General Biology

Pinnipedia is a suborder of carnivorous mammals found exclusively in aquatic habitats. The suborder includes three recognized extant families: Odobenidae (walrus), Otoriidae (sea lions and fur seals), and Phocidae (true seals). Members of this suborder are easily recognized by their general body shape and the morphology of the appendages that are adapted to the aquatic environment. Both thoracic and pelvic appendages are modified for effective locomotion in water and take the form of flippers. In otoriids, the thoracic appendages are the dominant ones and also allow the animals to support the weight of the trunk on land and effectively “walk,” whereas the phocids do not; and odobenids, although able to support their trunks with the forelimbs, are less agile. Members of this suborder are found around the entire globe, including the Arctic and Antarctic regions. One genus, Pusa (which includes the Ladoga ringed seal, the Saimaa ringed seal, and the Baikal seal) has established a niche in fresh water habitats, whereas the remainder is marine. The smallest pinniped, Pusa sibirica (the Baikal seal), which weighs about 45 kilograms (kg) when full grown, grows to about 1 meter (m) long; the largest, Mirounga leonina (the male Southern elephant seal), grows to about 5 m long and weighs up to 3200 kg.⁴ Many pinnipeds are typically found foraging and resting in and near the relatively shallow waters of the littoral zone; others spend most of their lives as pelagic animals, and still others are pagophilic (ice loving).

Many members of this suborder have been exploited by humans for centuries. Several are now extinct, and many more have been hunted to near extinction. Notably, the Northern elephant seal population was estimated at only around 20 individuals at the end of the 1890s.⁵ This severe population reduction is thought to have contributed to significant loss of genetic diversity and is clinically important. Rates of congenital defects in Northern elephant seals are highest compared with any other of the pinniped species presented to rehabilitation facilities in North America (Figure 44-1), presumably as a result of defect heritability and lack of genetic diversity.⁶⁸ A study of tissues collected from 371 stranded California sea lions (Zalophus californianus) found that sick animals have higher-than-normal parental relatedness, suggesting that these animals too have been impacted by human exploitation.¹ Recently, low major histocompatibility complex (MHC) allelic diversity was shown to be a strong predictor of pup survival in the gray seal (Halichoerus grypus) on the Isle of May, in the United Kingdom.¹⁵

The most endangered of the current extant species are the Mediterranean monk seal (Monachus monachus), the Hawaiian monk seal (Monachus schauinslandi), and the Caspian seal (Pusa caspica). Data are deficient, and for a number of other species, no status determination may be made at this time.³⁸ At the time of writing this chapter, approximately 1100 individual Hawaiian monk seals were estimated to make up the entire population, which is declining at a rate of approximately 4% per year.⁵³ In contrast, many other pinniped populations are thriving and increasing in numbers and resulting in conflict with human interests or activities. This, at times, results in harassment, injury, or death of animals. Negative human interactions (fishing gear or trash entanglement, gunshots, boat strike, etc.) account for as much as 8% to 10% of the cases admitted to rehabilitation centers in North America annually.

In their natural habitats, pinnipeds share with humans many risk factors for diseases, and many are thus excellent sentinel species used in efforts toward better understanding of several diseases, including domoic acid intoxication, leptospirosis, urogenital carcinoma, and malnutrition.^3,28,56 Pinnipeds possess exquisite anatomic, physiologic, and behavioral adaptations to the aquatic environment. These adaptations such as gas management during diving, thermoregulation, and environmental vigilance are fascinating from a comparative perspective. In addition, pinnipeds are generally easy to train for operant conditioning, and many are capable of complex behavioral modification, including learned behaviors conducive to human endeavors such as performance or utility work. As a result, pinnipeds frequently present to clinical veterinarians and require health assessments, medical care, or support during investigations.

General descriptions of the unique aspects of anatomy, physiology, and behavior important to clinical care and management of the most commonly clinically encountered pinnipeds, as well as typical patient presentations, comprise the remainder of this chapter. Much of the information included here is derived from the experiences of the author and colleagues at The Marine Mammal Center (TMMC) in Sausalito, California, and elsewhere, and a fair amount of personal clinical opinion is also included. TMMC typically treats around 600 to 800 marine mammals annually, the vast majority being California sea lions, elephant seals, and harbor seals. This chapter material should provide the general practitioner of zoologic medicine with enough information to make sound diagnostic and treatment plans for the most commonly encountered pinniped patients and health problems.

