Last week, people watching the the live webcam at Alaska’s Katmai National Monument got more of a show than they probably expected: two uninterrupted minutes of grizzly bear sex.

Though it looks kind of awkward, they can keep at it for most of an hour. And it’s effective – that female is very likely to wind up pregnant because male grizzly bears induce ovulation, probably thanks to some stiff stimulation of her cervix from the 6-and-one-half-inch long baculum embedded in the male’s glans, seen below.


The female won’t give birth until she’s in her winter den, sometime in January or February the following year. And although you might expect that a big animal with a pregnancy as long as our own would pop out some sizable cubs, you’d be wrong. Newborn bear cubs are tiny – they weigh less than a pound, their eyes are still sealed shut, and they can barely crawl.

The reason is one of the coolest tricks in bear reproduction. Even though there’s a nine-month gap between fertilization and birth, the cubs only develop for 6 to 8 weeks before they’re born. That’s because their mother puts them into suspended animation between May and November. Inside her uterus.

The process is called delayed implantation. It works like this: sex makes female bears ovulate, and if that egg gets fertilized it will start to develop as it travels toward the uterus. But once the embryo gets there, the mother bear arrests its development, keeping the little ball of cells floating inside her uterus while she fattens herself up over the summer and fall. When she reactivates the embryos in late fall, they’ll finally get to implant in her uterus and resume their development.


Why did this convoluted reproductive pattern evolve? Biologists think it could be a strategy that lets more mothers and cubs survive the winter. By “freezing” their embryos in late spring, female bears get more time to eat and put on weight without the strain of pregnancy. By keeping pregnancy short, females minimize their developing cubs’ demands on maternal muscle and bone, opting instead to use their fat stores to make milk after they’re born. Fatter females even give birth sooner than thinner females, giving their cubs extra time to nurse and grow. And giving birth in late winter means that the cubs are ready to leave the den in early spring, with three seasons to grow and gain weight ahead of them before they face another winter.

[Sources: Bronson 1989 | Lopes et al. 2004 | Robbins et al. 2012 | Steyaert et al. 2012]


Contact the author at Image: Ursus arctos Baculum | Didier Descouens via Wikimedia | CC BY-SA 4.0


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