By Nadine Pinnell and Doug Sandilands

Two species of porpoise, Dall’s porpoise (Phocoenoides dalli) and harbour porpoise (Phocoena phocoena), are commonly seen along the B.C. coast. Although their ranges overlap, the two species rarely interact and do not form mixed groups. This made the discovery that they do hybridize (mate and have offspring) even more surprising.

Prior to 1994, unusually coloured porpoises had been seen around the southern Gulf Islands. These porpoises were all grey, all black or had colouring similar to harbour porpoises, but they generally behaved like Dall’s porpoise, bow riding and swimming and surfacing quickly. It had been assumed that these were just oddly coloured Dall’s porpoises; but then in 1994, the Stranded Whale and Dolphin Program of B.C. found a dead pregnant Dall’s porpoise that had washed ashore. Genetic analyses revealed that the fetus was fathered by a harbour porpoise. The hybrid fetus was the first cross documented in porpoises, and the second hybrid seen between cetacean (whales, dolphins and porpoise) species in the wild. [The first documented occurrences of cetacean hybridization were between fin whales and blue whales.]

Prompted by the discovery of the hybrid fetus, researcher Pamela Willis and colleagues set out to determine whether or not the unusually coloured porpoises seen in southern B.C. were in fact hybrids. The results of their study were published this year in the Canadian Journal of Zoology. While conducting surveys in the Haro Strait area, they saw up to four suspected hybrids a day. These individuals were seen swimming with Dall’s porpoises, alone, or with other suspected hybrids. Genetic testing revealed that these strange porpoises were indeed hybrids, the offspring of female Dall’s porpoises and male harbour porpoises. These hybrid porpoises seem to stay with their mothers or associate with other Dall’s porpoises rather than travelling with harbour porpoises. Based on the number of hybrids seen, the study authors estimated that hybrid individuals make up 1-2% of the Dall’s porpoise population in the southern Gulf Islands.

Two female hybrids seen during the study had calves traveling close to them, suggesting that female hybrids are fertile and can produce offspring. However, none of the male hybrids seen had the prominent hump that male Dall’s porpoises develop when they are sexually mature. This could mean that hybrid males are similar to harbour porpoise males in that they do not develop this hump, or that hybrid males are infertile and never reach sexual maturity. It is not uncommon for hybrid males to be sterile; for example, the first documented cetacean hybridization, which occurred between a fin whale and blue whale, produced a sterile male hybrid. Having sterile males would reduce the overall fertility of hybrids, making it less likely that they will persist in a population. However, further research is needed before it is clear if male hybrid porpoises are actually sterile.

Hybridization only seems to occur between female Dall’s porpoises and male harbour porpoises. Differences in mating strategies between the males of each species offer a possible explanation. Male Dall’s porpoises are not promiscuous and are quite protective of the females they mate with; an individual male’s strategy for reproductive success is to ensure that he is the only male to mate with a particular female. In contrast, harbour porpoise males are extremely promiscuous and will breed with as many females as possible. Their mating strategy is to try to produce more sperm than other males, increasing the chance that they will successfully fertilize the females they mate with. During the breeding season the testes of the male harbour porpoise enlarge significantly, becoming 4% of their body mass – one of the largest testes to body size ratios in the animal kingdom. By attempting to mate with as many female porpoises as possible, no matter what species they are, male harbour porpoises are likely the driving force behind porpoise hybridization in British Columbia.

Harbour porpoise habitat disturbance could also be causing hybridization. In other situations where two species have hybridized, one species’ population is often in decline due to disturbed habitat, making it difficult for members of that population to find individuals of their own species as mates. Anecdotal evidence indicates the harbour porpoise populations in southern British Columbia may have declined over the last 60 years. Thus, male harbour porpoises could be mating with Dall’s porpoises because there are fewer female harbour porpoises available for mates. The fact that most reports of hybrid porpoises are from southern British Columbia, where high population densities increase human disturbance to harbour porpoise habitat, supports the theory that harbour porpoise habitat disturbance could be related to hybrid porpoise abundance. However, it could also be that hybrid porpoises occurring in other areas are not being recognized as hybrids. The B.C. Cetacean Sightings Network at the Vancouver Aquarium has recieved two sightings of a hybrid porpoise in the Johnstone Strait area over the past two years, and a sailor from the area reports having seen strangely coloured porpoises before. More data is still needed to determine if hybrid porpoises are less common in other areas of the coast, or if observers in those areas are just not aware that the strangely coloured Dall’s porpoises they are seeing are hybrid porpoises.

Therefore, we encourage anyone who sees odd-looking porpoises (or any other whale, dolphin, or porpoise) to report their sighting to the B.C. Cetacean Sightings Network by phone (1 866 I SAW ONE), email, or through our web entry form. We’re also interested in any photos of hybrid porpoises you may take.

Tips on how to distinguish hybrid porpoises from other species:

Harbour porpoise
Length: to 1.8 m
Colouring: dark brown to gray, lighter colour on the belly
Dorsal fin: triangular; same colour as body
Behaviour: swims slowly; little splashing
Other characteristics: generally travels in small groups, avoids boats

Dall’s porpoise
Length: to 2.2 m
Colouring: black body with conspicuous white patch on either lower flank
Dorsal fin: triangular; white frosting on top, dark bottom
Behaviour: swims very quickly, creating a distinctive rooster-tail spray; will bow ride
Other characteristics: large hump before tail, especially in males

Hybrid porpoise

Length: varies
Colouring: all grey, all black, or similar to harbour porpoises (dark back, light belly)
Dorsal fin: triangular; colour varies
Behaviour: seen alone or with Dall’s porpoises; has rooster-tail spray; may bowride;
Other characteristics: no large hump before tail

Pacific white-sided dolphin
Length: to 2.3 m
Colouring: dark grey back with light gray sides and a white belly
Dorsal fin: curved or sickle-shaped; white on back of fin, dark otherwise
Behaviour: often leaps out of the water, bringing whole body out of water
Other characteristics: generally travel in large groups (25-100+)

References:

Baird, R.W., P.M. Willis, T.J. Guenther, P.J. Wilson and B.N. White, B.N. 1998. An intergeneric hybrid in the family Phocoenidae. Canadian Journal of Zoology, 76: 198-204. [PDF]

Willis, P.M., B.J. Crespi, L.M. Dill, R.W. Baird and M.B. Hanson. 2004. Natural hybridization between Dall’s porpoises (Phocoenoides dalli) and harbour porpoises (Phocoena phocoena). Canadian Journal of Zoology, 82: 828-834. [PDF]