Steno bredanensis
rough-toothed dolphin

Rough-toothed dolphins, Steno bredanensis have a broad geographic range which ecompasses tropical and subtropical oceans. They have been frequently sighted along various coastal areas such as Kaua’i, Ni’ihau, and O’ahu of the Hawaiian Islands, the Mediterranean Sea, the Sicily Channel, Tahiti, Moorea, and the Windward Islands. (Gannier and West, 2005; Kuczaj II and Yeater, 2007; Ritter, 2007; Shirihai and Jarrett, 2006; Watkins, et al., 1987)

Although rough-toothed dolphins reside in both shallow and deep ocean waters, they prefer deep waters greater than 1500 m in depth. They have been found at depths of up to 2000 m. Their location is often driven by the amount of nutrients in a given area. Rough-tooth dolphins are most commonly spotted in temperate waters. They prefer sea surface temperatures of 25 ̊C during the warm season but have been discovered during the cold season in waters ranging from 17 to 24 ̊C. Rough-toothed dolphins are rarely seen ranging north of 40 degrees latitude or south of 35 degrees latitude. (Baird, et al., 2008; Gannier and West, 2005; Jefferson, et al., 1993; Kuczaj II and Yeater, 2007; Ritter, 2007; West, 2002)

The average adult rough-toothed dolphin reaches a length of 2 to 2.65 m and a mass of 90 to 160 kg. This species is the only dolphin species that possesses a long beak. The elongated beak is dual colored; the upper jaw is blue and gray while the lower is pale pink and white. Their colorings may vary geographically. The body of rough-toothed dolphins is dark grey with white or light colored spots on their sides. The belly, lips, and parts of the lower jaw are white. Rough-toothed dolphins have a distinctive color pattern, consisting of a dark narrow cape which passes over the eyes and arches high on the sides of the body. Some of these animals show white and yellowish scars, due to encounters with large squid, cookie-cutter sharks, other rough-toothed dolphins, and interactions with boats. Males and females are similar in appearance. However, some males can grow larger and possess a more pronounced post-anal hump and prevalent scars. Rough-toothed dolphins are commonly misidentified as bottlenose dolphins, spinner dolphins, and spotted dolphins, but closer examination of the beak, head shape, and jaw color can help distinguish these species. ("Rough-toothed Dolphin", 1998; Carwardine, 1995; Jefferson, 2002; Jefferson, et al., 1993; Kays and Wilson, 2009; Rouch and Poss, 1998; Shirihai and Jarrett, 2006; Watkins, et al., 1987)

Little information is available regarding the mating systems of rough-toothed dolphins in the wild. (Dohl, et al., 1974; Lodi, 1992; West, 2002)

The reproductive habits of rough-toothed dolphins are not well known, though captive studies provide some information. At birth, rough-toothed dolphins measure 1 to 1.3 m in length. Calves attempt to nurse within an hour of birth, but are initially unsuccessful, unable to connect to their mother’s mammary slits. Within the first 3 days, calves can successfully nurse, which takes place underwater and occurs throughout the day. Calves nurse, rest, and play on a daily basis. Play time generally follows nursing and includes exploration to the surface while staying in close proximity to the mother. Calves rest around midday for about 60 minutes. At 2 months of age, calves begin to eat fish and decrease nursing time. ("Rough-toothed Dolphin", 1998; Dohl, et al., 1974; Jefferson, 2002; Lodi, 1992; Ritter, 2007; West, 2002)

Rough-toothed dolphins exhibit sexual dimorphism, and mature males are longer than mature females. In both sexes, the most rapid growth occurs in the first 5 years. Females reach sexual maturity at 9 to 10 years of age at a length of 212 to 217 cm and a weight of 101 to 108 kg. Males reach sexual maturity at 5 to 10 years of age at a length of about 216 cm and a weight of 92 to 102 kg. (Dohl, et al., 1974; Jefferson, 2002; Ritter, 2007; West, 2002)

