Meet the hourglass dolphin
Small and stocky, with a striking blank and white pattern that gives it its common name, the hourglass dolphin is one of the least studied cetaceans in the world. Scientists know very little about its behavior and biology: as of 2010, less than eight specimens had ever been studied.
Hourglass dolphins are very rare, and yet they are not included in the threatened or endangered species list. Why? First, their habitat is the cold and remote subantarctic and Antarctic waters. Secondly, they are not hunted commercially, and thirdly, they are timid and usually avoid human contact.
Many names, one dolphin
Its scientific name is Lagenorhynchus cruciger. Lagenorhyncus means ‘jar-beaked’ referring to their small, jar shaped rostrums and ‘cruciger’ means cross carrier, which references their unique cross like color scheme on their flank. This dolphin may also be classified as Lagenorhynchus wilsoni, for Edward Wilson, an Antarctic naturalist in the early 20th century. Other names for this species are skunk dolphin, Wilson's dolphin, and Southern White-sided dolphin. They are also nicknamed “sea cows” due to their distinctive black and white coloring.
How to identify an hourglass dolphin
Its distinctive coloring sets it apart from other dolphins. But above the Antarctic Convergence, they might be confused with dusky and Peale's dolphins, which are also black and white. Below the Convergence, though, the hourglass dolphin is the only small cetacean with a dorsal fin, which is tall, hooked and with a broad base. Some animals, thought to be adult males, have a marked backward bend approximately half way up the fin. Their unique coloration and the shape of their fin makes it very easy to identify these dolphins at sea.
The hourglass dolphin's habitat
It is most commonly seen around the Antarctic Convergence, between South America and Macquarie Island, although it has also been seen off the south coast of New Zealand, near the South Shetland Islands, and around the Tierra del Fuego province. The largest concentration of sightings has been in the Drake Passage.
It is usually found in southern waters during the summer months and northern waters during the winter. This seems to suggest that the dolphins migrate seasonally following the cold-water currents.
Friends will be friends
Gregarious and sociable, the hourglass dolphin will typically congregate into pods ranging in size from 1 to 60 individuals, with their average group size being around seven. Not only these dolphins are sociable with their own species, but they have been seen “hanging out” in the company of fin whales, sei whales, southern bottlenose whales, Arnoux's beaked whales, killer whales, long-finned pilot whales, and southern right whale dolphins.
They enjoy bow riding the waves of these much larger animals, and they have been spotted altering their direction to catch the waves created by traveling boats and ships. They swim in long, low leaps and, from a distance, this undulating motion makes them look like a swimming penguin. Whalers have historically looked for this characteristic behavior in order to locate the fin whale.
What's for dinner?
The hourglass dolphins are known to feed on small fish including lanternfish, squid, and crustaceans, and they have been spotted feeding in aggregations of seabirds and plankton swarms. They have conical teeth, 26-34 teeth in their upper jaws and 27-35 teeth in their lower jaws, and they use echolocation for orientation and prey location. A recent study showed that they produce clicks with higher intensity, which allow them to detect their prey at more than twice the distance of other dolphin species.
Again, very little is known about the parental behavior of the hourglass dolphin. In studies of their social habits, only 3 calves were identified out of a group of 1,634 individuals. The scientists believe that this may be due to a winter birthing season when the weather prevents studies from being conducted or due to the fact that the mothers protect their calves by avoiding research vessels.
Threats to the hourglass dolphin
As we said at the beginning, the hourglass dolphin is not hunted commercially, and it is likely that their numbers are at or near original levels (the estimated global population is 144,300). There has never been any systematic exploitation, and there have been only a few accidental catches in net fisheries.
Scientists believe that the species is probably preyed upon by killer whales, but there is no documented evidence of this.
Hopefully, the increasing influx of Antarctica vacation tourism from southern South America to the Antarctic Peninsula will produce awareness and further sightings of the beautiful and shy hourglass dolphin.
Photo credit: Lomvi2-CC BY-SA 3.0