Claims of interspecies hanky-panky have unfairly sullied polar bear & Neanderthal reputations

One big question I asked before writing my book on polar bear evolution was this: did interbreeding with grizzlies, aka brown bears, profoundly impact polar bear history, as geneticists insist? Or is something else going on?

Polar bear hybrid, 3/4 grizzly (offspring of a hybrid female backcrossed to a grizzly male), Ulukhaktok, Victoria Island, Canada, 2010.

Virtually all genetic studies done in recent years, which I review in my book, conclude that hybridization with grizzlies has happened to various degrees over the course of polar bear history (e.g. Cahill et al. 2013, 2018; Cronin et al. 1991; Edwards et al. 2011; Hailer 2015; Kumar et al. 2017; Miller et al. 2012). Two of the most recent studies claim the most complicated hybrid ancestry for polar bears yet, invoking tales of “extensive” past hybridization events between the two species (Lan et al. 2022; Wang et al. 2022).

But does their interpretation of the genetic data represent reality or does it simply fit the authors’ preferred but false narrative that climate change is to blame for recent hybridization events and therefore likely to happen more often in a warmer world? And if, as I argue in my book, grizzly hybridization isn’t needed to explain polar bear evolutionary history, what does that say about similar claims that there has been a significant amount of Neanderthal interbreeding with humans in our past? Put another way, are geneticists everywhere going overboard with claims of interspecies hanky-panky?

One of many suggested scenarios to explain polar bear evolutionary history that involve hybridization with grizzlies. This one is from Edwards et al. 2011 (Fig. 4B), where the scale on the bottom is years before present & the vertical lines indicate hybridization events.

Questions I had to ask

And their short answers (long answers in the book):

  1. Is polar bear hybridization common in the wild? No. We have scientific evidence of an isolated event that spanned more than a decade, involving one female polar bear and two male grizzlies (Pongracz et al. 2017). There is no genetic evidence of any other hybridization events in modern polar bears, despite testing of individuals across the Arctic (e.g. Cronin et al. 2012). Offspring of this mating have polar bear mitochondrial DNA (mtDNA) inherited from their mother. Numerous suspected hybrids have proven to be blonde grizzlies.
  2. What are the barriers to hybridization in the wild? Considerable, including a mismatch in mating times but also differences in aggression between the two species. A hybrid cross can only occur between a very late breeding polar bear and a very early breeding grizzly, and it seems very few such individuals are present in any population (e.g. Smith and Aars 2015). Also, grizzlies are considerably more aggressive than polar bears, such that even very large male polar bears actively avoid contact with much smaller grizzly females outside the breeding season (Miller et al. 2015). This means that mating between a polar bear female (the less aggressive species) and a male grizzly is much more likely to occur in the wild. Polar bear males do not expect their mating partners to be as aggressive as a grizzly female is by nature, and are more likely to run than copulate.
  3. What do hybridization events between captive (zoo) bears tell us? Two documented hybridization events happened in zoos that involved a male polar bear X female grizzly, the opposite cross to the example documented in the wild (Davis 1950; Preuß et al. 2009). In both cases, the individual bears were housed together or lived side-by-side, giving them years to become acquainted. In one case, mating occurred only after 24 years of being housed together, suggesting it took considerable forced familiarity to break down the natural avoidance behaviour of the polar bear male to the grizzly female. Offspring of this mating have grizzly mtDNA inherited from their mother.
  4. Has a male polar bear ever mated with a female grizzly in the wild? Not that we know about. There has been no grizzly mtDNA found in wild polar bears (Cronin et al. 2012).
  5. Would rare hybridization events be easy to detect in a population genetic study of modern animals? No. Even if a hybridization event like the one documented in 2006 occurred once every hundred years or so, it is very unlikely that descendants of the individuals involved would be selected for sampling, even when hundreds of samples are used. That’s another way of saying that rare hybridization events will have no significant impact on the genetic history of a species.
  6. Would rare hybridization events be easy to detect from ancient samples? No. It would be even more unlikely to find mtDNA evidence of a rare hybridization event when there are only a few samples to examine. Detecting past hybridization events based on nuclear DNA analysis is a numbers game, where statistical probabilities are all you get for an answer.
  7. Is there another, equally plausible, explanation for finding modern polar bear DNA in some grizzly bear genetic samples? Yes. It’s called “shared common ancestry.” The simplest explanation is that both modern and ancient polar bears, and some modern grizzlies, are descendants of an ancient linage of brown bears that was once wide-spread around the world but is now extinct. That is, polar bears evolved from that ancient lineage into a new species and ABC Island brown bears that live in Southeast Alaska (Heaton et al. 1996; Talbot and Shields 1996) are an isolated relict population of that ancient lineage (Crockford 2023).
  8. Is ‘shared common ancestry’ a known and accepted phenomenon? Yes. Several polar bear genetic studies acknowledge it’s importance as an alternative explanation to hybridization (e.g. Cronin et al. 1991, 2014; Davison et al. 2011; Kumar et al. 2017; Kutchera et al. 2014; Kitchener et al. 2020). But this has been routinely dismissed or ignored as a viable option in favour of the hybridization scenario, especially after the recent hybridization event in Canada occurred in the early 2000s.
  9. Is shared common ancestry acknowledged as an alternative explanation in human/Neanderthal hybrid studies? Yes, but rarely (e.g. Erikson and Manica 2012; Meneganzin and Bernardi 2023; University of Cambridge 2012), even though shared genes from their common ancestor is the more plausible explanation. As for polar bears, the interbreeding narrative is given more weight in the human evolution field, probably because it makes for a more interesting story that’s ready-made for media promotion (e.g. Stringer and Crété 2022). What scientist isn’t thrilled to have their work splashed across the pages of newspapers world-wide, when the alternative is obscurity?

