Grolar Bears and Narwulga Whales
Published: February 24, 2011
Normally thought of as just "flukes" of Nature or the manipulated outcomes of zoo breeding experiments (see: 'Liger'), examples of such out-breeding in the wild are not very common, but can result from dramatic changes in habitat. But it appears as though inter-breeding is becoming more common; in 2009, a bowhead (Balaena mysticetus) and right whale (Eubalaena spp.) hybrid was spotted, and, more recently, sightings of spotted seal (Phoca largha) and ribbon seal (Histriophoca fasciata) crosses have been made.
These two examples are biologically noteworthy in another regard: they represent cross-breeding between two different Genera, which is a wider taxonomic grouping than specie (note: the polar and grizzly bears share the same Genus, but are different species).
Off-spring of these couplings are generally fertile and healthy. This is because, in each case, the cross-breeding creatures possess the same number of chromosomes (incompatible chromosome number results in mal-developed gametes), but differ in the types of alleles, or gene variants (making the pairing up of recessive alleles, and thus the emergence of deleterious traits, less likely).
Getting back to the seeming rise in hybrid occurrences...
It is theorized that the breaking up and melting of Arctic sea ice due to warming is the basic driver of this emerging, inter-breeding trend.
As land and water warm they become more habitable for creatures adapted to warmer conditions. Also: this 'El Norte' migration of land animal species may be buttressed by pressure from shrinking habitat in more southerly climes (e.g., grizzly habitat is diminishing due to several factors, including huge loss of montane pine forest habitat due to decimation from pine beetles, also partly blamed on warming impacts).
A team of ecologists lead by Brendan Kelly of the National Marine Mammal Laboratory, in Juneau Alaska, has been documenting cases of cross-breeding and out-breeding and they have identified 34 potentially cross-breeding animal pairs - including two separate, cross-breeding pairs of porpoises, seals and whales. One of these latter pairings is that between the narwhal (Monodon monoceros) and the beluga whale (Delphinapterus leucas) - yielding the "narluga" whale of this post's title. Recently, sightings of narlugas have been made in the Bering Sea.
The team's findings and theories were recently published in the journal Nature, under the title: 'The Arctic melting pot.'
As climate change continues, then, these hybrid animals will increase in number. Initially, off-spring of crosses (especially between different Genera) are likely to be fertile. Further, there are fewer chances for pairing up of similar, recessive alleles (or gene variants), at least initially.
According to Kelly, the biggest threat in the near term is to already endangered, native species, like the polar bear. Already shrinking in numbers as their habitat shrinks, polar bears will come under increasing pressures from the influx of a competing species*, as well as from the new cross-breeds. Kelly and his team are concerned that, if this hybridization trend continues, the polar bear and other imperiled animals native to the arctic will soon disappear in the wild.
The inter-breeding trend, if it is indeed a trend, may be an even worse threat to the right whale, whose numbers hover around 200.
But not all scientists in the field agree here. Many ecologists think that hybrids are less fit genetically and others point out the lack of solid evidence that hybridization in the Arctic is in fact increasing. Further, the very idea of a hybrid being somehow a "corruption" of the "pure" species has garnered its share of criticism from fellow scientists.
But perhaps, the concern here is preservation of the original, the most long-standing, of creatures. At least until people came along. Does the polar bear get some credit? Some respect? Enough to help protect its existence?
Is preserving the "bloodline" acceptable when it comes to amazing examples of animal life? Do the same rules of "survival of the fittest" apply when it is willful human industrial activity that is partly to blame?
And it's not all good news for the hybrids. According to theory, although the initial off-spring of cross-breeds are generally fertile, over time and continuous mating, recessive alleles (present in every genome) will build up in a population, increasing the likelihood of recessive pairing (what is needed for a recessive, non-dominant, trait to appear), resulting in less genetic fitness and, decreased chances of long-term survival.
Developing effective survival traits takes time (and perhaps also a stable environment). An accelerating rate of evolutionary change may simply be too much for some species.
Already, their have been signs of maladaptation: a narwhal was recently spotted off Greenland lacking its spiral tusk (which is important for sexual selection), and, in a German zoo, grizzly-polar hybrids exhibit seal-hunting behavior, but lack the polar bear's swimming capability.
Alternatively, these could be simply the result of male fighting, on the one hand, and environmental adaptation, on the other.
These could be merely exceptions, of course. Much more data is needed to validate these theories. But, if larger numbers of these hybrids are being driven by accelerated Climate Change/Arctic Warming, then we have a cautionary lesson: that the speeding up evolutionary pressures, the forcing of animals into rapid adaptive modes, may not produce biologically favorable outcomes.
Quoting Kelly from an interview with Live Science: "This change is happening so rapidly that it doesn't bode well for adaptive responses."
As the Arctic warms, and the great ice barrier to cross-breeding melts away, hybrids will no doubt grow more common; in time, will these hybrids also become endangered? The Endangered Species Act (ESA), as it is currently framed, makes no mention of protections for hybrids.
* One criticism here is the claim that grizzly distribution is shifting (northward) in response to the climate, when grizzly and polar bear domains over-lap considerably and have for thousands of years.
Some source material for this article came from the msnbc.com article 'Hybrid polar-grizzly bear a sign of Arctic's future?' by Janelle Weaver
Top Photo: Canadian Wildlife Service via AP; This polar bear was legally hunted by Jim Martell, left, in 2006 in Canada's Northwest Territory. DNA tests later showed the bear had a polar bear for a mother and a grizzly bear for a father.