The Smallest Large Thing
by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station
Over the past few weeks we have seen some rare visitors to our island that illustrate our distance from the mainland and our location along feeding and migration routes for whales and dolphins. On July 28, a dwarf sperm whale beached itself near Low Beach (south shore near ‘Sconset) and after its brief struggle for life was lost, found a final resting place on the beach near the old sewer beds. The Nantucket Marine Mammal Stranding Team (NMMST) and the New England Aquarium (NEAq) identified the creature as Kogia sima, or dwarf sperm whale, the smallest of the whales. This is such a rare occurrence that the NEAq scientists decided to come down to the island to conduct a necropsy on the animal to determine the cause of death and obtain some more information regarding its age, sex, and condition. This is the first time in recent memory that this species of whale has washed ashore.
Senior Biologist Kate Sardi of the NEAq and two assistants along with a contingent of trained volunteers from the Nantucket Marine Mammal Stranding Team took tissue samples and measurements of the dwarf sperm whale. This specimen was a young male approximatively seven feet long and weighing around 350-400 pounds. In much the same way that an autopsy tells us what happens to a human who passes away; biologists use forensic science techniques to determine the cause of death and underlying health issues of deceased stranded animals. Pollutants commonly found in all mammals concentrate in either the fatty tissues, the brain, or internal organs such as the liver or kidneys depending on the chemical nature of the pollutant. The heads of these creatures are often taken back to the lab because so much information can be gathered from the teeth, skull, and brain. Measurements of blubber thickness, overall weight and length, and skin condition are helpful external clues regarding the health of an animal. Obvious wounds, whether made by predators or an indication of human-animal interactions such as netting or entanglement or propeller scars, are also noted so that scientists can record these interactions and form policy (in the case of human involvement) to reduce them. These mammals also suffer from some of the same ailments that plague humans including infections, cancer, disease, and neurological disorders. If you have a strong stomach and want to find out more about how a detailed marine mammal necropsy can uncover evidence of illness or contributing factors, then visit this web site: www.nmfs.noaa.gov/pr/health/mmume/event2005jan.htm.
I found it fascinating and an extremely useful report of a multiple species stranding event.
The dwarf sperm whale (Kogia sima) has often been lumped in with its look-alike cousin, the pygmy sperm whale (Kogia breviceps) in the Kogiidae family and Kogia genus. It was not until 1966 that the two species were regarded as separate when a biologist at the Smithsonian definitively placed the dwarf sperm whale in its own family, although spirited taxonomic debates are still occurring. There is also quite a bit of debate on the name Kogia as bestowed by J.E. Gray in 1846. Some biologists think that name is not descriptive enough, but I like it, especially as it may be derived from the Persian word kucek and there are not many scientific names that originate from that part of the world. Dwarf sperm whales (Kogia sima) were previously known as Kogia simus. The name was changed due to rules in the use of the Latin language, specifically switching to the feminine form for the genus name.
The dwarf sperm whale is the smallest of all whales. It grows up to 2.7 m (9-10 feet) in length – making it smaller than the bigger dolphins. They weigh between 136 to 272 kg (299.2 to 598.4 lbs), with an average weight of 204 kg (448.8 lbs). The dwarf sperm whale is physically very similar to the pygmy sperm whale. Identification may be close to impossible at sea; however, the dwarf is slightly smaller, has a smaller snout and has a considerably larger and more anteriorly located dorsal fin. The body is mainly bluish gray on the dorsal (top) side. The ventral side is paler with whitish to pinkish coloration that gives the animal a counter-shading effect. On the throat below the jaw may be several short longitudinal grooves or creases. Their "bulging" eyes are dark with a light circular mark above them. Dwarf sperm whales have a mark on either side of the head known as a "false gill" because of its resemblance to the gill slits of fish which is also seen on pygmy sperm whales. Besides size, the two whales are distinguished by their different jaws and teeth. The dwarf sperm whale has large curved sharp teeth in the lower jaw. Small non-functional teeth may be present in the upper jaw. The pygmy sperm whale lacks functional upper jaw teeth, but the dwarf sperm whale can sometimes have up to three pairs of small maxillary teeth. The top of the snout overhangs the lower jaw, which is small. These unusual snout and teeth have led to the species being described as the "rat porpoise" in the Lower Antilles. Some researchers describe this unusual mouth as a “shark like” mouth and I can see the resemblance.
Like the other sperm whales, the dwarf sperm whale has a spermaceti organ in its forehead although it does not contain nearly the volume of oil as its much larger relative. These guys can be aggressive like their famous cousins and have been known to ram small boats when caught in seine nets. Like the pygmy sperm whale, the dwarf is able to expel a dark reddish syrupy substance (intestinal tract liquid or poop, yum!) when frightened or attacked that disguises their presence (similar to the inking defense mechanisms of squid and octopi) to throw off predators. The whale will then hide in the cloud; mother calf pairs have been observed to do this when encircled in nets. Normally, the species makes slow, deliberate movements and will usually lie motionless (a behavior called logging) when at the sea's surface. Before diving, they will slowly roll or sink and disappear from view without displaying their flukes. Dwarf sperm whales, like their much larger cousins, can dive to great depths and feed on the ocean bottom to depths of possibly 300 meters and therefore they are not seen at the surface as much as the baleen whales. There are very few images of these creatures alive and swimming around due to their stealthy nature and rarity near shore. Most of our information comes from stranded animals or incident by-catch in nets.