In captivity, female rough-toothed dolphins protect their calves by swimming in close proximity to their young and positioning themselves between the calf and other dolphins. The length of the mother-calf relationship is unknown. A female rough-toothed dolphin, presumed to be the mother, was observed supporting a dead calf at water's surface for several days. During this time, she was escorted and protected by a number of male rough-toothed dolphins. This may demonstrate a prolonged mother-calf association in rough-toothed dolphins. Such behavior has been observed in the tight social groups of other marine mammals. ("Rough-toothed Dolphin", 1998; Dohl, et al., 1974; Kuczaj II and Yeater, 2007; Lodi, 1992; Ritter, 2007; West, 2002)

Rough-toothed dolphins off the coast of Japan can live to be 32 to 36 years of age, though it is presumed that members of this species may live considerably longer. The oldest individual was estimated to be 48 years old and was found stranded from the Florida coast. The longest lived individual in captivity, however, was only 12 years of age. (Jefferson, 2002; West, 2002)

Rough-toothed dolphins are most commonly seen in groups of 10 to 30 individuals, though pods of up to 160 dolphins have been spotted containing eight smaller groups. They have also been spotted with pilot whales, bottlenose dolphins, spotted dolphins and spinner dolphins. Rough-toothed dolphins are great divers and can stay submerged for as long as 15 min. They are also fast swimmers and often swim just under the surface with their dorsal fins clearly visible. They enjoy "surfing" or bow-riding, but not as readily as other tropical dolphins. ("Rough-toothed Dolphin", 1998; Carwardine, 1995; Dohl, et al., 1974; Jefferson, et al., 1993; Lodi, 1992; Ritter, 2007)

Female rough-toothed dolphins protect their calves by swimming in close proximity to their young and positioning themselves between the calf and any other dolphin. This behavior can be characterized as epimeletic behavior, or behavior to attract support or attention from individuals in the group, species, or genus, and is commonly observed in rough-toothed dolphins. This species is also thought to demonstrate reciprocal altruism, as they swim below injured dolphins, supporting them at the surface so they can breathe. Rough-toothed dolphins have also been observed supporting deceased dolphins at the water's surface for several hours up to several days. ("Rough-toothed Dolphin", 1998; Carwardine, 1995; Dohl, et al., 1974; Jefferson, et al., 1993; Lodi, 1992; Ritter, 2007)

The most commonly observed behaviors of rough-toothed dolphins are travelling, social physical contact with pod members, feeding, resting, object play, sexual activity, and milling. In a study by Kuczaj and Yeater (2007) in their natural habitat, travelling comprised 26.6% of total time the dolphins were observed. In the same study, travel with a boat accounted for 17.5% of time. Social behaviors accounted for 11% of behaviors, which include pod members engaging in physical contact and surface swimming behaviors oriented toward one another. Feeding comprised 14.9% of their activity budget. Feeding behaviors involved tossing fish, diving for fish, and forming feeding circles. Rest occupied 9.7% of their time and was characterized as drifting in one constant direction. Milling (9%) is defined as the absence of physical contact between pod members, and swimming in a stationary location at the surface. Object play (interactions with foreign objects) represented 7.1% of observed behaviors and sexual behavior encompassed just 3.8% of observed behaviors. (Kuczaj II and Yeater, 2007)

A stranded and then rehabilitated rough-toothed dolphin was reported to travel over several hundred kilometers to 1,500 kilometers in a five month period. Members of this species do not defend a territory. (Baird, et al., 2008; Jefferson, 2002)

Rough-toothed dolphins communicate with other dolphins through echolocation clicks, whistles, burst pulse signals, and synchronous swimming patterns. Echolocation clicks help provide a sense of location, directionality, and with identifying objects. Burst pulse signals, which can be heard by the human ear, can be social or reinforce echolocation functions. Whistles are used socially among dolphins. Rough-toothed dolphins often travel in a close school with either synchronous or asynchronous swimming patterns. In the group of synchronous dolphins, a single dolphin produces higher frequency echolocation calls than the rest of the group. Rough-toothed dolphins are also found traveling alone, and these dolphins produce lower frequency echolocation calls. (Gotz, et al., 2006; Oswald, et al., 2007)

