Everyone wants to save the Maui’s dolphin. It’s either the world’s smallest dolphin or smallest marine mammal (depending who you read), and supposedly the most endangered: there are only about 55 left. A petition is doing the rounds to save the “most critically endangered marine species in the world” from extinction, partly by stopping iron ore mining on the seabed.
There’s one big problem, though. Maui’s dolphins don’t really exist, not in the same way that takahe or kakapo or short-tailed bats do. They’re not a distinct species, just a subspecies of Hector’s dolphins—in fact, it would be perfectly accurate to call them “North Island Hector’s dolphins”. And this has implications for their conservation.
Part of the problem is that people who should know better use technical terms like subspecies and species as if they were the the same thing, but there’s a world of difference. A subspecies is a population of animals or plants that looks a bit different from its relatives nearby. Critically, members of the subspecies are still perfectly-well able to breed with other members of their species. If a subspecies is isolated for long enough and become so different they it can’t interbreed any more, we call it, by definition, a different species. A species is a real thing, reproductively isolated and distinct from its nearest relative; a subspecies is just a formally-named variety, and what counts as a subspecies can be a matter of personal taste, reflecting how picky a biologist is. Some biologists refuse to even countenance subspecies and say, with some justification, that they’re not real.
Because biologists, like everyone else, are fond of large charismatic animals, it’s mostly mammals and birds that get fussily sorted into subspecies. This whole debate would be a non-issue if we were talking about an insect, or reptile, or fungus. The problem is if you split something into multiple subspecies you are almost guaranteed to create at least one rare, localised population, which then becomes in urgent need of conservation, and it’s problematic if ever-decreasing conservation funds have to be allocated to saving something that doesn’t really exist.
So what sort of thing is a Maui’s dolphin, then? Alan Baker and two colleagues measured bones of a dozen or so Hector’s dolphins from the North Island and few dozen from the South, and also compared their genetics. North Island Hector’s dolphins are a bit bigger, have a longer snout, and show some differences in their DNA—the sort of differences you would see if they’d been evolving separately from the southern dolphins for a few thousand years. (Thousands of years sounds like a lot, but it’s really nothing in evolutionary time.) Baker et al. (2002) named the northern population Cephalorhynchus hectori maui, and decided it would henceforth be called Maui’s dolphin, with the South Island population, confusingly, remaining Hector’s dolphin (Cephalorhynchus hectori hectori). Notice the triple-barrelled Latin names, which indicate both forms are just subspecies of Cephalorhynchus hectori.
(Maui’s dolphin is actually a pretty terrible name, as Maui didn’t identify, know about, or even see them, being, as he was, mythical. Baker et al. were naming them after the North Island, Maui’s fish—Te Ika a Maui—as that’s where they live, but the South Island is Maui’s canoe, and they live there too; it would be more appropriate, but a bad idea, to call them fish dolphins, or ika dolphins. Personally I think we should go back to cumbersome-but-accurate North Island and South Island Hector’s dolphins.)
Does all this pickiness about the difference between species and subspecies matter? Yes. Cephalorhynchus hectori are distinctive, unique beasts, not very much like other dolphins, and they’re only found here in New Zealand. Losing them would be a tragedy, because there’s no way to get them back. Maui’s dolphins are just those slightly-bigger Hector’s dolphins that live in the North Island, and if they died out the species would be be a little less varied, but, critically, it would still be here. It’s awful to lose a subspecies, but it doesn’t even compare to an actual species extinction; we’ve had plenty of those in New Zealand, and will be hard-pressed to prevent more.
It’s also important to look at these subspecies on a longer, evolutionary, timescale. The north/south split is, as you would guess, caused by Cook Strait; it isn’t a complete barrier, but Hector’s dolphins are coastal animals that prefer water less than 100 m deep, and most of the Strait is deeper. They can and do cross it, though, as the 2010 DoC survey found some South Island dolphins along the North Island coast. This species prefers cool water, so Maui’s dolphins are currently living close to the edge of their comfortable range, and population numbers might never have been very high.
But the North Island/South Island split is quite recent, and only temporary. Over the last couple of million years, New Zealand has been through about 20 ice ages, in which the world sea level dropped dramatically and our three main islands were joined into a single land mass. Back then, Hector’s dolphins would have lived as one continuous population in the cool waters around the entire coastline of that big island. Twenty times there has been a comparatively short warm period, called an interglacial (we’re in one right now), when the waters have risen, the islands have separated, and the North Island dolphins have been isolated from their kin and begun to evolve slightly differently. Twenty times they’ve been reabsorbed back into the fold. Maui’s dolphins have appeared and disappeared repeatedly for millions of years.
The species Cephalorhynchus hectori is in trouble: it’s been classified as Endangered since 2000 (IUCN 2011), and the population has dropped by 75% in the last 40 years, down to about 7000, largely from accidental bycatch in fishing nets. The response has been to ban commercial gillnetting in much of the dolphin’s habitat—coastal waters less than 100 m deep—though numbers continue to drop. I think it’s more important to stabilise the dolphin population than worry about whether or not they persist in the North Island. Most whales and dolphins, as their numbers recover, can recolonise their former range without assistance, and the fact that South Island Hector’s dolphins are still crossing Cook Strait suggests that would happen here too. Rather than worrying about seabed mining near Raglan, which is surely a comparatively minor threat, we should be fighting for Liz Slooten’s proposal: a gillnet and trawling ban across Cook Strait shallow waters so dolphins can get to the North Island unhindered.
Hector’s dolphins do need protection, but conservation organisations, journalists, and even the Department of Conservation are treating the North Island dolphins as if they were a distinct species about to go extinct, even comparing them to the moa. That’s irresponsible, and plays fast and loose with the facts at a time when we need to have those facts on our side.
Baker, Alan N., Smith, Adam N. H., and Pichler, Franz B. 2002. Geographical variation in Hector’s dolphin: recognition of new subspecies of Cephalorhynchus hectori. Journal of the Royal Society of New Zealand 32(4): 713–727
IUCN. 2011. Cephalorhynchus hectori. IUCN Red List of Threatened Species. Accessed 20 March 2012.
And for journalists who claim Cephalorhynchus hectori is the “smallest” or “rarest”:
- Smallest marine mammal: the sea otter (Enhydra lutris), up to 45 kg.
- Smallest cetacean: probably the vaquita (Phocoena sinus), from the Gulf of California, 40–55 kg.
- Rarest cetacean species: the vaquita again (100–300 left).
- Rarest other marine mammals: the Hawaiian monk seal (Monachus schauinslandi), 1100 remaining, and the Mediterranean monk seal (Monachus monachus), less than 600.