The Galápagos sea lion (Zalophus wollebaeki) is a type of sea lion that lives and has its young on the Galápagos Islands. It also lives in smaller numbers on Isla de la Plata in Ecuador. This sea lion is the smallest of all sea lion types.
Taxonomy
This species was first described by E. Sivertsen in 1953. Some scientists thought it was a type of Zalophus californianus, called Z. c. wollebaeki. However, genetic studies show that Z. wollebaeki is a separate species. It belongs to the Otariidae family and the Zalophus genus.
Physical characteristics
Galápagos sea lions are slightly smaller than their Californian relatives. They measure between 1.5 and 2.5 meters (4.9 to 8.2 feet) in length and weigh from 50 to 400 kilograms (110 to 880 pounds). Males are generally larger than females. There is a small difference in size and features between males and females. Males have thicker, more muscular necks, chests, and shoulders compared to their slender bellies. Females have longer, thinner necks and broader bodies. When males reach sexual maturity, a bump-like projection develops on their forehead due to the growth of the sagittal crest. Adult females and young sea lions do not have this feature and instead have flat heads with little to no forehead. Compared to California sea lions, Galápagos sea lions have a smaller sagittal crest and shorter muzzles.
Both males and females have pointed, whiskered noses and long, narrow muzzles. Baby sea lions resemble dogs in shape. A key feature of sea lions is their external ears, which have visible pinnae, distinguishing them from seals. On their front flippers, short fur grows from the wrist to the middle of the dorsal fin. Otherwise, the flippers are covered in black, leathery skin. The first digit of each flipper is the largest and curves backward, giving the flipper a swept-back appearance. Each digit ends with a claw, which is usually small and rarely visible. Although they move awkwardly on land, sea lions are very agile in water. Their streamlined bodies and flipper-like feet help them swim quickly through waves and rocky coastal areas. They can also move their flippers independently, allowing them to change direction easily.
When wet, sea lions appear dark brown. When dry, their color varies. Females are usually lighter than males, and pups are chestnut brown. Newborn pups have long, dark brown fur called lanugo, which fades to brown within five months. At this time, they shed their baby fur and grow their adult coat. Galápagos sea lions reach sexual maturity at about 4 to 5 years old. Their lifespan is estimated to be between 15 and 24 years.
Distribution
Galápagos sea lions live on all the main islands of the Galápagos archipelago and many smaller islands nearby. Fewer than one-fourth of them are found on San Cristobal Island. These sea lions also live on Isla de la Plata, which is located just off the coast of mainland Ecuador. They can be seen along the coast of Ecuador, extending north to Isla Gorgona in Colombia. Galápagos sea lions have also been spotted on Isla del Coco, an island about 500 kilometers southwest of Costa Rica. However, the population on Isla del Coco is not permanent and is considered to be temporary visitors.
Diet and feeding patterns
Galápagos sea lions are carnivores, meaning they eat meat. Their main food is sardines, but they also eat crustaceans, squid, other fish (such as sardines and lanternfish), and octopus. Because they eat fish, they are classified as piscivores. These sea lions sometimes travel 10–15 km (6.2–9.3 mi) from the shore over several days to hunt. During these trips, they may encounter their main predators: sharks and killer whales. Scars and injuries from these attacks are often visible on their bodies.
During El Niño events, which occur when Pacific Ocean water temperatures change, sardine populations decrease or move away. This forces Galápagos sea lions to dive deeper to eat lanternfish instead. The lack of food during El Niño events often causes their populations to shrink. Scientists have observed Galápagos sea lions hunting yellowfin tuna in groups, guiding the fish into rocky areas or even onto land. This behavior was shown in a TV series called Blue Planet II.
Living near humans has given Galápagos sea lions access to more food sources. In addition to hunting, they sometimes take food from fishing boats and use the bright lights on these boats to help them find prey.
Behavior and male competition
Galápagos sea lions are very vulnerable to human activity. Their curious and social behavior makes them more likely to approach areas where humans live, leading them to come into contact with human waste, fishing nets, and hooks. These sea lions live in many different shoreline areas, including steep, rocky cliffs and flat sandy beaches. To stay cool during the day, sea lions seek shelter under plants, rocks, and cliffs to avoid the sun.
