Galpagos sea lion - Zalophus wollebaeki

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Galapagos sea lions are similar in appearance to California sea lions, but differ in size, behavior, and skull morphology. Galapagos sea lions are sexually dimorphic, with males growing larger than females and having several secondary sexual characteristics. The degree of sexual dimorphism appears to be less than in California sea lions, although few weights and measurements are available for adults to confirm this observation.

Adult males are robust in the neck, chest, and shoulders and are proportionately much smaller in the abdomen. As males sexually mature the enlarging sagittal crest becomes evident as a bump on the crown. The crest grows until the male reaches physical maturity, at which time it forms a prominent ridge behind the eyes and creates a steep forehead. Galapagos sea lion males are said to lack the pale pelage coloration on top of the sagittal crest common in California sea lions, although this feature appears to be present in one published photograph. Also, the skull of adult male Galapagos sea lions has a 20-25% smaller sagittal crest, a shorter muzzle, is about 10% shorter overall and is narrower than the skull of male California sea lions. As in all otariids the canine teeth of adult males are larger and thicker than those of females.

Adult and subadult male Galapagos sea lions bark in often long repeated sequences. The bark is loud, rapidly repeated, and distinctive. Females and juveniles do not produce the repetitive bark. Juveniles, subadults and adults of both sexes will also growl.

Adult females and juveniles do no have a sagittal crest. When viewed in profile, juveniles have a nearly flat head with little or no forehead. Adult females have a slight forehead formed by a gentle slope from the crown to the muzzle. In contrast to adult males, adult females have a long relatively thin neck and a wide torso.

Both sexes have a long and somewhat narrow muzzle that tapers to a slightly pointed nose. In profile, the face of younger animals is dog-like. The fore flippers have a sparse short fur that extends beyond the wrist onto the middle of the dorsal surface of the flipper in a “V” pattern and does not reach the rounded tip. The rest of the dorsal surface, and the palms of both fore flippers are covered with a hairless black leathery skin. The first digit is the longest, widest and thickest, and curves posteriorly, giving the flipper a swept back look. Digits 2-4 are successively shorter. There is a small opening in the skin at the end of each digit for a claw that is usually reduced to a vestigial nodule, and rarely emerges above the skin. The claw openings are set back from the free edge of the flippers by cartilaginous rods that extend the length of each digit, and expand the size of the flippers. The hind flippers also have cartilaginous rods that extend the length of each toe. The bones of the three central toes terminate at the position of the small nails that emerge through the skin on the dorsal surface, set back from the end of the flipper. The first and fifth toes are longer than the three middle toes, and the first toe, or hallux, is longer and wider than the fifth toe. The hind flippers have short hair covering part of the proximal end of the flipper, and the rest of the dorsal surface, and the entire sole is covered in black leathery hairless skin.

Color of Galapagos sea lions is highly variable. When dry, the coat of adult males ranges from grayish and golden brown to the common dark brown, and most bulls appear blackish or very dark when wet. Darkening begins when males are subadults and is generally complete when a bull reaches physical maturity. Adult males can have light gray coloration on their backs. Adult females, juveniles and young subadult males are pale colored above, and can be many shades of tan to light brown. There are often light colored areas on the muzzle, and around and above the eyes in both sexes. The sparse short fur covering a portion of the tops of the flippers can be the same color or darker than the color of the body. Pups are born with a longer brownish-black lanugo coat that fades to pale brown by three to five months. Pups go through their first molt at around five months and emerge with the pelage of adult females and juveniles.

There is little information on the lengths and weighs attained by Galapagos sea lions, but they are said to be somewhat smaller than California sea lions. Adult males are estimated to weigh up to 250 kg, but this has not been confirmed through direct study. Four adult females caring for pups weighed from 50 to 100 kg. Pups of both sexes are born at approximately six kilograms and weaned at approximately 25 kg.

The dental formula is I 3/2, C 1/1, PC 6/5 (~75%), PC 5/5 (~25%).

Galapagos sea lions share the Archipelago with Galapagos fur seals. As vagrants they may show up in the range of South American sea lions and South American fur seals, and there is a record of the former from the Galapagos.

Galapagos and South American fur seals have thick pelage and look shaggier, especially when wet, than Galapagos sea lions. Both fur seals are dark gray to brown and darker than similar-sized sea lions, with the exception of a fully-grown male Galapagos sea lion. Both fur seals have a more pointed muzzle and proportionately larger eyes and longer ears that stand out farther from the head when they are in the water (or otherwise wet). Adults of both fur seals have long pale conspicuous vibrissae. Galapagos fur seals are the smallest otariids, only reaching 1.5 and 1.2 meters for males and females respectively, and are stockier with a shorter neck and body than Galapagos sea lions.

Galapagos sea lions are found throughout the Archipelago on all the major islands and on many smaller islands and rocks. A colony was established in 1986 at Isla de la Plata, just offshore of mainland Ecuador, and vagrants can be seen from the Ecuadorian coast north to Isla Gorgona in Columbia. There is also a record from Isla del Coco approximately 500 km southwest of Costa Rica.

