Researchers in Canada have conducted an extensive genetic study on polar bears living in the Western Hudson Bay, producing a number of findings that contribute to our previously limited understanding of these elusive ice bears’ mating patterns. Among the most surprising discoveries was the detection of a pair of genetically identical twins – the first such case ever reported in any species of bear.
Though polar bears are often born in litters of two or three cubs, all of those sampled in previous studies were found to vary in their genetic makeup, suggesting that they had come from separate fertilized eggs. However, after analyzing the DNA of 4,449 bears collected between 1966 and 2011, the team uncovered evidence of a single pair of monozygotic polar bear twins – meaning siblings originating from a single fertilized egg.
The pair, which were two years old at the time of their capture in November 2003, were found to share the same genes on all 25 loci – or positions on a chromosome – that the researchers tested. The probability of this occurring by coincidence has been calculated at 1.64 x 10–11, or 164 billionths of a percent, enabling the researchers to reliably conclude that they are indeed monozygotic twins, as reported in the journal bioRxiv.
In addition to the unexpected discovery of these identical twins, the data also provided evidence of six cases of cub adoption, whereby adult females were found to be accompanied by infants to whom they were not related.
Such a phenomenon has been reported before, although the reasons for this are somewhat uncertain, especially since females reduce their own survival chances – as well as those of their natural cubs – by giving up their milk to non-related cubs.
One explanation put forward by the researchers is that mothers may have picked up stray cubs without noticing, possibly during periods of high bear density, such as the springtime den emergence or the autumn fasting period, when bears congregate on the shore.
Alternatively, they suggest that some mothers whose natural litters have died may adopt stray cubs because their maternal instincts compel them to do so. However, they have ruled out the possibility of cubs becoming separated from their family groups due to the temporary immobilization of mothers by scientists for research purposes. Indeed, none of the biological mothers of the adopted cubs were captured between the time of the cubs’ birth and their first observation with their adoptive mothers, implying that this switch could not have occurred when these genetic mothers were scientifically incapacitated.