With delayed implantation the egg is fertilized and grows into a blastocyst (microscopic ball of cells) at which point development is suspended. The embryo floats free in the female’s uterus until her body signals for it to implant and continue development. Over 100 mammalian species display delayed implantation.
There are two types of delayed implantation: obligate and facultative. Obligate delayed implantation always occurs while with facultative delayed implantation the fertilized egg may or may not attach to the uterus wall depending on the situation.
Many reasons for delayed implantation have been proposed including:
Nutritional – Bears mate in early summer and implant in late November or early December. If food supplies are scarce and the female does not have enough nutritional reserves (fat) to support a litter, she will not implant and abort the fetuses.
Seasonality – Seals come ashore once a year to give birth and breed before returning to the sea. Delayed implantation assures that all the pups are born at the same time when the seals are ashore. If born at sea the pups would drown.
Excessive Stress – Nursing and being pregnant puts excessive stress on a female. Rodents often become pregnant before the young are weaned. Delayed implantation in rodents occurs only after the female stops nursing.
Optimal Breeding Conditions: Males expend large amounts of energy during the breeding season. For example, bear males are at their prime in the summer so this is the best time for them to pursue a mate. Then the male can replenish his fat reserves before winter. If all the females are receptive at this time the males have a shorter breeding season and expend less energy than if he were breeding throughout the year.
Optimal Survival Conditions: If a pregnant female can time the birth so that the young are born in the spring when food is more available, the babies have a better chance of survival. This timing also allows the young to grow and develop survival skills of their own before winter comes again.
Superfection: The topic of my next post.
All the exact triggers for implantation are not completely understood. The uterine environment and hormones influence when implantation occurs. Day length, light intensity, temperature and food availability are some factors which appear to have an effect on the timing of implantation.
This black bear was photographed in Lassen Volcanic National Park (California) not far from Upper Kings Creek Meadow.