Effects of a Foreign Male on Pregnancy and Pup Survival
The fertilized embryo is a free-floating entity in the female reproductive tract for the first four and a half to five days of development. It is during this pre-implantation period that external events can play a role in determining whether a successful implantation will occur. Obvious disturbances to the mental health of the pregnant female - such as erratic lighting, extremes in temperature or humidity, high noise levels, or insufficient food and water - can cause a failure to implant. In addition, there is one other less obvious disturbance that is highly significant in the eyes of the female - the introduction into her cage of a male other than the one with whom she had mated. If the foreign male is not genetically identical to her partner, he can cause a premature termination of the pregnancy through a mechanism that is almost certainly a hormonally induced block to implantation (Bruce, 1959; Bruce, 1968). This pregnancy block is also known as the Bruce Effect (after its discoverer) and it provides an obvious selective advantage by ensuring that females will use their resources only to raise offspring who carry the genes of the intruding male (who is presumably more fit since he has displaced the original mating male). With the previous pregnancy terminated, the female can quickly become pregnant again with her new partner.
It is interesting that females do not recognize males from the same inbred strain as foreign (Bruce, 1968). On the other hand, a pregnancy block is induced in nearly all other cases. These findings indicate that one or more genetic differences are responsible for the distinction between the original and the intruder male, but in addition, they clearly show that the genetic recognition system is highly polymorphic. Further studies with congenic and coisogenic strains have demonstrated conclusively that a major component of this recognition system is the highly polymorphic class I family of genes in the major histocompatibility complex (Yamazaki et al., 1986).
The Bruce effect has important implications for the management of a
breeding mouse colony. Quite simply, if a mating event has occurred and
one wants to recover live-born offspring from this mating, for the first
five days that follow the pregnant female should not be placed either
into a cage with a foreign male or in contact with bedding that has been
soiled by a foreign male. After this initial stage there is no longer
any problem. In fact, if one wishes to quickly set up a new mating pair,
one should be sure to do it before the litter is born. If a foreign male
is in the cage at the time of birth, he will normally accept the newborn
pups. (Presumably, he 'thinks' these pups are his own.) On the other hand,
if a male is placed into a new cage that already has newborn pups, he
is likely to kill them ('knowing' that he couldn't possibly have been
the father).
|
||
|
Back to Breeding Click on a heading below to go to another section: Breeding |
