Callosities (marine mammals)

 

Perhaps no external feature on any baleen whale is as distinctive as the hardened patches of skin, called callosities (pronounced cal-OS-it-ies), found on the head of North Atlantic, North Pacific, and southern right whales (Eubalaena glacialis, E. japonica, and E. australis, respectively; Fig, 1). These features are characteristic of the genus and are immediately notable and visible upon sighting the whale. Old whalers called the most visible callosity, on the tip of the rostrum, the “bonnet”; that name has stuck through the present day.

The head of a northern right whale (Balaena glacialis) showing the most prominent callosity (usually called the bonnet).

Figure 1 The head of a northern right whale (Balaena glacialis) showing the most prominent callosity (usually called the bonnet).

The term callosity gained acceptance in the first part of the 20th century. The word extends from the term “callus,” which refers to a variety of thickened tissues in many species. A variety of terms have been used in the past, primarily based on the function assumed by the author. Speculation included the possibility that the callosity was an “excrescence” (a commonly used term in the late 19th century) from barnacles found on the head, abrasions from rubbing the head, or that they were irritations from whale lice. A number of whalers and scientists have noted the coincident occurrence between hair clusters on the whale and callosities, and Payne et al. (1983) noted that callosities also occur in the same locations as facial hair in humans, e.g., above the eyes, between the nostrils (blowholes) and upper lip, and on the skin covering the mandible. In fact, there are several locations where callosities are found and hairs are not present, and vice versa (e.g., small callosities are often also found immediately posterior to the nostrils). However, large callosities have at least a scattering of hairs over their surface, and smaller ones often have a single hair near the center of the callosity.

Callosities are a naturally occurring physical feature of the whale. They have been reported to be visible on both late-term (2.5 m) fetuses and newborn calves, although in some cases they may develop shortly after birth. In calves, the callosities are smooth and gray in color, but quickly acquire a pitted, jagged texture. It is thought that this may come from whale lice (Cyamid spp.), which, through eating a portion of the skin which comprises the callosity, create an area of lowered laminar flow in which they could adhere more easily to the whale. Certainly whale lice are widely present over the surface area of callosity tissue. To date, Cyamus ovalus and C. gracilis have been found on both northern and southern right whales. In addition, C. erraticus has also been reported from southern right whales. While the callosities maintain their gray color throughout life, they often appear white, yellow, or orange because of the coloration of the whale lice living there.

The function of callosities remains unknown. Male right whales have, on average, a greater portion of the surface area of their head covered by callosity tissue than females. These may be used by males in mating competition, and observations have been reported where males in mating groups deliberately ran the dorsal side of their heads along the backs of other males, with the recipient of the scrape reacting by “twisting and writhing” (Payne and Dorsey, 1983). Given the sensitivity of cetacean skin, it would be likely that contact from the callosity of another animal would be painful. While use in competition may account for the greater amount of callosity tissue in males, it does not explain why callosities are also present in females.

In the past 30 years callosities have received increased attention from cetologists photo-identifying individual right whales, as the shape and area of the callosities vary between individuals. In the southern right whale, individual identification is facilitated by a configuration where the bonnet covers only the front portion of the rostrum, and there are several additional “rostral islands” between the bonnet and the blowholes. This is referred to as a “broken” callosity. Researchers can then use the shape of the bonnet in addition to the number, location, and shape of the rostral islands to identify individuals. In the North Atlantic right whale, however, the bonnet can cover the entire area between the tip of the rostrum and the blowholes, referred to as a “continuous” callosity (found rarely, but occasionally, in southern right whales). Identification is further confounded because whale lice on and around the callosities are mobile, significantly masking the true edge of the callosity. Callosity edges in the same animal can then appear different in several sightings of the same individual. By using additional distinctive features, including the three-dimensional configuration of the callosities, additional scars or marks, and crenula-tions on the lower lip. North Atlantic researchers have still been able to reliably identify each individual. Photographic catalogs of identified right whales, primarily of their callosity patterns, have been published for the North Atlantic (by the New England Aquarium, Boston, MA) and Peninsula Valdez, Argentina (by the Whale Conservation Institute, Lincoln, MA).

Additional collections of photographs of individual right whales based on callosities and other natural markings exist in various institutions around the world.

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