All pinnipeds are generally fusiform in body shape with variations in the morphology of the head and appendages. The appendages are adapted to aquatic mobility and thermoregulation, being flattened with little or no individualization between digits. The proximal bones of the pelvic appendage (femur, patella, tibia, and fibula) are incorporated within the trunk at the level of the pelvis, leaving only the tarsus, metatarsus, and phalanges to serve as the hind flipper. Similarly, the proximal caudal vertebrae of the tail are incorporated into the trunk at the pelvis, and only a relatively short, protruding tail is present. Vibrissae and eyes are the prominent features of the head. Male elephant seals and hooded seals have prominent saccular adaptations to the nasal passages, and male walruses have diverticulae of the oropharynx. In all probability, these adaptations are important in acoustic or visual signaling during male-associated reproductive behavior. Pinniped kidneys are discretely multireniculated, composed of multiple functional subunits, presumably an adaptation to life in a hypertonic environment.⁵⁸ All pinnipeds are also monogastric, with very long small intestines (averaging 25 times the body length for elephant seals). The ratio of small intestine length to body length tends to be higher than in most terrestrial herbivores. This anatomic feature is most likely related to the relative high body mass, high energy requirements, diet content, and motility rate of the digesta.³³ Pinniped dentition is polyphydont and heterodont, although deciduous teeth are resorbed prior to or at birth, and the permanent dentition erupts shortly after birth.

Otariids

Sea lions and fur seals are quite similar to each other in overall body shape. They possess pinnae, and the dominant appendages are the thoracic appendages, which flex laterally at the carpus to provide for locomotion on solid surfaces. Otariids do often “drag” their hind flippers when moving quickly; it would seem a more effective method for higher speeds on land. Amputation of pelvic appendages causes no significant disability, but amputation of a fore flipper is not recommended. Although arthrodesis of the carpal joint has been effective, an individual without a fore flipper would be significantly impaired on land and in water.

Closer inspection reveals the difference between the pelages of sea lions and fur seals. Sea lions have a short, stout, slick hair coat that lies flat against the skin when wet. Fur seals have a highly specialized hair coat with prominent guard and secondary hairs that trap air and is an important means of thermoregulation. Fur seals thus need to “groom” and may be seen lolling at the water surface, rolling and rubbing, as sea otters do. Sea lions rely primarily on subcutaneous adipose tissue to provide insulation.

With practice, the dentition of California sea lions can be used to distinguish between “pups” (animals less than 1 year old) and “yearlings” (animals between 1 and 2 years of age). During the first year of life, the prominent maxillary tooth is the corner incisor, which has the appearance of a canine tooth and can easily be confused with it. During the second year of life, the canine tooth erupts beyond the corner incisor and becomes the dominant tooth in the arcade.

The carina of otariids is located near the level of the thoracic inlet, not over the base of the heart as in phocids (Figure 44-2). This may lead to bronchial intubation during anesthesia if caution is not used. At the other end of the sea lion, the perineum and distal anus are made up of multiple folds of loose tissue, presumably to assist in preventing water intrusion during dives. This may make it difficult to collect samples or insert instruments into the rectum, and care must be taken to avoid causing trauma. Conscious individual animals often are more resistant to attempts to pass something per rectum than to blood sampling.

Nails are present on all flippers but are much reduced on the fore flippers. Otariids also possess a prominent hepatic sinus, but the abdominal plexuses are not nearly as significant as in phocids.

Special Housing Requirements

All pinnipeds require access to water and dry haul-out areas to rest. In the United States, the specifications for enclosure space provisions in permanent collections are mandated by law (the Animal Welfare Act) and include specific formulas based on average adult body length for the species and number of individuals housed within the enclosure. Standards for temporary housing of animals in rehabilitation settings have been developed by the National Oceanic and Atmospheric Administration (NOAA) Fisheries, and these standards contain useful information regarding the special housing requirements for this taxon.²³ In the author’s opinion, it would be difficult to provide an animal too much dry haul-out space or too much water to swim in. Pinnipeds tend to be gregarious, often associating in groups, particularly when hauled out. The tendency for direct contact between hauled-out individuals seems to vary with species. California sea lions are often observed resting while in contact, even on top of one another. Harbor seals, in contrast, appear to desire a gap between each other and, as mentioned earlier, will often “swat” at each other if they get too close. Interestingly, weather conditions may impact the distance between individuals. During rain, resting sea lions have been observed to separate, leaving some space between each other. Sea lions, in particular, appear to enjoy climbing up on structures and will often be seen resting on pool walls, transport crates, shelves, and almost anything that allows them purchase to climb.