Geneticists prefer the polar bear hybridization story but that doesn’t make it the truth.

As for the human/Neanderthal example, it simply makes a more interesting story and provides a way to frame otherwise boring genetic research as something the general public will find compelling. Interspecies sex sells, whether recent or in the past!

More recently, genetic studies have been framed around a preferred narrative that falsely blames recent hybridization on human-caused climate change and concludes that even warmer Arctic temperatures, as predicted by climate models, will cause the extinction of polar bears through interbreeding with grizzlies (e.g. Lan et al. 2022; Wang et al. 2022). This scary hypothetical scenario has been repeated often by the media, the message reaches a public who would never read the jargon-heavy scientific papers (e.g. Cassella 2022; Dickie 2017; Popescu 2016; Stephens 2022; White 2022).

Bottom line

I contend that extinct and living ABC brown bears have a genetic similarity to modern polar bears because they are both descendants of an ancient linage of brown bears that was once wide-spread around the world, and because polar bears, having evolved only about 140kya, are an especially young species. There is no reason to invoke hybridization to explain polar bear or human evolutionary history (Crockford 2023).


Cahill, J.A., Green, R.E., Fulton, T.L., et al. 2013. Genomic evidence for island population conversion resolves conflicting theories of polar bear evolution. PLoS Genetics 9(3): e1003345.

Cahill, J.A., Stirling, I., Kistler, L., et al. 2015. Genomic evidence of geographically widespread gene flow from polar bears into brown bears. Molecular Ecology 24:1205–1217.

Cassella, C. 2022. Hybrid ‘brolar bears’ could spread through the Arctic as the planet warms. ScienceAlert, 17 November.

Crockford, S.J. 2023. Polar Bear Evolution: A Model for How New Species Arise. Amazon Digital Services, Victoria.

Cronin, M.A., Amstrup, S.C. and Garner, G.W. 1991. Interspecific and intraspecific mitochondrial DNA variation in North American bears (Ursus). Canadian Journal of Zoology 69:2985–2992.

Cronin, M.A. and MacNeil, M.D. 2012. Genetic relationships of extant brown bears (Ursus arctos) and polar bears (Ursus maritimus). Journal of Heredity 103(6):873–881.

Cronin, M.A., Rincon, G., Meredith, R.W., et al. 2014. Molecular phylogeny and SNP variation of polar bears (Ursus maritimus), brown bears (U. arctos), and black bears (U. americanus) derived from genome sequences. Journal of Heredity 105(3):312–323.

