Run silent, run deep

(This post was originally published September 7, 2021. Posts have been manually reordered for more logical storytelling. To go to the next post in the sequence, click “Previous Post” at bottom.)

The term “cetacean(s)” refers to all whales, dolphins, and porpoises. During this expedition, we are searching for beaked whales in the genus Mesoplodon (accent on the second syllable). Although small by whale standards, mesoplodonts can weigh 1–1.5 tons, which still makes them among the largest animals on earth. Currently, there are 16 recognized species of Mesoplodon, and surely more yet undescribed. Most of what we know about this group comes from dead specimens stranded on beaches, and by my count, 6 species have never been identified alive in the wild.

This photo shows an adult male Mesoplodon densirostris, a known species of beaked whale. The beak (mouth) is at the front and the two horn-like projections at the top of the skull are its two teeth protruding from the lower jaw. Photo: D Claridge.

Mesoplodonts live in open ocean waters and dive 1000–2000 meters for up to 1.5 hours at a time, mostly searching for small squids and midwater fishes. They are not very social. Group size is typically 2–3, usually a mother and calf, sometimes with a suitor. For the most part, they come to the surface only for a few quick gulps of air before descending back into the cold, dark depths that they inhabit most of the time.

Their main enemies are killer whales that patrol the surface waters. Beaked whales vocalize at depth, to communicate with each other and to echolocate their prey, but as they return to the surface, they observe radio silence so as not to tip-off any eaves-dropping orcas. Mesoplodonts are rarely seen at the surface: they seem wary of any strange sounds and usually slip under the waves at the approach of a vessel.

This photo shows a male Mesoplodon layardii skull. In adults, the lower teeth actually grow over the top of the jaw, constricting it so that it can open only a few centimeters. Photo: R. L. Pitman.

The name Mesoplodon roughly translates to “armed with a middle tooth.” Mesoplodonts are suction feeders that do not require teeth for feeding; females remain toothless their entire lives. Among maturing males, however, a single pair of teeth erupts from the lower jaw at various locations, depending on the species. Males use these teeth to spar with each other to determine access to breeding females. Instead of having horns or antlers for fighting, mesoplodonts use their out-sized teeth, which grow above the level of upper jaw. Although these battles have never been witnessed, they must be fierce, because when jousting males rake each other with their tusks, they sometimes leave deep, bleeding furrows along each other’s bodies.

In one of these species, the male has a tooth shaped like the leaf of a ginkgo tree — Mesoplodon ginkgodens (see photo in our first blog post). It is one of the species that has never been identified alive in the wild, and we suspect that it, or something like it, occurs in the Eastern Pacific Gyre. Our little expedition (GyreX) has come looking for it.

~ Bob Pitman

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