How and why do some animals delay pregnancy?

Find out how mothers make sure conditions are just right for their offspring

In a remarkable feat of evolution, the females of around 100 mammal species are able to delay or pause pregnancy. This reproductive strategy is called delayed implantation, and increases the survival chance of the young. When a female mates, a small ball of cells known as a blastocyst forms in the uterus, but it is then kept in a dormant state and develops no further until conditions change.

 

Kangaroo mums can keep a joey in reserve; while one baby is growing in the pouch, a second one waits in the uterus ready to take its place.

It’s all down to a feedback loop in their hormones. If there’s already a joey in the pouch, the action of suckling triggers the release of a hormone called prolactin, which halts the development of the embryo. The fertilized kangaroo egg can divide until it reaches a ball of 100 cells, but after that it stops until there is room. When the bigger joey is old enough to leave its mother’s pouch, the hormone balance changes and the embryo can continue developing.

This clever adaptation means that the female kangaroo will always have room for her new arrival, but she won’t need to wait until her older joey has left to fnd a mate, maximizing the number of little kangaroos she can successfully raise.

This form of delayed implantation is known as facultative diapause or lactational delayed implantation, and is also seen in other marsupials and some rodents.

 

The other form of this strategy is obligate diapause; delaying development of the embryo after conception allows the overall gestation time to be increased so that young are born in favourable conditions. If a female mates in autumn but has a short gestation time, this pause means that her offspring will be born in spring and not be forced to compete with harsh winter conditions. This occurs regularly in armadillos, seals, bears, mustelids like weasels and badgers, roe deer and a species of fruit bat.