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California sea lion

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California sea lion
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Mammalia
Order:
Carnivora
Suborder:
Pinnipedia
Family:
Otariidae
Subfamily:
Otariinae
Genus:
Zalophus

Gill, 1866
Species:
Z. californianus
Binomial name
Zalophus californianus
(Lesson, 1828)
California sea lion breeding range in dark blue, total range light blue

The California sea lion (Zalophus californianus) is a coastal sea lion of western North America. Their numbers are abundant (188,000 U.S. stock, 1995 estimate),[2] and the population continues to expand about 5% annually.[3] They are quite intelligent and can adapt to man-made environments. Because of this, California sea lions are commonly found in public displays in zoos and marine parks and trained by the U.S. Navy for certain military operations. The California sea lion is the classic "circus seal," although it is not a true seal.[4]

Classification

The California sea lion is classified with the other sea lions and fur seals in the family Otariidae. Otariids, also known as eared seals, differ from true seals in having external ear flaps, and proportionately larger foreflippers and pectoral muscles. Along with the Galapagos sea lion and the extinct Japanese sea lion, the California sea lion belongs to the genus Zalophus, which derives from the Greek words za meaning "intensive" and lophus meaning "crest".[5] This refers to the protruding sagittal crest of the males, which distinguishes members of the genus.[6] Traditionally, the Galapagos sea lion and Japanese sea lion were classified as subspecies of the California sea lion. However, a genetic study in 2007 found that all three are in fact separate species.[7] The lineages of the California and Japanese sea lion appear to have split off 2.2 million years ago during the Pliocene.[8] The California sea lion differs from the Galapagos sea lion with its greater sexual dimorphism.[6] The Steller sea lion is the closest extant relative of the Zalophus sea lions, being a sister taxon.[9]

Appearance, physiology and movement

California sea lion skeleton

Being sexually dimorphic, California sea lions differ in size, shape and coloration between the sexes. Males are typically around 2.4 m (7.9 ft) long and weigh up to 350 kg (770 lb) while females are typically around 1.8 m (5.9 ft) and weigh up to 100 kg (220 lb).[6] Females and juveniles have a tawny brown pelage,[6] although they may be temporarily light gray or silver after molting.[10] The pelage of adult males can be anywhere from light brown to black but is typically dark brown.[6] The face of adult males may also have may also be light tan in some areas. Pups have a black or dark brown pelage at birth.[10] Although the species has a slender build, adult males have robust necks, chests and shoulders.[10] Adult males also have a protruding crest which gives them a "high, domed forehead",[11] and is tufted with white hairs.[6] They also have manes, which are less developed than those of adult male South American and Steller sea lions.[11] Both sexes have long, narrow muzzles.[10]

As an otariid, the California sea lion relies on its foreflippers to propel it when swimming and can turn its hindflippers forward and walk on all fours, which gives it maneuverability on land. The sea lion’s form of aquatic locomotion, along with its streamlined body, is effective in reducing drag. Movement of the foreflippers is not continuous, and the animal glides inbetween each stroke.[12] It's spine is so flexible that it can bend its neck backwards far enough to reach its hindflippers. This allows the animal to make dorsal turns and maintain a streamlined posture.[13] When moving on land, the sea lion moves the foreflippers in a "transeverse" rather than "sagittal" fashion. In addition, it also relies on moving its head and neck more than its hindflippers for terriestial locomotion.[14]

Ecology

Range and habitat

Sea lions at Santa Cruz, California

The California sea lion ranges along the western coast and nearby islands of North America, from southeast Alaska to central Mexico. Mitochondrial DNA sequences in 2009 have identified five distinct California sea lion populations; the US or Pacific Temperate stock, the Western Baja California or Pacific Tropical stock, and the Southern, Central and Northern Gulf of California stocks.[9] The US stock breeds mainly at the Channel Islands (although some breeding sites may be established in northern California, and females are now commonly found there.[1]) while the Western Baja California stock mainly breeds near Punta Eugenia and at Isla Santa Margarita. These populations are divided by the Ensenada Front. The stocks of the Gulf of California live in the shallow waters of the north, (Northern stock) the tidal islands near the center (Central stock) and the mouth of the bay (Southern stocks). The status of the sea lions at the deep waters of the central bay has not been analyzed.[9] Sea lions may travel at speeds of around 10.8 km/h.[15]

File:Sea lion with salmon.gif
Sea lion taking a chinook salmon near Bonneville Dam tailrace

During the breeding season, sea lions gather on both sandy or rocky shores. On warm days, they are closer to the water. At night or whenever it is cool, the sea lions can be found farther inland or at higher elevations.[10] Non-breeding adult males, subadult males and juveniles may gather at marinas and wharves, or even navigational buoys. California sea lions can also live in fresh water for periods of time. They may swim near the Bonneville Dam in the Columbia river.[16] In 2004, a healthy sea lion was found sitting on a road in Merced County, California, almost a hundred miles upstream from San Francisco Bay and half a mile from the San Joaquin River.[17] California sea lion can dive at depths of 274 m (0.170 mi) and lasting 9.9 minutes but most dives are typically 80 m (0.050 mi) and last less than 3 mintues.[18]