Galapagos sea lions are essentially non-migratory. They are unafraid of humans when ashore and will investigate and climb on backpacks and other things people leave lying around. Haul-out sites can be on rugged shoreline types, including steep rocky shorelines, ledges and offshore stacks, but rookeries are mostly on gently sloping sandy and rocky beaches. Sea lions will use shade from vegetation, rocks, and cliffs, and wade into tidal and drainage pools or move into the ocean, as needed during the heat of the day to avoid overheating.

Pupping and breeding take place across an extended period from May through January. Because of the protracted breeding season and extended care provided to the pups by females, there are dependent pups on the rookeries year-round. Females usually wean pups in 11-12 months, but some continue to suckle yearlings along with newborn pups. Pups are attended continuously for 6-7 days, after which the female goes to sea to feed, and begins a cycle of daily, diurnal foraging trips that last an average of 12 hours. Pups will enter the water and begin to develop swimming skills 1-2 weeks after birth. Females return at night to nurse their pup, departing again the next morning. Females and pups recognize each other and reunite based on calls and scent. Galapagos sea lion females feed during the day, in contrast to Galapagos fur seals, which primarily feed at night. In addition to foraging niche separation, female sea lions reduce their thermoregulatory challenges by being at sea during the heat of the day.

Galapagos sea lions are polygynous and males hold territories both on land and in shallow water near shore that they vociferously and aggressively defend. Male tenure on territories usually lasts from ten days to three months, and most copulations occur in the water. Adult males have been observed to mob Galapagos sharks that approach rookeries.

Diving has been studied in four females. The maximum depth of dive recorded was 186 m and duration of 6.0 minutes. Average depth of dive was approximately 37-38 m and duration of less than 2 minutes. At sea they will raft at the surface and rest on their sides with one or more flippers held vertically in the air.

Very little information exists on Galapagos sea lion prey. The remains of small sardines have been observed in vomit found on beaches. Galapagos sea lions have been seen smashing octopus on the surface of the water, presumably to stun or break them up to facilitate swallowing. Foraging dives by lactating females occur predominantly during the day, and are only to relatively shallow depths. This usually precludes Galapagos sea lions from foraging on vertically-migrating species, such as myctophids, midshipmen, and other deeper living prey routinely taken by California sea lions. However, during El Niño events prey includes green-eyes and myctophids, suggesting a change in foraging strategy.

The majority of the Galapagos sea lion population lives in the Archipelago, which is an Ecuadorian National Park surrounded by a marine resources reserve. Tourism occurs on a large scale but is strictly controlled to protect wildlife from disturbance.

The population fluctuates between 20,000 and 50,000 animals. Die-offs and cessation of reproduction during El Ni&btilde;o events, when marine productivity collapses, have caused episodes of population decline. Irruptions of a sea lion poxvirus have occurred during El Niño events, adding to the stress on individuals from starvation. Feral and uncontrolled dogs have been reported to kill sea lion pups, and could transmit diseases to the population. Shark predation is evident from animals seen with injuries and scars from attacks, and killer whales are presumed to be another predator on Galapagos sea lions.

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Heath, C.B. 2002. California, Galapagos, and Japanese sea lions Zalophus californianus, Z. wollebaeki, and Z. japonicus. pp. 180-186 in W.F. Perrin, B. Würsig, and J.G.M. Thiewissen (eds.), Encyclopedia of Marine Mammals. Academic Press.

Kooyman, G.L. and F. Trillmich. 1986. Diving behavior of Galapagos sea lions. pp. 209-219 in R.L. Gentry and G.L. Kooyman (eds.), Fur Seals Maternal Strategies on Land and At Sea. Princeton University Press.

Odell, D.K. 1981. California sea lion - Zalophus californianus Lesson, 1828. pp. 67-97 in S.H. Ridgway and R.J. Harrison (eds.), Handbook of Marine Mammals, Vol. 1: The walrus, sea lions, fur seals and sea otter. Academic Press.

Orr, R.T. 1967. The Galapagos seal lion. Journal of Mammalogy 48(1):62-69.

Trillmich, F. and K.G.K. Trillmich. 1984. The mating system of pinnipeds and marine iguanas: Convergent evolution of polygyny. Biological Journal of the Linnean Society 21:209-216.

Trillmich, F. 1986. Attendance behavior of Galapagos sea lions. pp.196-208 in R.L. Gentry and G.L. Kooyman (eds.), Fur Seals Maternal Strategies on Land and At Sea. Princeton University Press.

Trillmich, F. and T. Dellinger. 1991. The effects of El Niño on Galapagos pinnipeds. pp. 66-74 in F. Trillmich and K.A. Ono (eds.), Pinnipeds and El Niño: Responses to environmental stress. Ecological Studies 88. Springer-Verlag, Berlin.