The search for “black dolphins” refers to large, dark-colored marine mammals that share the social behaviors and body shapes of dolphins. While few species technically classified as “true” dolphins are purely black, the distinction between dolphins and small whales is often blurred. Many cetaceans are jet black or dark gray and belong to the oceanic dolphin family, Delphinidae. Several very dark cetacean species are part of the dolphin lineage.
Cetacean Species That Are Jet Black
The most prominent examples of jet-black cetaceans are the Pilot Whales and the False Killer Whale. Both are taxonomically members of the oceanic dolphin family (Delphinidae), despite their common names.
The Long-finned Pilot Whale (Globicephala melas) is a large cetacean characterized by a robust body and a distinctive bulbous forehead, or melon. Adults are almost entirely dark brown to black, with only a small, light anchor-shaped patch on their belly. These whales inhabit cooler, temperate, and sub-polar waters.
The Short-finned Pilot Whale (Globicephala macrorhynchus) shares a similar dark coloration but prefers tropical and subtropical waters worldwide. Both species are often grouped with other large, dark dolphins and referred to collectively as “blackfish.”
The False Killer Whale (Pseudorca crassidens) is also slender and primarily black, with a rounded head and no discernible beak. They are found in deep, offshore waters across tropical and warm temperate seas. They typically grow to about six meters in length and are among the largest members of the Delphinidae family.
Color Variations and Melanism in Typically Gray Dolphins
Most well-known dolphin species, like the Bottlenose and Common Dolphins, exhibit a pattern called countershading. They are dark gray on their back and lighter on their belly, which helps them blend into the water column when viewed from above or below.
A dolphin that appears completely black is an extremely rare occurrence, typically resulting from a genetic condition called melanism. Melanism is characterized by an overproduction of melanin, the dark pigment responsible for coloration in the skin. This genetic anomaly causes an individual from a normally lighter species to display a hyperpigmented, all-black phenotype.
Documented cases of melanistic individuals have been observed in typically gray species, including the short-beaked common dolphin. Such individuals lack the usual distinctive patterns that define their species, appearing uniformly dark. The presence of these melanistic variants demonstrates that a truly black dolphin is possible, though it is an exception to the species’ typical appearance.
The Biological Purpose of Dark Coloration
The deep, dark pigmentation seen in species like the Pilot and False Killer Whales serves several important biological functions related to their environment.
One primary function is camouflage, known as crypsis. These species spend much time in deeper, darker offshore waters where little light penetrates. Uniformly dark coloration helps the cetacean blend into the low light environment, making it difficult for deep-dwelling prey or predators to detect their outline. This deep-water camouflage differs from the countershading used by surface-dwelling dolphins.
Dark coloring also plays a part in thermoregulation, the ability to maintain a consistent body temperature. Darker surfaces absorb solar radiation more efficiently than lighter ones. For large cetaceans, especially those inhabiting colder temperate and sub-polar zones, this enhanced heat absorption can be beneficial for conserving warmth.