Diet and predation

California sea lions feed on a wide variety of seafood, mainly squid and fish, and sometimes even clams. Commonly eaten fish and squid species include salmon, hake, Pacific whiting, anchovies, herring, schooling fish, rock fish, lampreys, dog fish, and market squid.[19] They mostly forage near mainland coastlines, the continental shelf, sea mounts and, to a lesser extant, along the ocean bottom.[10] California sea lions may eat alone or in small to large groups, depending on the amount of food available. They will cooperate with other predators (dolphins, porpoises, and seabirds) when hunting large schools of fish.[20] Adult females feed between 10–100 kilometres (6.2–62.1 mi) from shore.[15] Males may forage as far as 450 km (280 mi) from shore when water temperatures rise and productively drops.[21] They also have learned to feed on steelhead and salmon below fish ladders at Bonneville Dam and at other locations in the Columbia River.[22]

Sea lions themselves are preyed on by killer whales and large sharks. At Monterey Bay, California sea lions appear to be the more common food items for mammal-eating "transient" killer whale pods.[23] The sea lions may respond to the doral fin of a killer whale and remain vigilant even when encountering fish-eating "resident" pods.[24] Sea lions are also common prey for white sharks. They have been recorded with scars made by attacks from both white sharks and shortfin mako sharks. Sharks attack sea lions by ambushing them while they are resting at the surface.[25] Sea lions that are attacked in the hindquaters are more likely to survive and make it to shore.[26]

Life history

Reproductive behavior and parenting

Sea lion rookery

California sea lions are gregarious breeders and breeding takes place between May and August, when the sea lions arrive at their breeding rookeries. When establishing a territory, the males will try to increase their chances of breeding by staying on the rookery for as long as possible.[27] During this time, they will fast, using their blubber as an energy store. Size is a key factor in winning fights as well as waiting. The bigger the male, the more blubber he can store, and the longer he can wait. A male sea lion usually keeps his territory for around 27 days. Females have long parturition intervals and thus the males do not establish their territories until after the females give birth.[27] Most fights occur during this time. After this, the males rely on ritualized displays (vocalizations, head-shaking, stares, bluff lunges, etc) to maintain their territoral boundries. Since temperatures can reach over 30 °C (86 °F) during this time, males must include water within their territories. Some territories are mostly in the water, particularly those near steep cliffs.[27] Sea lions that fail to establish a territory are driven out to sea or gather at a nearby beach.[6]

Sea lion mother with pup

Before mating begins, females gather into "milling" groups of 2–20 individuals. The females in these groups will mount each other and even a male. These groups begin to disintegrate as the female begin to mate.[6] The territorial and mating system of the California sea lion has been described as similar to a lek system, as female appear to choose their mates while moving though different territories.[28] They avoid males that too aggressive, rowdy or attentive. Males are usually unable to prevent females from leaving their territories,[6] particularly in water.[29] Mating may even occur outside the rookeries, between non-territoral males and females as the latter move to and from the breeding site. While in some rookeries, copulations may be monoplized by a few males, at others, a single male may sire no more than four pups.[29]

The females have a 12-month reproductive cycle, 9-month actual gestation with a 3-month delayed implantation of the fertilized egg before giving birth in May or June. Interbirth intervals are particularly long for this species, being 21 days for sea lions off California and more than 30 days for sea lions in the Gulf of California.[29] Females remain with their pups on shore for 10 days and nurse them. After this, females will go on foraging trips which can last as long as three days, returning to nurse their pups for up to a day. Pups left at shore tend to gather in nurseries to socialize and play.[10] When returning from a trip, females call their pups with distinctive calls to which the pups will rely in kind. A mother and pup can can distingush each other's calls from those of other mothers and pups. At first, reunions largely depend on the mothers. However, as pups get older, they get more involved in reunions.[30] In addition, older pups may sometimes join their mothers during their foraging trips.[10] Pups are weaned by a year but can continue to suckle for another year.[6]

Sea lions gathering at sea

Communication

California sea lions communicate with a repertoire of vocalizations. The commonly used one is their characteristic bark. Territorial males are the loudest and most continuous callers, and barks are produced constantly, day and night, during the peak of the breeding season.[31] Barks are especially rapid when sea lions are excited but are slower when they are calmer. The barks of territorial and non-territorial males sound similar, although those of the former are deeper. Males may bark when threatening other males or during courtship. The only other vocalization made by territorial males is a "prolonged hoarse grunt sound" made when an individual is startled by a human. This vocalization is also made by aggregating non-reproductive males.[31]

Female sea lions are less vocal. Their barks, high-pitched and shorter than those made by males, are used in aggressive situations. Other aggressive vocalizations given by females include the "squeal", the "belch" and the "growl". The sound a female sea lion gives when calling her pups is called a "pup-attraction call", described as "loud" and "brawling".[31] Pups respond with a "mother-response call" which is similar in structure. Pups will also bleat or bark when playing or in distress.[31] California sea lions can also produce vocalizations underwater. These include "whinny" sounds, barks, buzzings and clicks.[32]