Access to water is critical. These animals behaviorally thermoregulate. They are adapted to maintain body temperature while spending much of their lives in the perennial “heat sink” of the ocean. Core body temperature of most is approximately 37° C. Even in “warmer” water environments, the water temperature is lower than core body temperature, and the thermal conductivity of water is about 25 times that of air. Thus, the insulation and circulatory adaptations discussed above are designed to maintain core body temperature; when the animal is out of the water, the risk of hyperthermia is always present. A sea lion that has restricted access to water and is producing metabolic heat because of muscular activity such as pacing or struggling against restraint may quickly develop heat prostration. When in doubt, the animal should be cooled off with a dousing of water. It is very difficult to get a pinniped too cold or too wet. Emaciated young animals are an exception to this rule (animals under anesthesia are another). This special class of patients lacks the insulation, is often metabolically challenged, may have immature physiologic mechanisms for thermoregulation and is thus prone to hypothermia when ambient air temperatures are low, particularly if the animals are wet. Protection from the elements must be provided for this special class of patients. Thermal stress risks are the opposite for older animals in fair to good body condition. This author is aware, some concern is felt that a wet animal will heat up faster than a dry one. Recommendations have been made to avoid wetting the animals when they are lying in the sun. These recommendations have been based on published observation that a wet hair coat will be darker in coloration compared with a dry one and thus will absorb more solar radiation.⁴⁰ This report did not take into account evaporative cooling or measure core temperatures or any thermal flux from the animals themselves; rather, it was focused on enclosure design. The authors of that paper also made several assumptions based on measures from terrestrial animals. Pinnipeds are not terrestrial animals. Individual pinnipeds often appear to be less anxious and more comfortable when in water. Animals transported from one location to another unfamiliar location will often stay in the water almost exclusively (sometimes swimming for days) until they become comfortable in the new surroundings. Wetting an animal will also often stimulate defecation. If physiologically stable, young orphans should be exposed to water following feeding to encourage normal GI motility. Adult sea lions with reduced appetites may sometimes be enticed to eat by directing a steady stream of water from a hose to the face.

Ocular disease is one of the most common presenting complaints of pinniped patients in managed collections, leading to a large number of challenging medical treatments and complicated surgical procedures, sometimes with less-than-desirable outcomes. Protection from exposure to solar radiation is probably significant with regard to the ocular health of pinnipeds. Evidence that keratopathies and secondary intraocular disease are associated with solar radiation exposure is increasing. Captive pinnipeds that did not have access to shade were almost 10 times as likely to develop lens luxations, cataracts, or both compared with those provided shade.²¹ From an empirical perspective, this makes sense. Pinniped eyes, as is the case with all marine mammal eyes, are particularly adapted for low-light visual acuity. These animals forage in the water column, some at great depths with low ambient light. When the animals are kept away from these depths and fed in air, as is often the case with collection animals, their eyes are exposed to more solar radiation than in their natural habitat. Husbandry conditions are a likely contributing factor, so preventive measures should be a priority. Provision of shade; darkened, nonreflective surfaces; feeding underwater; and stringent attention to water quality must be considered in a preventive medicine plan for pinnipeds. In one published study of 111 captive pinnipeds, 48.6% had lens disease, cataracts, or both.¹¹ Another study documented ocular pathology in 81.5% of 27 captive pinnipeds and only 35% of wild submitted ones.⁴⁹ The conclusion was that diseases of the eye are “common” in wild and captive pinnipeds, although a higher prevalence exists in captive populations. However, a retrospective search of the medical records of documented pathology from 10,919 California sea lions presented to TMMC revealed only 272 (2.5%) results with recorded ocular disease. This topic has received considerable attention in the literature (refer to the discussion by Gage in the seventh edition of this text).

