During the winter months in British Columbia the number of cetacean (dolphins, whales, and porpoises) and sea turtle sightings we receive usually takes a nose dive. This isn’t surprising to us – there are often far fewer boats out on the water and thus fewer eyes to spot marine animals – but what did catch us off guard was an extremely unusual news report from Mexico this week: conjoined grey whale twins.

This report represents the first ever documentation of its kind for the species. The twins were found in Mexico’s Laguna Ojo de Liebre, a large lagoon on the west side of the Baja California peninsula and a known wintering area for eastern Pacific grey whales – the same population that travels along the B.C. coast. Although it’s likely the twins were stillborn (the calves were only about half the length of typical grey whale newborns at 2.1m), the fact that the twins were found and collected for research presents scientists with the highly unusual opportunity to study conjoined cetaceans.

Mysterious multiples

Researchers estimate that less than 1% of cetacean births are multiples, with even fewer being conjoined, although very little information is available on the phenomenon. According to Norris (1966), sei whales, the third largest whale species on earth, have the highest rate of multiple births out of all cetaceans, at 1.09%. The majority of cetacean multiple births result in fraternal, or dizygotic, twins – meaning the fetuses developed from two separate eggs. Norris also writes that the highest ever number of developing fetuses in a cetacean uterus was six, although the fate of those calves and their mother is unclear.

Prior to this recent discovery, only six cases of conjoined cetacean twins have been recorded. Out of those, four unborn conjoined twin fetuses were collected from pregnant female cetaceans caught by Japanese or Russian whalers between 1969 and 1987. No information was available on the other two cases. Adding to the mystery surrounding these unusual events is the fact that no pair of conjoined cetacean calves has been known to survive past birth and even less information is available on mothers that birthed them.

A case of killer whale twins

Killer whales are no different from the species mentioned above with regards to multiple births – for them the events also occur far and few between. No reports of conjoined killer whale twins exist, but there is one case of possible twins believed to have been born into the northern resident community in 1980. The pair, born to the cow I15, consisted of a boy (I41) and a girl (I4), and was documented in Killer Whales: The Natural History and Genealogy of Orcinus orca in British Columbia and Washington State by John Ford et al (1994).

Interestingly, the book notes that twinning may actually occur more frequently in killer whales than we think. Most calves are born in the colder months of the year, during winter and fall, when few boats are out on the water, so one or both calves may not survive long enough to be identified by the time researchers hit the water in the spring.

Could you be the next to report killer whale twins? Although very unlikely, it’s not impossible! Keep a lookout at all times of the year – you never know what you might see. Report all sightings to the B.C. Cetacean Sightings Network and you can be a part of our citizen science research project. Click here to report any sighting of a whale, dolphin, porpoise, or sea turtle in B.C.


Ford, J. K. B., Ellis, G. M., & Balcomb, K. C. (1994). Killer Whales: The Natural History and Genealogy of Orcinus orca in British Columbia and Washington State. (1st ed.) Canada: University of British Columbia Press and USA: University of Washington Press, Seattle.

Kompanje, E. J. O. (2005). A case of symmetrical conjoined twins in a bottlenose dolphin Tursiops truncatus (Mammalia, Cetacea). DEINSEA, 11, 147-150. Retrieved January 7, 2014 from http://erwinkompanje.files.wordpress.com/2010/10/deinsea-conjoined-tursiops.pdf

Norris, K. S. (1966). Whales, Dolphins, and Porpoises. USA: University of California Press.

Sieto Verver

In the spring, grey whales leave the winter breeding grounds in Mexico and migrate north towards Alaska, passing through B.C. waters.

Arjan Bronkhorst

Grey whales have distinctive mottled grey skin, and much of their bodies are covered with barnacles and whale lice.

Joan Lopez

It is estimated that less than 1% of cetacean pregnancies, including killer whales, bear more than one calf.

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