The dwarf sperm whale prefers relatively deep water, but favors coastal areas more than the pygmy sperm whale. Its favorite habitat appears to be just off the continental shelf. In the Atlantic, animals have been observed across the entire ocean basin and as far south as southern Brazil and the tip of Africa. In the Indian Ocean, specimens have been found on the south coast of Australia and on many places along the Indian Ocean's northern coast. In the Pacific, the known range includes the Japanese coast and British Columbia. They do not exchange much between the Pacific and Atlantic Oceans and are considered by some researchers to be genetically diverse between ocean basins. They are seen throughout the year off of Africa which scientists seem to think implies they are not very migratory unlike some other whale species.
The rarity of the dwarf sperm whale, Kogia sima, keeps it from being targeted by commercial whalers; however, scientists theorize that their scarcity may be due to extensive hunting in the past. Occasional harpoon kills are made by Indonesian and Japanese fishermen and one paper documented significant mortality in drift gillnets. Because dwarf sperm whales live in coastal habitats as opposed to pygmy sperm whales, they may be more vulnerable to human interference such as fishing and pollution. No data exists at this time as to whether such activities are threatening the long-term survival of the species and on the International Union for Conservation of Nature’s Red List, they are listed under “data deficient” to indicate that we simply don't know whether to be concerned or not about their population levels.
The dwarf sperm whale, Kogia sima, feeds near the ocean bottom on delicacies like deep sea cephalopods (squid and octopi), fish, and crustaceans. They prefer squid, which explains their location near Nantucket as a fairly large mass of squid is fished by several boats off the southern shore of the island during the summer. This large shoal of squid may explain the recent stranding of a Risso's dolphin along the south shore (eventually meeting its demise on the south shore of Tuckernuck) and some reports of other large dolphins and small whales. Dwarf sperm whales use a suction feeding technique which involves rapid jaw-opening, a powerful retraction of the tongue and expansion of the throat to generate negative pressure and suck their squid prey into their undersized mouths. Both species (dwarf and pygmy) have been reported with plastic bags in their stomachs that may have prevented digestion of food and ultimately brought death, so plastic pollution in the ocean effects them because it mimics the appearance of squid.
Little is known about the reproductive cycle of the dwarf sperm whale. Males are sexually mature when they reach lengths of 2.1-2.2 m. Females are sexually mature when they are about 2.7 meters long. Gestation is thought to last about 9 months followed by a calving season of 4-5 months although calves may nurse for up to a year. Females are thought to give birth to one calf measuring about 1.0 -1.2 m long at birth. Juveniles do not change much in body shape as they mature. Although these whales can be gregarious and may form small groups of ten or fewer animals, they are also seen traveling alone. Sexually mature males and females are found in the same groups, and there is some evidence that immatures form their own groups. Intraspecies fighting has been reported, but the nature of these conflicts is not known. Perhaps they are arguing over the remote control. Despite the fights, the estimated lifespan for this species may be up to 22 years.
The population of these whales in the western North Atlantic Ocean is estimated to be 395 of both species, according to the International Union for Conservation of Nature’s Red List. But these creatures are very abundant near Hawaii and in other parts of the Pacific Ocean. Scientists attribute large counting errors in population estimates for these whales because their sneaky swimming mode and preference for offshore environments can easily contribute to underestimates. According to the NOAA National Marine Fisheries Service web site: “For management purposes, dwarf sperm whales inhabiting U.S. waters have been divided into four stocks: California/Oregon/Washington Stock, Hawaiian Stock, Northern Gulf of Mexico Stock, and Western North Atlantic Stock. The estimated abundance for Kogia sp. (dwarf and pygmy sperm whales) is about 300-400 animals for the Western North Atlantic stock, and 600-750 for the Northern Gulf of Mexico stock. The estimated abundance for dwarf sperm whales in the Hawaiian Islands Exclusive Economic Zone (EEZ) is 11,000-19,000. No population size estimate exists for the California/Oregon/Washington stock. In the eastern tropical Pacific there is an estimated 11,000 animals. There are insufficient data for this species to determine the population trends.”
Very little is known about dwarf sperm whales. Although unfortunate, last week's stranding and the subsequent research conducted on the animal will add precious knowledge to the slim scientific file that exists for this creature.
Race Week is coming up and for our next issue we'll talk about efforts to conduct a Clean Regatta and keep our harbor clean while enjoying sailing and boating.