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Introduction

The hourglass dolphin is one of the most populous, yet poorly understood cetaceans. The species resides in off-shore Antarctic and sub-Antarctic water, a remote and often treacherous location making studies difficult. The habitat of the strikingly beautiful hour glass dolphin makes it a difficult species to study. They live in deep water (between 31 to 55 degrees Fahrenheit) near the Antarctic convergence, with the largest concentrations in the Drake Passage, a difficult place for researchers to get to! Although they are almost always found in deep pelagic waters, they may rarely be sighted in fairly shallow water near the Antarctic Peninsula or around islands and banks, and one (likely vagrant) individual was seen in the inland waters of Southern Chile in 2013. They may complete north-south migrations at different times of the year, but the extent of this is unknown.

These little dolphins are easily distinguishable from the other few cetaceans in the same range (i.e., the southern right whale dolphin). A smaller species at 5.9 feet and averaging 200 lbs (although few animals have been able to be measured), the hourglass dolphin is black with a white belly and white side stripe. This stripe is thick at the head and peduncle areas and very thin below the dorsal fin – resembling the shape of an hourglass (hence the name!). The dorsal fin is broad at the base and fairly hooked when compared to other species, with some males exhibiting a dorsal bending towards the body.

Hourglass dolphin showing clear markings along the flank. Photo: Roger Tidman.

Behavior

Hourglass dolphins are found in groups ranging from 1-12, with an average of about 7 individuals, though they can be seen in groups up to 100. 

Information on the life history of this species is very, very limited. For species that are hard to study in the field we often use stranded individuals to help fill in the gaps, but these dolphins (due to their pelagic lifestyle) rarely strand (there are about 5 on record). Thus their lifespan is unknown, but is thought to be similar to Atlantic white-sided (27yrs) and Pacific white-sided (46yrs).

Not surprisingly, virtually nothing is known about hourglass dolphin reproduction. Calves seem to be sighted less often, with one study observing only 3 calves in 1,634 individuals. However, this may be due to birthing season not coinciding with field work (there is limited time researchers can get to those waters!), or that females actively avoid research vessels. It is thought that their reproduction may be similar to other dolphins in the Lagenorhynchus genus that give birth about every 2.5 yrs, have a gestation of 12-13 months and nurse for 12-18 months.

One thing we do know is they love to ride bow waves – whether that be on boats, or large whales! They are often seen associated and riding on the bow waves of larger baleen whales. Hourglass dolphins are sociable with other species and have been sighted with a variety of dolphin and whale species including: fin whales, sei whales, minke whales, Arnoux’s beaked whales, southern bottlenose whales, long-finned pilot whales, Orca, southern right whale dolphins, southern right whales. Hourglass dolphins can swim up to 22km/hr or 13.5 mi/hr.

Diet

Research on what an animal eats is often conducted on stomach contents of stranded individuals, of which there is very little information for hourglass dolphins. But what has been identified is fish, squid and crustaceans. They sometimes feed in large groups in plankton swarms which also attract sea birds – this is helpful to researchers as it helps them find the dolphins! They use echolocation clicks to find food, like other dolphin species. Interestingly their clicks are narrow bandwidth high frequency (NBHF), but are more similar to their closely related cousins in the Cephalorhynchus genus (specifically Hectors dolphins) than other dolphins in their Lagenorhynchus genus.  However, they produce their sounds at a higher source level meaning they can detect prey at more than twice the distance compared to Hectors dolphins. This is likely important for their pelagic environment, whereas Hector’s dolphins are found close to shore. There is still so much to learn about this beautiful species!

This hourglass dolphin demonstrates their acrobatic nature! Image credit: Quotev.

Status

There are thought to be more than 140,000 hourglass dolphins in their range, with the last population count putting their population at approximately 144,300…however this count was from the late 1980’s so has likely changed somewhat! They are currently listed as a “Species of Least Concern” by the IUCN.

Threats

For once, there’s not a long list of threats for this petite dolphin species! In fact, aside from predatory killer whales the hourglass dolphin has no known direct threats to its population, likely due to their remote range that is inaccessible by human populations. However, due to their location, the hourglass dolphin is likely to be impacted by climate change and the implications that will have on its prey, movement patterns, and behavior.

Warming ocean temperatures continue to threat the polar regions, and could put hourglass dolphins at risk for prey shifts, greater anthropogenic activity, pollution, increasingly violent storms and currents, shifts in migration patterns, or other knock-on effects.

Fun Facts

  • Hourglass dolphins are the only dolphins living below the Antarctic convergence zone with a dorsal fin!
  • Their Latin name, Lagenorhynchus cruciger, literally translates to “bottle beak” (from lagenos meaning “bottle”, and rhynchus meaning “nose or beak”) “cross-carrier” (from cruciger). Cruciger refers to the area of black coloration on the flank which resembles a horizontal cross or, when viewed from above, a Maltese cross. So similarly to its common name, the hourglass dolphin’s Latin name is very apt!
  • Did you know these dolphins were discovered about 200 years ago, and that they are the only cetacean to have been classified based only on an eye-witness account?! It’s true! On an expedition to the Antarctic in 1824, two French naval surgeons and naturalists made a sketch of an unusual dolphin they observed, and due to the unique markings they declared it a new species. Thankfully they weren’t wrong, and given the overt nature of the hourglass dolphin’s markings, it’s unlikely they would be misidentified in that region!

cindy.elliser@pacmam.org

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