Delayed implantation

As you may know, some grizzlies spent much of June mating. Well, the products of those matings are finally about to take hold. The gestation period in bears is approximately 235 days, though it is a somewhat atypical 235 days, at least by human standards. A grizzly female exhibits what is called “delayed implantation”, which means that while she may have successfully conceived more than once during her springtime copulations, the fertilized blastocysts (eggs) float freely in her uterus until this time of year. In the late autumn, as long as she has put on sufficient weight to support the pregnancy, one or more of these embryos will implant into her uterine wall. If she is in good condition, 2, 3 or even 4 of the embryos will implant. If she is in poor condition, none of the embryos will implant, ensuring that she will have sole claim to all of her resources, and thus have a better chance of surviving the long winter ahead. Essentially it is a safety valve protecting the female.

If indeed she does conceive, her cubs will be born sometime around the end of January while she is in the depths of her winter sleep. So the embryos actually only undergo growth and development for about 2 months, a very short pregnancy.

Check back next month for another interesting wildlife fact.