Delayed implantation in grizzly bears
Grizzly bears typically mate between May and early July. Although grizzlies have a gestation period of only 6-8 weeks, birth doesn't actually occur until 6 months later in dead of winter. How is this possible? Upon conception a fertilized egg begins to divide until it gets about the size of a pinhead and is called a blastocyst. In some mammals like humans, the blastocyst implants into the uteran wall within a week of conception where it continues to grow until birth. Female bears however have evolved "delayed implantation" whereby the blastocyst does not implant into the uteran wall until late fall, just before a grizzly mother enters her den for the winter. Within a week of conception (during the spring) the blastocyst enters a state of dormancy. In late fall the blastocyst will implant into the grizzly mother's uterus if she is fat and healthy enough to support cubs during the winter hibernation. If she is under severe stress, and under nourished, the blastocyst will simply be reabsorbed into her body. The number of cubs in her litter is also determined by how healthy and fat the mother is in the fall. A really fat and healthy mother may give birth to up to four cubs, whereas a marginally fat mother may only give birth to one or two cubs.
Hibernation lasts about 6 months and during this time the mother grizzly will not eat, drink, urinate, or defecate. Since grizzly cubs are born during hibernation it is absolutely essential that the mother have enough stored fat to be able to nurse her cubs through the winter. Fall is the best time to determine whether or not a grizzly mother has enough fat reserves to support newborn cubs through hibernation. This is why the body's decision to implant or not implant a blastocyst happens in the fall. The other benefit of delayed implantation is it allows bears to mate in the spring thus freeing up the fall for concentrated eating. It's during the fall that bears gain most of their new fat to sustain them over the upcoming winter hibernation. Mating requires a lot of energy, especially for the males which seldom even eat while pursuing females, so it makes sense for mating to occur during the spring.
© 2008 John & Jessica Teel