Why are polar bear pregnancy rates declining in Western Hudson Bay?

We talk to researchers about a surprising drop in polar bear pregnancy rates in Western Hudson Bay, Canada.

When researchers first looked at pregnancy rates among the polar bears of Hudson Bay in the early 90s, the results were promising. Between 80-100% of adult females were pregnant between 1982 and 1990, with some slight variation from year to year. However, a recent study has uncovered a striking decline. We talked to study authors Andrew Derocher and David McGeachy about what they’re seeing and why.

What did you learn from this study?

Compared to the earlier study, we found a 12% drop overall in pregnancy rates for these polar bears in 1991 to 2021. What’s more, we now see a significant increase in the year-to-year variation, ranging from 50-100%. In other words, now an average of 25% of lone females on land aren’t pregnant — and that means fewer new cubs each year. Another concerning finding: the number of 4-year-olds that were pregnant dropped by 27%. Collectively, fewer cubs and a delayed age of first breeding are symptoms of a population in decline.

Why are pregnancy rates dropping?

As is so often the case in wildlife studies, it’s one thing to see change when monitoring over many decades but it’s another to figure out why. We found that fatter and prime aged females were more likely to be pregnant, so there’s a link to body condition and experience that is important. Are there other factors such as food availability, contaminants or perhaps increased cub mortality we don’t know about? Perhaps males are having a harder time finding females with sea ice being more fragmented with climate change. It’s clear from our analyses that the variation in pregnancy rates between years is increasing and the overall pregnancy rate is declining from the “good old days” in the 80s. 

While polar bears are extremely well adapted to the dynamics of Arctic ecosystems, there are limits to what the bears can do. Our findings provide new insights but frustratingly, also more questions.

How can you tell if a polar bear is pregnant?

Measuring the pregnancy status of polar bears is straightforward - as long as you can collect a blood sample near implantation in September. A lab analysis measures the progesterone and at a certain level, a bear is pregnant and below that, they’re not. This isn’t a new method but because polar bears are only routinely sampled in autumn in Hudson Bay, it’s not widely applied. 

How does a polar bear find a mate?

In the lifecycle of female polar bears, the obvious precursor to producing cubs is mating and becoming pregnant. This sounds simple until you ponder that a female in breeding condition has to find, or be found by, a male. This process isn’t trivial given the low density of bears on the sea ice — your mate may be far away. 

Nonetheless, the drive to mate prevails and males spend most of the spring looking for females in breeding condition. Due to the long mother-offspring bond, most females only mate every three years but any male over a few years old, given the chance, can sire young. With many females busy raising cubs, there are more males capable of breeding than females available. This skew during the mating season results in intense competition between males for mates. The biggest and strongest males tend to win these battles and, as a result, we see one of the most pronounced differences in size between males and females to be found in the animal kingdom.  Perhaps unsurprisingly given the competition, the prime males in their mid-teens do most of the breeding.

Does successful mating always result in cubs?

Following a week or two of romance (polar bears are surprisingly frisky and mate frequently because the physical process of mating is what releases eggs - a process called induced ovulation), a female is technically pregnant. However, the fertilized eggs won’t implant until autumn, and only then if conditions are right. 

At implantation, hormones change to deal with the pregnancy and a spike in blood progesterone levels indicate a pregnancy is moving ahead. This delay in implantation explains the tiny size of cubs at birth (about 600 grams / 1.5 pounds). They’ve only undergone 60 days of active gestation — about the same as dogs, and we know how small puppies are at birth. 

But some females, perhaps due to low body condition, don’t implant and return to the ice in autumn to breed again the following spring. Meanwhile, the pregnant females find or dig a maternity den which they enter from August through October. 

Other females that are solitary in the fall may not have bred during the mating season if they recently lost their cubs either shortly before or after migrating to shore. 

We’re not sure why some females may fail to implant. It doesn’t appear to be related to sea ice conditions, but it appears physiological mechanisms based on body mass and age are involved. There’s no point in attempting to produce cubs if a female lacks the resources to fast for up to eight months, gestate, and then nurse cubs to a viable size. This research was only possible because of  Environment and Climate Change Canada’s long-term research program in Western Hudson Bay established almost 50 years ago by the late Dr. Ian Stirling.