Risso’s dolphins (Grampus griseus) are distinguished by their heavily scarred bodies, a unique characteristic that makes them easily recognizable in marine environments. Born with dark gray skin, these dolphins progressively accumulate white marks throughout their lives. The striking appearance of these scars often prompts curiosity about their origin and what they signify for the animals.
The Canvas of Their Skin
The prominent scarring seen on Risso’s dolphins is a direct result of their unique skin properties. Their skin is relatively soft and susceptible to being marked, and crucially, these marks heal as depigmented, white scars. Unlike many other dolphin species where scars may fade over time, Risso’s dolphins lose pigmentation in their skin during the healing process, causing the white marks to persist. This process means that older individuals often appear almost entirely white due to the accumulation and overlapping of countless scars.
The mechanism behind this permanent white scarring involves the disappearance of melanocytes, the pigment-producing cells, in the scarred areas. Healed white scars show a significant reduction or complete absence of these cells, leading to the lasting pale marks.
Life’s Etchings: Sources of Scars
Risso’s dolphins acquire their extensive scarring from a variety of interactions throughout their lives. A significant portion of these marks originates from interactions with other Risso’s dolphins. These intraspecific encounters often involve “rake marks” inflicted by the teeth of conspecifics during social behaviors such as play, mating, or dominance displays. Males, in particular, tend to be more scarred than females, suggesting a role for these interactions in male-to-male competition and social hierarchy.
Another primary source of scars comes from their main prey, cephalopods like squid and octopus. The beaks and suction cups of these prey can leave distinctive circular or linear marks on the dolphins’ bodies, especially around the head. They possess only a few teeth in their lower jaw, which are primarily used for grasping squid rather than chewing.
While less common, encounters with predators also contribute to their scarred appearance. Sharks and killer whales are known predators of Risso’s dolphins, and survivors of such attacks may bear wounds. Environmental factors like rubbing against rough surfaces can cause marks. Human activities, such as entanglement in fishing gear or vessel strikes, can also leave scars.
Scars as a Life Story
The accumulation of scars on a Risso’s dolphin’s body tells a detailed story of its life experiences. The extent of scarring generally correlates with age, with older individuals displaying a greater degree of white markings. This makes the pattern of scars a visible indicator of its survival and encounters. The density and type of scars can also reflect an individual’s social history and its participation in intraspecific interactions.
Researchers use these unique scar patterns for individual identification, a technique known as photo-identification. By cataloging and tracking these distinct markings, scientists can monitor populations, study their movements, and gain insights into their social structures and longevity. The scars provide valuable data for understanding the lives of these marine mammals.