The dwarf sperm whale is similar in appearance to the pygmy sperm whale, but has a larger dorsal fin, generally set nearer the middle of the back. Also, the dwarf sperm whale’s blowhole is positioned further forward. Like its congener, the dwarf sperm whale has a shark-like profile (but with a more pointed snout than the pygmy sperm whale), gray and white countershading, and a light pigment block resembling a shark’s gill slit on the side of its head. Generally, a pair of short grooves similar to those in beaked whales is present on the throat. There are 8-11 (sometimes up to 13) pairs of teeth in the lower jaw; sometimes teeth are present in the upper jaw as well.
Adults of this species are up to 2.7 m long (although in the northwestern North Atlantic they generally do not exceed 2.5 m) and may weigh up to 272 kg. Length at birth is about 1 m.
Can be Confused With
Dwarf sperm whales are most likely to be confused with pygmy sperm whales, which are very similar in appearance. Besides being generally smaller in length, dwarf sperm whales have taller, more dolphin-like dorsal fins, usually set more toward the middle of the back. Also, the position of the blowhole can help (being more than 10% of the way back from the snout tip in pygmy sperm whales, and less than 10% in dwarf sperm whales). However, because sizes overlap and dorsal fins are variable in size and position, many at-sea sightings of Kogia whales may not be identifiable to species.
The dwarf sperm whale, like the pygmy sperm whale, is known mostly from strandings. It is generally not commonly seen at sea, although this may have more to do with its cryptic appearance than actual rarity. It appears to be distributed widely in tropical to warm temperate zones, apparently largely offshore.
Ecology and Behavior
Dwarf sperm whales inhabit oceanic waters. Group sizes tend to be small, most often less than five individuals (although groups of up to 10 have been recorded). This species, like the pygmy sperm whale, is shy and undemonstrative when observed at sea. They often drift motionless at the surface. When startled, dwarf sperm whales may leave a large rust-colored cloud of fecal material behind as they dive. In at least one area, there appears to be a calving peak in summer.
Feeding and Prey
Dwarf sperm whales appear to feed primarily on deep-water cephalopods. This species is thought to feed in shallower water than does K. breviceps. They feed by seizing prey.
The dwarf sperm whale is not listed as threatened or endangered by either the IUCN or the United States government. Dwarf sperm whales are sometimes killed in directed fisheries in the Caribbean and Indo-Pacific regions, and a few have been known to die incidentally in fisheries throughout the range. They have a habit of eating plastic discarded or lost at sea, and their tendency to lay quietly at the surface may make them vulnerable to vessel strikes.
The United States National Marine Fisheries Service considers animals in United States waters to be members of several distinct stocks, and assesses them separately. Delineations between stocks are often difficult to determine, therefore assessments should be considered ongoing processes. In the case of the dwarf sperm whale, concern that sightings may be confused with or for the cogener K. breviceps (the pygmy sperm whale) further complicates the estimation of abundance. Stocks are estimated as follows:
All Kogia sp. (K. breviceps and K. sima) in the northern Gulf of Mexico – 547 (CV = 0.28) 1991-1994 shipboard surveys
All Kogia sp. in the western North Atlantic stock – 536 (CV = 0.45) 1998 shipboard and aerial surveys
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Dos Santos, R.A. and M. Haimovici. Cephalopods in the diet of marine mammals stranded of incidentally caught along southeastern and southern Brazil (21-34° S). 2001. Fisheries Research 52:99-112.
McAlpine, D.F. 2002. Pygmy and dwarf sperm whales Kogia breviceps and K. simus. pp. 1007-1009 in W.F. Perrin, B. Würsig and J.G.M. Thewissen, eds. Encyclopedia of marine mammals. Academic Press.
Nagorsen, D. 1985. Kogia simus. Mammalian Species 239:1-6.
Ross, G.J.B. 1979. Records of pygmy and dwarf sperm whales, genus Kogia, from southern Africa, with biological notes and some comparisons. Annals of the Cape Provincial Museums (Natural History) 15:259-327.
Willis, P.M. and R.W. Baird. 1998. Status of the dwarf sperm whale, Kogia simus, with special reference to Canada. Canadian Field-Naturalist 112:114-125.