You ever think about polar bear milk? Yeah, us neither… but a recent email from the folks over at Polar Bears International got us thinking about their lactation situation. With Polar Bear Week being celebrated every year on the first week of November, now seemed as good a time as any to get into it.
It’s no secret that decreasing ice sheets are negatively impacting polar bears as they run out of space to hunt, and the longer females spend on land the longer they are forced to fast – meaning the moms often stop producing milk for cubs or make a product that’s substantially less fatty. Gathering samples to test the health and composition of polar bear mom milk involves expressing sedated females – after all, we wouldn't much fancy your chances of getting any from an awake one.
As you might imagine, the fishy diet of polar bears gives rise to an unusual brand of milk. Its composition was studied by PBI scientific advisor Dr Andrew Derocher, a researcher who even went as far as to taste the stuff, describing it as having a "rich, marine, earthy, chalky taste with a finish vaguely reminiscent of fish" in his book Polar Bears: A Complete Guide To Their Biology And Behavior.
That creamy texture comes from the high-fat content of polar bear milk, demonstrating the immense energy that goes into making it. Unfortunately for polar bear moms, new research has revealed that making it is getting harder in the current climate, and it could be about to get even worse. We spoke to study author Dr Louise Archer, a PBI Postdoctoral Fellow at the University of Toronto Scarborough, to find out more.
Where do polar bear cubs come from?
Dr Louise Archer: For polar bears, mating usually happens in the spring on the sea ice. The female and the male bear will spend a couple of weeks together, mating, and then they'll go their separate ways. But that fertilised egg that the female carries doesn't naturally begin developing straight away. The female will carry that egg inside her uterus, but it doesn't implant and begin developing.
Polar bears show something that's called delayed implantation and that means the development is delayed until later in the year. That means that the polar bear moms have time to build up their energy reserves. They continue feeding on the ice and only come the fall – so typically around October or November time, if they've got enough body fat accumulated at that point – only at that stage will the egg implant and begin developing.
How long is a polar bear’s gestation?
LA: Only around 60 days or so of development – it's a really short period, but it allows them a bit of flexibility. If they haven't gotten fat enough, then they won't continue with the pregnancy. Instead, they'll go back out onto the sea ice for another year, continue feeding and hopefully be in better condition to be able to support cubs the following year.
This is important for polar bear moms because giving birth and supporting cubs is really challenging for her, it requires a lot of investment. Anywhere from October through to November, dens are built in the snow. When she's in the den she doesn't eat, she doesn't drink, so she has to be fat enough to support herself while she's fasting.
During that time, she'll give birth to her cubs – anywhere from one to three cubs in midwinter. So around December/January time, and those clubs are very, very small, about the size of a block of butter.
By the time they leave the den [three months later], they're about 20 times bigger. A female polar bear can lose up to around 44 percent of her body mass during that time in the den. And in some parts of the Arctic, they can fast for about eight months, so it's really quite a challenge.
And what has your research revealed about the challenges they face feeding their cubs?
LA: For this study, we were really interested in what actually happens when polar bear mums who are supporting their cubs are forced to go without food. When polar bears leave the den with their cubs, the cubs will stick with them for about two and a half years and during that time, moms are providing them with milk.
In parts of the Arctic, polar bears move on to land when sea ice melts in the summertime. That can be challenging for polar bear moms who are continuing to provide milk for their cubs while they're not eating themselves. That’s because when they move on to land, there's nothing that's energetic enough for polar bears to feed on that they can catch easily. So, they really are fasting for most of that time on land.
We looked at polar bear milk samples taken from bears that were on land with their cubs and then we measured whether the energy content in the milk changed the longer they spent on land. We also looked at whether they're more likely to stop producing milk altogether because some of the polar bears sampled weren't actively providing any milk for their cubs, even though they had cubs with them.
What we found from that was that the longer polar bears spent on land, the more likely it was they had stopped lactating or providing milk altogether. Also, the longer they spent on land, they produced milk that was lower in energy for their cubs. This actually helped the females because they saved energy when they stopped producing milk.
Polar bear milk is very fatty, so it does require a lot of energy to produce it. So, when they stopped providing milk, they saved energy, thus, that benefited the moms but it had a negative impact on the growth of the cubs. So that could have a knock-on impact down the line on cub survival and potentially impact the population in the long term as well.
Find out how you can help these animals during Polar Bear Week on the PBI website.