Water quality also has a powerful impact on overall pinniped health and welfare. These animals evolved over millions of years to spend most of their waking hours (and many of their sleeping hours) in the sea, a thick soup of microbial life that has evolved over billions of years; the so-called “rare biosphere.” This biomass is immense; 98% of the primary production of the oceans and the mediation of all biogeochemical processes in the oceans are accounted for by these microbes.⁶⁰ Pinnipeds are merely visitors to this microbial world, but they visit often. They bathe in it, bask in it, swallow it, cover their bodies with it, soak their mucous membranes in it, even inhale it. They have successfully established biologic mechanisms to live in harmony with it, most of the time. It is all about balance. When conditions tip the balance in favor of the microbes, disease may result. The clinical veterinarian will do well to keep in mind the triad of factors that influence the development of disease: the host, the agent, and the environment. Numerous reports of microbial surveillance in free-ranging animals and those that have presented as patients have been published. In a study of healthy pinnipeds off the California coast directed at detection of Salmonella spp., 21% of California sea lion pups and 87% of Northern elephant seal pups were culture positive for Salmonella spp.⁶³ What is really needed for effective management of pinniped health is better understanding of the influences on the aquatic system that favor the microbes, and pressures on even the natural system appear to be increasing. Perhaps this is the explanation for the observation that blooms of diatoms responsible for the production of domoic acid and the resulting intoxication in animals appear to be increasing in frequency and magnitude off the coastal North American Pacific.⁴¹

More importantly for clinical veterinarians, artificial habitats may always be expected to place some selection pressures on the microbial ecology of the water within. Alterations of the abiotic water parameters are often performed with the presumed aim of creating a “healthier” environment for inhabitants. In the United States, the water quality of permanent marine mammal enclosures is mandated by the Animal Welfare Act of 1972, as amended and promulgated by the Animal and Plant Health Inspection Service of the United States Department of Agriculture (USDA). With respect to the microbial content of water, the mandate states: “The coliform bacteria count of the primary enclosure pool shall not exceed 1000 MPN (most probable number) per 100 milliliters (mL) of water.”⁹ This is, in fact, a fairly easy standard to meet through oxidation, as detailed in an instructional pamphlet published by the USDA titled “The Sterilization of Marine Mammal Pool Waters.”⁶¹ However, this author argues that in all things, artificial and controlled environments should aim to mimic the natural environment, whenever possible, and that excessive oxidation may work against this goal. The author’s experience with both natural and artificial systems has provided strong evidence for just how different the aquatic microbial ecology is between the two types. Evidence also indicates that these differences are manifested clinically as animal health issues. The author has suggested that this situation is similar to the “Hygiene Hypothesis,” as discussed in human medicine, and it has been termed the “Aquatic Animal Hygiene Hypothesis.”^8,70 While many pinnipeds have been, and some continue to be, housed in fresh water systems, the author and colleagues believe that this is not the most appropriate environment for a marine mammal and that it undoubtedly places some significant physiologic stress on the animals. Much work is needed to revise water quality standards for marine mammals using science-based, sound data.

Feeding

Pinnipeds are carnivorous and largely piscivorous, although most species have been documented to eat prey items other than fish. Walruses are well known to forage in the benthos for a variety of crustaceans and other invertebrates, making use of their prominent vibrissae and perhaps their tusks as well. Stomach contents and scats of phocids and otariids often contain remnants of invertebrates as well. Squids, octopoids, and crustaceans are commonly consumed, in addition to fishes of all sorts. In permanent collections, most pinnipeds are fed diets similar to those of cetaceans (dolphins and whales), generally a teleost fish–based ration comprising commercially available “bait” species such as herring (Clupeidae), mackerel (Scombridae), and smelt (Osmeridae). Squids are often included, but other invertebrates are offered on occasion, except to walruses. Captive pinnipeds appear to do well on these diets, and, in contrast to captive cetaceans, diseases with a presumed iatrogenic nutritional etiology have not been reported, with the exception of vitamin deficiencies. Thiamin deficiency was experimentally induced in seals in studies conducted 40 years ago and is still a concern today.^13,24 Over 30 years ago, vitamin E deficiency was experimentally induced in harp seals.²⁰ In addition, the interaction of vitamin A supplementation on vitamin E levels has been investigated in captive fur seals.⁴⁵ As a result, most facilities supplement rations fed to pinnipeds, with multivitamins, especially thiamin and vitamin A and E, at levels based on these original studies.

