Risso’s Dolphin

Physical Characteristics

Risso’s Dolphin (Grampus griseus) is a large, robust oceanic dolphin known for its heavily scarred body and blunt head shape.

Coloration:
They are born dark gray or brownish but lighten with age. Adults often appear pale gray to nearly white, covered in a network of scars from social interactions and squid encounters.

Head and Face:
They have a blunt, rounded head with no beak, distinguished by a vertical crease along the forehead. Their mouth contains 7 to 14 pairs of peg-like teeth in the lower jaw only.

Body:
The body is deep and stocky, with a tall dorsal fin located mid-back. The skin is often covered with scratches and scars, making each individual uniquely identifiable.

Fins:
They have long, pointed flippers and a sickle-shaped dorsal fin, one of the tallest among dolphins relative to body size.

Size:

• Length: Adults typically measure 10 to 13 ft (3 to 4 m).

• Weight: They weigh between 660 and 1,100 lbs (300 to 500 kg).

Sexual Dimorphism:
Males and females are similar in size, though males may accumulate more scars over time due to aggressive interactions.

Risso’s Dolphins are instantly recognizable by their pale, scarred bodies, blunt heads, and tall dorsal fins, setting them apart from other dolphin species.

Reproduction

The reproductive cycle of the Risso’s Dolphin is slow, with a strong emphasis on long-term maternal care.

Mating and Courtship:
Courtship behavior is not fully understood, but males compete for access to females, often leaving scars during social and mating interactions. Vocalizations and physical contact may also play roles in courtship.

Breeding Season:
Risso’s Dolphins do not have a strict breeding season, though births are more common in winter and spring in some regions, likely influenced by prey availability.

Gestation:
The gestation period lasts about 13 to 14 months, one of the longest among dolphins.

Birth of Calves:
Females usually give birth to a single calf, measuring about 4 to 5 ft (1.2 to 1.5 m) long and weighing 45 to 75 lbs (20 to 35 kg).

Nurturing and Care:
Calves nurse for 12 to 18 months, though maternal care and social learning continue for several years. Mothers are highly protective, often supported by the pod.

Independence and Maturity:
Juveniles remain closely tied to their mothers before gradually becoming independent. Sexual maturity is reached at 8 to 10 years for females and 10 to 12 years for males.

Reproductive Rate:
Females typically give birth every 2.5 to 3 years, reflecting a slow reproductive rate similar to other large dolphins.

The Risso’s Dolphin’s long gestation, extended maternal care, and infrequent births make population recovery slow, emphasizing the importance of stable environments and conservation efforts.

Lifespan

Risso’s Dolphins are long-lived marine mammals, with survival shaped by their oceanic environment, prey abundance, and human impacts.

Lifespan in the Wild:
They typically live 25 to 35 years, though some individuals have been recorded at 40+ years. Females generally live longer than males, benefiting from less risk-taking behavior.

Lifespan in Captivity:
Risso’s Dolphins do not adapt well to captivity. Stress, reduced space, and difficulty replicating natural diets often result in significantly shorter lifespans than in the wild.

Threats to Longevity:

• Bycatch: They are vulnerable to entanglement in drift nets and gillnets, leading to injury or drowning.

• Pollution: High levels of heavy metals and persistent organic pollutants accumulate in their tissues, weakening immunity and reproduction.

• Ship Strikes: Vessel collisions pose a risk in busy shipping lanes.

• Climate Change: Shifts in ocean temperature may affect squid populations, their primary prey.

• Noise Pollution: Military sonar and industrial noise can disrupt navigation and communication, increasing stranding risks.

Conservation actions, including marine protected areas, sustainable fishing regulations, and pollution control, are essential to safeguard the Risso’s Dolphin’s natural lifespan and population stability.

Eating Habits

Risso’s Dolphins are squid specialists, with feeding habits adapted to deep-water hunting.

Diet:
Their diet consists mainly of cephalopods, especially squid, but also includes cuttlefish and octopus. Fish are consumed occasionally, but squid make up the majority of their intake.

Hunting Strategy:
They are deep divers, capable of reaching depths over 1,000 ft (300 m) in search of squid. Hunting is guided primarily by echolocation in dark waters.

Foraging Behavior:
They usually forage at night when squid migrate closer to the surface. Pods often spread out to hunt individually but regroup after feeding.

Feeding Technique:
Risso’s Dolphins have few teeth—usually 7 to 14 pairs in the lower jaw only—which are used more for grasping than chewing. Prey is swallowed whole.

Feeding Frequency:
They feed multiple times a day, requiring high protein intake to sustain their large bodies and energetic lifestyle.

Adaptations:
Their blunt head and strong jaws are well suited for suction feeding on slippery squid. Scarring on their bodies is often the result of encounters with squid tentacles and other dolphins during feeding competition.

The Risso’s Dolphin’s specialization as a squid hunter makes it a key predator in deep-water ecosystems, linking mid-ocean food webs from surface to depth.

Uniqueness

Risso’s Dolphin (Grampus griseus) is one of the most distinctive members of the dolphin family, recognized by its unusual appearance and behaviors.

Scarred Appearance:
Their pale, heavily scarred skin makes each individual uniquely identifiable. Scars result from interactions with squid and aggressive encounters with other dolphins.

Blunt Head:
Unlike most dolphins, they lack a beak. Instead, they have a rounded head with a vertical crease, giving them a distinctive facial profile.

Minimal Teeth:
They possess only 7 to 14 pairs of teeth in the lower jaw, fewer than most dolphins, reflecting their specialization in soft-bodied prey like squid.

Deep Divers:
They are capable of deep and prolonged dives, often exceeding 1,000 ft (300 m), to reach squid in mesopelagic zones.

Social Structure:
Risso’s Dolphins form stable pods of 10 to 50 individuals, though super-pods of hundreds may gather. They often mix with other species, including pilot whales and bottlenose dolphins.

Surface Behavior:
Despite being deep divers, they are also highly active at the surface, frequently breaching, spyhopping, and slapping their flippers.

Global Range:
They are widely distributed in temperate and tropical oceans worldwide, often found along continental slopes and offshore waters.

The Risso’s Dolphin’s scarred skin, blunt head, and squid-specialist lifestyle make it one of the most easily recognizable and fascinating dolphins in the open ocean.