Expert: John Locke - 5/2/2009

Question
Can any or all mammals have multiple concurrent births from different sperm donors?

Answer
Thanks for using AllExperts. In certain, specific circumstances, it would be possible for different males to inseminate a single female concurrently, but there are biological and practical obstacles to this happening. Carrying a pregnancy to term is a time- and labor-intensive process, so various strategies have evolved to limit the number of pregnancies that can occur at one time; these strategies also have the effect of limiting the number of males that could inseminate a female to one for any single pregnancy.

Many mammals release only a single egg during one ovulation, humans included; this limits the number of possible offspring to one, and thus the number of potential fathers is also limited to one. Note that a single fertilized egg can split and produce separate embryos that will make identical twins; in this case, there will be more than one offspring but only a single father. When fertilization occurs, the production of estrogen and progesterone quickly inhibits further ovulation for the course of the pregnancy. In these animals, once fertilization has occurred, there is no opportunity for additional fertilization by other males.

Some mammals release multiple eggs routinely during ovulation (dogs), and some will occasionally release multiple eggs (humans, as occurs with fraternal twins). In these circumstances, it would be theoretically possible for multiple males to inseminate a single female within a short period of time, with the resulting pregnancies having different biological fathers. Generally, however, the first male to inseminate a female has the advantage, as his sperm will probably reach the eggs first. Females often become less receptive to additional mating after a single episode; since it takes about 24 hours for fertilization to occur after intercourse, this effect may be more than enough to ensure that only a single sperm donor will become the father.

I should make the point that this scenario does occur. This picture is of two dogs from a single litter with strikingly different physical characteristics, which suggests multiple fathers:
http://en.wikipedia.org/wiki/File:DogsMixedBreedlittermates.jpg