Sea lion

Sea lions are pinnipeds characterized by external ear flaps, long foreflippers, the ability to walk on all fours, short and thick hair, and a big chest and belly. Together with the fur seals, they make up the family Otariidae, eared seals. The sea lions have six extant and one extinct species in five genera. Their range extends from the subarctic to tropical waters of the global ocean in both the Northern and Southern hemispheres, with the notable exception of the northern Atlantic Ocean.
Sea lions have an average lifespan of 20–30 years. A male California sea lion weighs on average about and is about long, while the female sea lion weighs and is long. The largest sea lions are Steller's sea lions, which can weigh and grow to a length of. Sea lions consume large quantities of food at a time and are known to eat about 5–8% of their body weight at a single feeding. Sea lions can move around in water and at their fastest they can reach a speed of about. Three species, the Australian sea lion, the Galápagos sea lion and the New Zealand sea lion, are listed as endangered.

Taxonomy

Sea lions are related to walruses and seals. Together with the fur seals, they constitute the family Otariidae, collectively known as eared seals. Until recently, sea lions were grouped under a single subfamily called Otariinae, whereas fur seals were grouped in the subfamily Arcocephalinae. This division was based on the most prominent common feature shared by the fur seals and absent in the sea lions, namely the dense underfur characteristic of the former. Recent genetic evidence, suggests Callorhinus, the genus of the northern fur seal, is more closely related to some sea lion species than to the other fur seal genus, Arctocephalus. Therefore, the fur seal/sea lion subfamily distinction has been eliminated from many taxonomies.
Nonetheless, all fur seals have certain features in common: the fur, generally smaller sizes, farther and longer foraging trips, smaller and more abundant prey items, and greater sexual dimorphism. All sea lions have certain features in common, in particular their coarse, short fur, greater bulk, and larger prey than fur seals. For these reasons, the distinction remains useful. The family Otariidae contains the 15 extant species of fur seals and sea lions. Traditional classification of the family into the subfamilies Arctocephalinae and Otariinae is not supported, with the fur seal Callorhinus ursinus having a basal relationship relative to the rest of the family. This is consistent with the fossil record which suggests that this genus diverged from the line leading to the remaining fur seals and sea lions about 6 million years ago. Similar genetic divergences between the sea lion clades as well as between the major Arctocephalus fur seal clades, suggest that these groups underwent periods of rapid radiation at about the time they diverged from each other. The phylogenetic relationships within the family and the genetic distances among some taxa highlight inconsistencies in the current taxonomic classification of the family.
Arctocephalus is characterized by ancestral character states such as dense underfur and the presence of double rooted cheek teeth and is thus thought to represent the most "primitive" line. It was from this basal line that both the sea lions and the remaining fur seal genus, Callorhinus, are thought to have diverged. The fossil record from the western coast of North America presents evidence for the divergence of Callorhinus about 6 mya, whereas fossils in both California and Japan suggest that sea lions did not diverge until years later.

Suborder Caniformia
* Family Otariidae
**Subfamily Arctocephalinae
*** Genus Arctocephalus
*** Genus Callorhinus
** Subfamily Otariinae
*** Genus Eumetopias
**** Steller's sea lion, E. jubatus
*** Genus Neophoca
**** Australian sea lion, N. cinerea
*** Genus Otaria
**** South American sea lion, O. flavescens
*** Genus Phocarctos
**** New Zealand sea lion or Hooker's sea lion, P. hookeri
*** Genus Zalophus
**** California sea lion, Z. californianus
**** Japanese sea lion, Z. japonicus – extinct
**** Galapagos sea lion, Z. wollebaeki
* Family Phocidae: true seals
* Family Odobenidae: walrus
Physiology

