than one father per twin litter in a big, free-ranging hoofed mammal.

Several dads siring offspring in the same litter wouldn't be a surprise for small mammals such as ground squirrels, says Anna Bess Sorin, now at the University of Memphis in Tennessee. Only one previous genetic analysis of large ungulates, in this case, deer held in a farm pen, has ever shown evidence of more than one dad.

For the latest analysis, Sorin studied a deer herd, including about 40 adult males, in a fenced woodland in Livingston County, Mich. She collected tissue samples from temporarily sedated members of the herd and from any dead animals. When wildlife managers culled pregnant does, Sorin could also obtain clear evidence of parent-offspring relationships.

Using DNA from the deer fetuses and other tissue samples, Sorin was able to identify the fathers of 27 sets of two-fawn litters. In six of the pairs, the fawns had different sires. All the mixed-parentage cases involved a young male and a considerably older one, says Sorin. She speculates that one of the bigger, more dominant males in a herd may have intruded on a subordinate with a female.--S.M.