The diet of carnivorous rough-toothed dolphins includes silverside, saury, needlefish, mahimahi, and squid. Their preference, however, is mahimahi. Rough-toothed dolphins are excellent divers and are known to dive to great depths in search of cephalopods and large fish. They chase their prey and toss it around with their beaks. As their common name suggests, they have rough teeth, which allow them to tear apart their prey. Rough-toothed dolphins forage in groups of 3 to 5 for predator efficiency, and they share their meals. Members of this species are also known to forage on "bait balls" of schooling fish. ("Rough-toothed Dolphin", 1998; Baird, et al., 2008; Pitman and Stinchcomb, 2002; Shirihai and Jarrett, 2006)

Currently, there are no known predators of rough-toothed dolphins other than humans. Although they have been found with scars from bites of cookie-cutter sharks, there is no record of this species being consumed by a shark. Rough-tooth dolphins are, however, incidentally caught in fishing nets. Some humans eat this species of dolphin. (Jefferson, 2002; Monteiro-Neto, et al., 2000)

Rough-toothed dolphins help regulate adult populations of mahimahi. They also host a variety of parasites. Larvae and adult Anisakis have been found in their stomach, causing ulcers, internal bleeding, and gastritis. These nematodes may be transmitted by the sharing of food among dolphins and this parasite species' dependence upon various intermediate hosts. Several helminth parasites can also infect the intestines of rough-toothed dolphins, including the cestode Tetrabothrius forsteri. Several trematodes also parasitize rough-nosed dolphins, including Campula palliate in the liver and bile duct, Pholeter gastrophilus in the forestomach, and Synthesium tursionis in the intestines. (Carvalho, et al., 2010; Forrester and Robertson, 1975; Monteiro-Neto, et al., 2000; Pitman and Stinchcomb, 2002)

Steno bredanensis individuals are caught and consumed by humans in the West Indies, West Africa, Japan, and the Solomon Islands. (West, 2002)

There are no known adverse effects of rough-toothed dolphins on humans.

Although listed as a species of list concern on the ICUN Red List, Steno bredanensis is experiencing an increase in mortality rate. Fisheries along the coast of Ceara, Brazil incidentally catch rough-toothed dolphins in gill nets. Individuals caught in gill nets are thrown overboard or used as bait for sharks. Small numbers of rough-toothed dolphins are targeted as food for humans by direct and drive fisheries, located in the West Indies, West Africa, Japan, and the Solomon Islands. Habitat destruction due to anthropogenic disturbances also threaten populations of this species. (Mann, et al., 2010; Monteiro-Neto, et al., 2000; West, 2002)

Severe to profound hearing loss was found in 5 out of 14 rough-toothed dolphins stranded or entangled in fishing gear from 2004-2009. Hearing loss in marine mammals is contributed to five factors: congenital genetic factors, intense chronic noise from boats, old age, intense noises such as explosions, and ototoxic drug treatments that are administered during rehabilitation from dolphin strands. Hearing loss in 2 out of the 5 rough-toothed dolphins affected was contributed to genetic factors because they were young dolphins. (Mann, et al., 2010; Monteiro-Neto, et al., 2000; West, 2002)

In captivity, mating has occured between a rough-toothed dolphin and a bottlenose dolphin resulting in a rare Steno and bottlenose, Tursiops hybrid. The hybrid resembled a bottlenose calf and was 60 cm long at birth. The calf's respiration rate at birth was 4 breaths per minute and stabilized to one breath every two minutes by the third month. (Dohl, et al., 1974; Lodi, 1992; West, 2002)