Sea lions are social animals and also very vocal. Adult males often bark in long, loud, and repeated patterns. Females and young sea lions do not make this type of repeated bark, but both male and female pups growl. From birth, a mother sea lion can recognize her pup’s unique bark and find it among a group of 30 or more barking sea lions.
Territorial males and non-territorial males behave differently. Territorial males vocalize more often than non-territorial males, and they begin vocalizing earlier. Vocalizations are important for choosing mates and help keep other males away from a male’s group of females. Most of the sounds made by territorial males are long-distance calls and are not aimed at specific targets.
On land, sea lions gather in groups at their resting areas. Adult males, called bulls, lead these groups. They can grow up to 7 feet (2 meters) long and weigh up to 800 pounds (360 kilograms). As males grow larger, they fight to gain control of a group of five to 25 females and the surrounding area. The dominant bull patrols his territory, swimming along the coastline and barking to show ownership. Nearby territorial males often display a "dear enemy effect," meaning they bark and act aggressively less often after repeated encounters with other males. Over time, males remember important details about their neighbors’ strength.
The average dominant bull holds his territory for only a few months. The dominant male cannot eat while defending his group, so he weakens and is eventually replaced by a stronger, healthier bull. On land, fights between bulls begin with both stretching their necks and barking to test each other. If this does not scare the opponent, they push and bite each other’s necks. Their thick, strong necks usually prevent serious harm, but injuries often occur, and many males have scars from these fights. The losing bull is chased away by the new dominant bull, often with splashing in the water.
Since only one male is in each group of females, there are always extra male sea lions without a territory. These males usually gather peacefully in less desirable areas of the coastline. One well-known place is the cliffs of South Plaza Island in the Galápagos. Males who lose their territory but stay on the island tend to bark less.
Breeding
The age at which Galápagos sea lions reach maturity is about 4 to 5 years. Their lifespan is estimated to be between 15 and 24 years. Breeding occurs from May to January. Because the breeding season is long and pups need a lot of care from their mothers, young pups are present in the colonies throughout the year. Each female in a group has one pup born about a year after mating. For about a week after birth, the mother stays with the pup constantly. After that, the mother returns to the ocean to search for food, and one week later, the pup joins her to begin learning how to swim. When the pup is two to three weeks old, the mother mates again. Mothers often take young pups with them into the water while nursing until about the 11th month, when the pups stop drinking their mother’s milk and start learning to hunt on their own.
The long-term relationship between mothers and their young is an important part of the social structure of these sea lions. A mother may care for a pup for up to three years. During this time, the mother and pup can recognize each other’s calls among the rest of the colony. Within the colony, young sea lions live together in a group called a rookery. Pups are often seen resting, playing, and eating together. It is common to see one mother watching over a group of pups while other mothers leave to find food.
Many mammals have their pregnancies occur at similar times to increase the chances of their young surviving, but this is not the case for Z. wollebaeki. Possible reasons for this lack of synchronization could include the absence of strong changes in daylight hours throughout the year, which is believed to control a pause in embryo development, and/or adaptation to an environment where food availability and prey populations vary.
Threats and status
The Galápagos Islands are part of an Ecuadorian national park and a protected marine area. These islands are a popular place for tourists, but strict rules are in place to protect wildlife. The number of sea lions in the Galápagos changes, usually between 20,000 and 50,000. Some dangers threaten their population. During El Niño events, ocean temperatures rise, and marine life that sea lions rely on decreases, leading to fewer sea lions and fewer babies being born. Sharks and killer whales are the main predators of sea lions. Adult sea lions are safer, but baby sea lions are more likely to be harmed. Rules about human behavior help reduce risks from people, but as more people live nearby, accidents and diseases still threaten sea lions. Sea lions sometimes stay near fishing areas to catch fish easily, but this puts them at greater risk from boats and fishing nets. Humans also affect sea lions indirectly. Stray dogs, introduced by people, form groups and attack sea lions. The pesticide DDT, used in some tropical countries to prevent malaria, builds up in the food chain and reaches harmful levels in sea lion pups. Between 2008 and 2012, more sea lions died from diseases. Galápagos sea lions are known to be affected by canine distemper virus, but this disease is not common in their population. A parasite called Toxoplasma gondii, which causes toxoplasmosis, has recently been found in sea lions.