Davis, M. 1950. Hybrids of the polar bear and Kadiak [sic] bear. Journal of Mammalogy 31(4):449–450.

Dickie, G. 2017. On the march: As polar bears retreat, grizzlies take new territory. The New Humanitarian, 28 June.

Edwards, C.J., Suchard, M.A., Lemey, P., et al. 2011. Ancient hybridization and an Irish origin for the modern polar bear matriline. Current Biology 21:1251–1258.

Erikson, A. and Manica, A. 2012. Effect of ancient population structure on the degree of polymorphism shared between modern human populations and ancient hominins. Proceedings of the National Academy of Science 109(35):13956–13960.

Hailer, F. 2015. Introgressive hybridization: Brown bears as vectors for polar bear alleles. Molecular Ecology 24(6):1161–1163.

Heaton T.H., Talbot S.L. and Shields G.F. 1996. An ice age refugium for large mammals in the Alexander Archipelago, southeastern Alaska. Quaternary Research 46:189–192.

Kitchener, A.C., Bellemain, E. Ding, X., et al. 2020. Systematics, evolution, and genetics of bears. In Bears of the World, Penteriani, V. and Melletti, M. (eds), pp. 3–20. Cambridge University Press, Cambridge.

Kumar, V., Lammers, F., Bidon, T., et al. 2017. The evolutionary history of bears is characterized by gene flow across species. Scientific Reports 7:46487

Kutschera, V.E., T. Bidon, F. Hailer, J.L. et al. 2014. Bears in a forest of gene trees: Phylogenetic inference is complicated by incomplete lineage sorting and gene flow. Molecular Biology and Evolution 31:2004–2017.

Lan, T., Leppälä, K., Tomlin, C., et al., including Lindqvist, C. 2022. Insights into bear evolution from a Pleistocene polar bear genome. Proceedings of the National Academy of Sciences USA 119(24):e2200016119.

Meneganzin, A. and Bernardi, M. 2023. Were Neanderthals and Homo sapiens ‘good species’? Quaternary Science Reviews 303:107975.

Miller, W., Schuster, S.C., Welch, A.J., et al. 2012. Polar and brown bear genomes reveal ancient admixture and demographic footprints of past climate change. Proceedings of the National Academy of Sciences 109(36):E2382–E2390.

Miller, S., Wilder, J. and Wilson, R.R. 2015. Polar bear-grizzly bear interactions during the autumn open-water period in Alaska. Journal of Mammalogy 96(6):1317–1325.

Pongracz, J.D., Paetkau, D., Branigan, M., et al. 2017. Recent hybridization between a polar bear and grizzly bears in the Canadian Arctic. Arctic 70:151–160.

Popescu, A. 2016. Love in the time of climate change: Grizzlies and polar bears are now mating. Washington Post, 23 May (interview with Andrew Derocher about polar bear hybrids and climate change).

Preuß, A., Gansloßer, U., Purschke, G., et al. 2009. Bear-hybrids: Behaviour and phenotype. Der Zoologische Garten 78(4):204–220.

Smith, T.G. and Aars, J. 2015. Polar bears (Ursus maritimus) mating during late June on the pack ice of northern Svalbard, Norway. Polar Research 34:25786.

Stephens, T. 2022. 100,000-year-old polar bear genome reveals ancient hybridization with brown bears. University of California Santa Cruz press release, 16 June.

Stringer, C. and Crété, L. 2022. Mapping Interactions of H. neanderthalensis and Homo sapiens from the fossil and genetic records. PalaeoAnthropology 2:401–412.

Talbot, S.L. and Shields, G.F. 1996. Phylogeography of brown bears (Ursus arctos) of Alaska and paraphyly within the Ursidae. Molecular Phylogenetics and Evolution 5:477–494.

University of Cambridge 2012. “Research raises doubts about whether modern humans and Neanderthals interbred.” ScienceDaily, 13 August.

Wang, M.-S., Murray, G.G.R., Mann, D., et al. 2022. A polar bear paleogenome reveals extensive ancient gene flow from polar bears into brown bears. Nature Ecology and Evolution 6:936–944.

White, R. 2022. Polar bears and grizzlies keep mating and could evolve into new species. Newsweek, 6 June.

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