Free-ranging wild pinnipeds do not consume supplements at all. Fat-soluble vitamins are abundant in marine fishes. One kilogram of herring may be expected to provide approximately 2000 international units (IU) of vitamin A, 8000 IU of vitamin D, and 40 to 60 IU of vitamin E, and marine mammals are expected to have a high capacity for storage of fat soluble vitamins.⁷⁴ So, if fresh-frozen, properly handled fish products are the base of the diet, supplementation may not be required at all, at least not daily. Proper handling of food fishes must include attention to time–temperature profiles. As a general rule, the shorter the time the product is held and the colder the temperature it is held at, the higher is the expected quality and the lower is the likelihood of nutrient degradation, particularly fat-soluble vitamin oxidation. It is this concern about degradation that apparently drives supplementation—not recognized deficiencies or toxicoses. All food handling from producer source selection, shipping, storage, “breakout,” feeding, and cleanup should be subjected to thorough oversight and review. A hazards analysis and critical control point (HACCP) approach, as used for human food service safety, is a wise approach and may, in fact, reduce the costs of future problems significantly. The advantage of reducing supplementation costs to one seventh of current costs by decreasing supplementation to once-weekly from once-daily is quite obvious. The responsible clinician should have a plan to monitor animal health status when any such changes are implemented.

The scientific literature is full of publications on the energetics of marine mammals, including investigations of hypothetical prey changes in association with species decline. One may easily find numerous formulas to calculate caloric requirements of various classes of marine mammals. The caloric content of individual lots of fish fed is also easily calculated from a simple proximate analysis of a sample, and many facilities use caloric density of fish lots to determine ration formulation. Practically, feeding decisions may be simplified by monitoring body weights. Assuming that the handling has been proper and monitored, if a variety of items expected to be found in the animals’ natural diet (a variety of fish types and perhaps an invertebrate or two) are fed, the diet may be expected to be balanced. If the fed diet meets energy requirements, the animals will grow or gain weight, depending on their stage of nutrition and health status. If not, the animals will lose weight or not grow. Therefore, an accurate means of monitoring body condition is essential. Regular body weights will suffice in most instances, although more detailed assessments, including ultrasonographic determination of “blubber” thickness or thermographic profiles over time may be useful in some animals. The clinician should also remember that a “lag time” often occurs with any ration change. An animal that has had an increase of caloric intake may not show any weight gain for a week or two and likewise an animal that has had a reduction in intake may not lose weight for a week or two.

Fasting in pinnipeds presents the clinical veterinarian with some unique concerns. Like other marine mammals, pinnipeds are well known to “mask” signs of pathology, appearing normal in spite of significant underlying disease. Appetite and attitude are often the first to change in an ill animal. In general, any collection animal that refuses food without an obvious explanation should be evaluated for a medical disorder. A reduction in appetite may be the only indication of serious illness. However, pinnipeds do fast at times during states of normal health. Elephant seal weanlings fast for 4 to 6 weeks following the departure of the dam prior to initiating foraging. They survive this entire period without any oral intake, neither food nor water, after which they “switch modes” to foraging and begin to seek and perhaps find solid foods to sustain themselves. Animals of this age group are frequently presented for rehabilitation and present a challenge in determining whether they should be fed or not. To date, no practical and specific biomarker of physiologic status that indicates the fasting or foraging stage has been recognized. Plasma hormone levels and fatty acid profiles have been investigated, and ratios of serum blood urea nitrogen (BUN) to creatine (Cr) have showed some promise but have not held up in clinical experience. Intact adult pinniped males will often go “off feed” during times of rut behavior and may not eat for weeks at a time. The clinician faced with a pinniped that refuses food must carefully weigh all the evidence at hand and, when in doubt, pursue an investigation that includes at a minimum a complete blood cell (CBC) count and a serum chemistry profile. Although pinnipeds appear to be exceptionally adept at metabolic water production via oxidative phosphorylation of fats, fresh water should be made available at all times.

