Genus Mesoplodon - Beaked whales
The species of the genus Mesoplodon are very poorly known. Many of the species are known only, or primarily, from study of skeletal material or a few stranded carcasses. Because the external appearance and behaviour of individual species is poorly documented, it is nearly impossible, even for experts, to identify whales of the genus to species from sightings at sea. Even with a specimen in hand, museum preparation is often required for positive identification (except for adult males of some species). Useful field marks for adult males are beak length, shape of gape, location and size of teeth, and to a lesser extent, size and shape of the dorsal fin.
The distribution of most species is also poorly documented, and is mostly known from stranding records. More information is available from the eastern tropical Pacific than for any other area because of the extensive survey effort associated with the tuna fisheries there. Even for this area, however, the picture is very blurry. There are 14 species of Mesoplodon currently recognized. The newest of these was only described in 1991, and it is possible, even likely, that other undescribed species exist. Most species are similar in appearance, and only adult males are likely to be identifiable to species. Coloration is usually brownish grey to olive, often with extensive white spots and scarring, especially in adult males. The pattern of scarring (which is thought to be caused by intraspecific fighting among adult males) may be useful in narrowing identifications to one or several species. For example, sets of closely paired scratches suggest 2 teeth located near the tip of the beak (such as in True's or Hector's beaked whales), while single or more widely spaced parallel scratches may implicate species with more widely separated, protruding teeth set farther back in the jaws (Blainville's, strap-toothed, Stejneger's, Andrews', or Hubbs' beaked whales). Finally, Gervais', Sowerby's, Gray's, and ginkgo-toothed beaked whales have teeth that are removed from the tip of the jaw, butwhich do not project above the snout. Thus, these species would not be expected to have paired scars. All species of Mesoplodon have low, inconspicuous (usually invisible) blows and most have a small dorsal fin set about two-thirds of the way back from the snout tip.
In general, mesoplodonts are slow and sluggish. Most sightings are brief, as these whales do not spend much time on the surface. They are presumed to pursue mostly squid at great depths. Groups are usually small, most often 7 or less. Almost nothing is known of their behaviour and social organization.