Nonbreeding activities

Outside of the breeding season, adult and subadult males and juveniles migrate to the northern ends of the species range to feed, while females and juveniles forage near the breeding rookeries.[6] Sea lions can stay at seas for as long as two weeks at a time. They make continous dives, returning to the surface to rest. Sea lions may travel alone or in groups while at sea and haul-out inbetween each sea trips. Molting takes place in autmun and winter for adult females and juveniles and during January and February for adult males.[10] Gulf of California sea lions do not migrate and stay in the Gulf year-round.[29]

Intelligence and trainability

Female balancing a ball on her nose at Blackpool Zoo, England
Zak, a 375 lb (170 kg) Navy sea lion

Marine biologist Ronald J. Schusterman and his research associates have studied sea lions’ cognitive ability. They have discovered that sea lions are able to recognize relationships between stimuli based on similar functions or connections made with their peers, rather than only the stimuli's common features.[33] Sea lions may also be able to understand simple syntax and commands when taught an artificial sign language. However, the sea lions rarely used the signs semantically or logically.[34]

Because of their intelligence and trainabilty, California sea lions have been used circuses and marine parks to preform various tricks; such as throwing and catching balls on their noses, running up ladders, or honking horns in a musical fashion. Trainers reward their animals with fish, which motives them to preform.[35] For ball balancing, trainers will toss a ball at a sea lion so it may accidentally balance it or hold the ball on its nose and so it will understand what to do. A sea lion may go trough a year of training before preforming a trick for the public. However, its memory allows it to preform a trick even after three months of resting.[35] Some orangizations, such as the Humane Society of the United States and the World Society for the Protection of Animals, object to using sea lions and other marine mammals for entertainment, claiming the tricks are "exaggerated variations of their natural behaviors" and distract the audience from the animal's unnatural environment, "so different from these species' natural habitat".[36]

The California sea lion is used in military applications by the U.S. Navy Marine Mammal Program, including detecting naval mines and enemy divers. In the Persian Gulf, the animals swim behind divers approaching a US naval ship, and attach a clamp, which is connected to a rope, to the diver's leg. Navy officials say the sea lions can get the job done in seconds, and the enemy does not know the clamp was attached to his leg until it is too late.[37]

Status

Photo of sea lions crowded together on dock
Hundreds of California sea lions sunbathe on Pier 39 in San Francisco. The sea lions here are notable because the city welcomes them as a tourist attraction.

In the United States, the Marine Mammal Protection Act (MMPA) protects California sea lions. Nevertheless, their population has increased and now conflicts with humans and other wildlife. They damage docks and boats (which, given sufficient numbers, can actually sink under the weight of the basking animals), steal fish from commercial boats, and attack and injure swimmers in San Francisco Bay.[38]

In 2007, legislation amending the MMPA to permit their lethal removal from near salmon runs when their population exceeds their maximum sustainable level was introduced (HR 1769: Endangered Salmon Predation Prevention Act). The purpose of HR 1769 is to relieve pressure on the crashing Pacific Northwest salmon populations. Officially, pinnipeds (including sea lions) account for only an estimated 4%[39] of salmon loss in 2007. However, that figure comes from actual surface observation. Much predation occurs underwater, leading marine biologists to conclude the true rate is higher.[40]

Short of lethal removal, attempts have been made to identify individual aggressive salmon predators, and to relocate them. Relocation generally fails because they simply return. In January 2008, at the request of Washington and Oregon, the National Marine Fisheries Service drafted a proposal to euthanize approximately 30 animals annually at Bonneville Dam.[40] The Humane Society of the United States brought suit, and the Ninth Circuit Court of Appeals ruled that the sea lions could be trapped and removed, but not killed.[41]

From the Humane Society court case—NMFS's decision to authorize the killing of these animals is impossible to reconcile with: (1) NMFS's 2005 decision finding that fishermen's annual take of up to 17 percent of listed salmon is not significant and has only 'minimal adverse effects on Listed Salmonid ESUs in the Columbia River Basin'; (2) the States' 2008 decision to increase fishing quotas from 9 percent to 12 percent of the total spring run; and (3) NMFS's 2007 decision finding that hydroelectric dam take up to 60 percent of listed juvenile salmonids and up to 17 percent of listed adult salmonids 'meet[s] or exceed[s] the objectives of doing no harm and contributing to recovery with respect to the ESUs.'" [42]

In 2009, strandings of recently weaned pups increased three-fold. Lesser winds reduced ocean upwelling, which in turn reduced food supplies. Worse, poisonous domoic acid from algal blooms got into the sea lions' food chain. Nearly 3/4 of that June's 341 stranded pups died. Strandings were only a small part of a much larger problem. The National Marine Fisheries Service estimated the 2009 pup mortality rates were twice as high as those in 2008, again at 3/4 of the estimated 59,000 newborns.[43]

References

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  • Bonner, N. (1994). Seals and Sea Lions of the World. New York: Facts on File.
  • Rice, D. W. (1998). Marine Mammals of the World: Systematics and Distribution. In Special Publications of the Society for Marine Mammals no. 4. Lawrence: The Society for Marine Mammalogy. ISBN 1-891276-03-4

Further reading

External links

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