Diving adaptations

There are many components that make up sea lion physiology and these processes control aspects of their behavior. Physiology dictates thermoregulation, osmoregulation, reproduction, metabolic rate, and many other aspects of sea lion ecology including but not limited to their ability to dive to great depths. The sea lions' bodies control heart rate, gas exchange, digestion rate, and blood flow to allow individuals to dive for a long period of time and prevent side effects of high pressure at depth.
The high pressures associated with deep dives cause gases such as nitrogen to build up in tissues which are then released upon surfacing, possibly causing death. One of the ways sea lions deal with the extreme pressures is by limiting the amount of gas exchange that occurs when diving. The sea lion allows the alveoli to be compressed by the increasing water pressure thus forcing the surface air into cartilage lined airway just before the gas exchange surface. This process prevents any further oxygen exchange to the blood for muscles, requiring all muscles to be loaded with enough oxygen to last the duration of the dive. However, this shunt reduces the amount of compressed gases from entering tissues therefore reducing the risk of decompression sickness.
The collapse of alveoli does not allow for any oxygen storage in the lungs, however. This means that sea lions must mitigate oxygen use in order to extend their dives. Oxygen availability is prolonged by the physiological control of heart rate in sea lions. By reducing heart rate to well below surface rates, oxygen is saved by reducing gas exchange as well as reducing the energy required for a high heart rate. Bradycardia is a control mechanism to allow a switch from pulmonary oxygen to oxygen stored in the muscles which is needed when the sea lions are diving to depth. Another way sea lions mitigate the oxygen obtained at the surface in dives is to reduce digestion rate. Digestion requires metabolic activity and therefore energy and oxygen are consumed during this process; however, sea lions can limit digestion rate and decrease it by at least 54%. This reduction in digestion results in a proportional reduction in oxygen use in the stomach and therefore a correlated oxygen supply for diving. Digestion rate in these sea lions increases back to normal rates immediately upon resurfacing.
Oxygen depletion limits dive duration, but carbon dioxide build-up also plays a role in the dive capabilities of many marine mammals. After a sea lion returns from a long dive, CO₂ is not expired as fast as oxygen is replenished in the blood, due to the unloading complications with CO₂. However, having elevated levels of CO₂ in the blood does not seem to adversely affect dive behavior. Compared to terrestrial mammals, sea lions have a higher tolerance to storing CO₂ which is what normally tells mammals that they need to breathe. This ability to ignore a response to CO₂ is likely brought on by increased carotid bodies which are sensors for oxygen levels that let the animal know its available oxygen supply. Yet, the sea lions cannot avoid the effects of gradual CO₂ build-up which eventually causes the sea lions to spend more time at the surface after multiple repeated dives to allow for enough built up CO₂ to be expired.

Parasites and diseases

Galapagos sea lions can be infected with Philophthalmus zalophi, an eye fluke. These infections have heavy impacts on the survival of juveniles. The disease appears to be compounded by global warming, as the number of infectious stages of different parasites species has a strong correlation with temperature change. The Galapagos Islands go through seasonal changes in sea surface temperatures, which consist of high temperatures from the beginning of January through the month of May and lower temperatures throughout the rest of the year. Parasites surfaced in large numbers when the sea temperature was at its highest. Furthermore, data published in 2015 was collected by capturing sea lions in order to measure and determine their growth rates. Their growth rates were noted along with the sightings of parasites which were found under the eyelid. The results were that sea lions are affected by the parasites from the early ages of 3 weeks old up until the age of 4 to 8 months.
The parasites found in the eye fluke did serious damage to the eye. From the data collected, 21 of the 91 survived; with a total of 70 deaths in just a span of two years. The parasites are attacking the pups at such young ages and causing the pups to not reach the age of reproduction. The death rates of the pups is surpassing the fertility rate by far. Since most pups are unable to reach the age of reproduction, the population is not growing fast enough to keep the species out of endangerment. Other parasites, like Anisakis and heartworm, can also infect sea lions.
Australian sea lions are also being affected by more frequent parasitic infections. The same method was used for the sea pups as on the Galapagos Islands, but in addition, the researchers in Australia took blood samples. The pups in Australia were being affected by hookworms, but they were also coming out in large numbers with warmer temperatures. New Zealand sea lion pups were also affected in really early ages by hookworms. The difference is that in New Zealand researchers took the necessary steps and began treatment. The treatment seemed to be effective on the pups who have taken it. They found no traces of this infection afterwards. However, the percentage of pups who do have it is still relatively high at about 75%. Those pups who were treated had much better growth rates than those who did not. Overall parasites and hookworms are killing off enough pups to place them in endangerment. Parasites affect sea pups in various areas of the world. Reproductive success reduces immensely, survival methods, changes in health and growth have also been affected.
Similarly, climate change has resulted in increased toxic algae blooms in the oceans. These toxins are ingested by sardines and other fish which are then eaten by the sea lions, causing neurological damage and diseases such as epilepsy.