Neonates present another challenge to the clinical veterinarian. Preweaned orphans require milk protein replacement and are usually gradually transitioned to a fish-based diet. Numerous references, recipes, and sources of practical information are available for the reader to consider when adopting a strategy for the situation at hand. Here only a few important considerations (in the author’s opinion) to be made when formulating a plan will be discussed. It must be kept in mind that these animals are predominantly piscivorous. Protein sources other than fish are not optimal, although some have a strong record of being used with success. Pinniped milk, like the milk of other marine mammals, is virtually devoid of carbohydrates. In fact, marine mammals really never ingest oral carbohydrates, so their inclusion in the fed diet is unnatural. Disorders of GI motility are not uncommon in artificially reared neonate seals and sea lions. Diarrhea, ileus, atony, and impaction have all been noted clinically. Carbohydrates included in the artificial diet have long been suspected to cause these problems. The microbial flora establishing itself in neonates, along with the brush border enzymes and anything else influenced by the biochemical milieu, may be disturbed by exposure to carbohydrate.

In health, pinnipeds acutely switch from suckling to solids without a transitional time of premasticated, regurgitated gruels fed by a parent, as in so many piscivorous birds. Young animals go from sucking milk one day to, in the case of phocids with a minimalistic maternal investment, fasting for a while, to eating solid fish. During attempts to artificially rear them at TMMC, often out of necessity, during a transition period gruel or “fish mash”—an unnatural diet—has been incorporated. Solid fish are generally swallowed whole or with little chewing in the normal weaned animal. The stomach is muscular and serves to masticate the food. Tube feeding gruel, although often necessary, may lead to motility disorders.

Lastly, “hand rearing” neonates increases the risk of abnormal socialization of the pup. For this reason, in rehabilitation facilities, most neonates are tube fed rather than trained to suckle a bottle delivered by hand. This is not a problem, of course, for animals destined to be placed in collections.

In situations where the required clinical information or patient inspection cannot be accomplished by simple observation from a distance or with trained cooperation of the animal, some sort of restraint is needed. For animals weighing less than 30 kg of body weight, with the aid of boarders, an experienced restrainer may wrap a towel about the animal’s head to block its vision and the ability to bite. The animal may then be pinned to the deck by grasping at the base of the skull, straddling the animal, and holding the fore flippers against the trunk with the thighs, knees, and shins so that the animal cannot gain purchase or stand. It is often useful to wet only the ends of the towel, providing weight that will facilitate wrapping the towel around the animal’s head. If the towel is not wet, the animal may often toss it easily; if the entire towel is wet, it may be difficult to achieve an effective wrap. Once the animal is immobilized in this fashion, the towel is removed from the nares and care taken to ensure the animal is breathing effectively. This method may be used in otariids and phocids alike (see Figure 44-1). Otariids are much more agile and apt to pose a bite hazard, whereas phocids often thrash the hind flippers back and forth with gusto. Additional restrainers to assist with fore flipper positioning or hind end control will often be needed with larger individuals or those that are healthier. Conversely, malnourished, stranded neonatal seals and sea lions alike may often be safely restrained by one person experienced in the use of this technique.

As the size of the patient increases or the overall condition of the animal improves, it is more difficult and dangerous to restrain it. Animals weighing up to 70 or 80 kg (or larger ones by experienced personnel) may still be effectively restrained without chemical agents. For these animals, the most effective means, in the author’s experience is a small mesh conical net designed to fit over the animals head with space only for the muzzle to protrude (Figure 44-3). These nets are tossed over the animal, again with the help of boarders or, in the case of animals on the beach, with a net frame and handle. As the animal moves into the net, the net diameter narrows until only the muzzle of the animal is exposed and the fore flippers are effectively pinned against the trunk. Experienced personnel may grasp the back end of the net, take several wraps to further tighten the net against the fore flippers, and immobilize the animal effectively by lifting the back end. Additional restrainers may then assist in pinning the animal against the deck, again paying close attention to effective breathing in the animal. Many simple procedures and sample collections may be conducted in this manner. If further restraint is needed, the animal may then be anesthetized safely with an inhalant agent delivered via a face mask. This method has proven so effective in the author’s experience that it has replaced the use of “Ridgway-Simpson”–style squeeze cages that were once popular.