An elephant seal rookery is a noisy place. The vocalizations of adults of both sexes and the pups are described as well as their functions. Elephant seals have good hearing in air and in water and are also unusually sensitive to sounds. This is especially the case because their deep diving puts them smack in the middle of the deep sound channel where low-frequency sounds travel most efficiently and can be heard thousands of miles away. This may make them prone to disturbance by sources such as ship traffic, air guns from petroleum exploration, and US Navy operations.

It is rare for an animal population to recover from near extinction, grow, and flourish at a time when so many species are going extinct. The remnant population of elephant seals increased slowly at first from approximately 30 individuals on a remote, volcanic island far from the coast of Mexico in 1890 to 300,000 animals breeding at 13 rookeries in Mexico and California today. The pattern of recovery is detailed. The settlement and growth of the Año Nuevo colony is described as well because of the concentrated study of the seals at this rookery from resettlement in 1961 to the present. The consequences of going through a population bottleneck and losing genetic variation implies that the present population is less adaptable to changes in the environment than the population that existed prior to 1800.

Since elephant seals are submerged 90% of the time they are at sea and they are at sea 8 to 10 months of the year, depending on sex, when do they sleep? They hold their breath while sleeping on land, as if they are diving. This suggests that they do not sleep at sea during the short periods at the surface. A good case can be made that they sleep during dives. This is reasonable because heart rate slows and metabolism decreases during diving just as it does during sleep. On land, elephants sleep whenever they can, day or night, that is, when the entire colony is quiet.

The elephant seal nomenclature is arbitrary, based on fallacies, and is impractical. Elephant seals are true seals that originated in the North Atlantic 15 to 20 million years ago. Today, the northern species is found along the west coast of North America and the southern species inhabits sites around the Antarctic continent. The two species have been separated for a minimum of 5,000 years but the exact duration is unclear. The northern elephant seal population was abundant before the seals were hunted for their blubber oil in the first half of the nineteenth century. The slaughter was so extensive and relentless that by the late-nineteenth century, the species was considered extinct. Approximately 20 individuals survived, however, on a volcanic island in Mexico. The population experienced a severe population bottleneck and lost genetic diversity. The southern species was also hunted by sealers, but its population was not reduced as drastically.

Although we know more about the natural history of elephant seals than most other animals, some things remain a mystery. The first is that most seals die at sea; the exception is numerous young pups and some weanlings that die on the rookery. We know some causes of death at sea but much more needs to be done to solve this mystery. The second unknown is when and with whom do virgin females mate. When future researchers determine this, the next question is “How important is it?” We conclude that tracking individuals throughout their lives is vital for understanding how natural selection operates. It shows us how individuals maximize their reproductive success and that the two sexes do this in entirely different ways.

Getting to the field site to study wild animals sets stringent limits on what can be done. Access to the seals at Año Nuevo Island from my university office was fast, inexpensive, and convenient but was adventurous and dangerous in the early years. The initial attempts to study the seals are described as well as monitoring the entire population by study of the largest extant rookeries. Field research at Año Nuevo was made easier when the seals started breeding on the mainland adjacent to the island. This change facilitated the long-term study of these animals, which is critical for a deep understanding of their natural history. Identifying individuals with marks, tags, or brands, as well as other manipulations such as measuring, weighing, and taking blood samples, was vital and the key start in determining the questions we could address and answer. We developed a system to identify individuals throughout their lives.

Development in elephant seals intrigues us because pups receive no training from adults or peers, they embark on sex-specific reproductive strategies early in life, and they must transition to foraging at sea. Young males obtain more milk energy than females, which increases their size and enhances the probability of breeding in adulthood. Pups learn how to swim and dive and forage on their own and they are naïve about prey and predators when they go to sea for the first time. Most pups die during the first two trips to sea; those that survive do not increase their weight, suggesting that foraging is difficult. By the end of the second year, the location and pattern of diving are similar to those of adults, and sex differences in dive pattern begin to appear. The transition to becoming a diver, both in behavior and physiology, is rapid.

Elephant seals gather in large groups on land throughout the year. In the nineteenth century and before, humans were keen to exploit large animal and bird aggregations for food or profit and this decimated many animal populations. In recent times, the focus has shifted from harvesting wildlife to viewing animal aggregations for pleasure, entertainment, or knowledge. The cultural and economic benefits of wildlife watching are immense, and such activities have increased exponentially in the last few decades. The downside is that too much tourism disturbs the animals being viewed, affecting foraging and reproduction. Elephant seals are robust against disturbance and can be observed from 10 m without disturbing them. Viewing sites are Año Nuevo State Park and Piedras Blancas in California and Península Valdes in Argentina. Seal activities are divided into four quadrants that describe the annual cycle: breeding season, female and juvenile molt, male molt, and juvenile haul-out.

Females are designed to reproduce, and mating is a small part of the process. Females that are most productive breed early in life, at every opportunity (annually), live long, and wean large pups that are most likely to survive and reproduce. These supermoms are rare but have the greatest influence on the next generations. In contrast, most female weanlings (75%) die before reaching breeding age, and most that survive to breed do so only a few times. Long-lived supermoms live up to age 23 and may produce 20 pups in life.

Elephant seals lead two entirely different lives: one on land and one at sea. The adaptations for feeding at sea, such as blubber for warmth in cold waters, streamlining to reduce drag, and a source of energy to see them through long fasts, impose difficulties on land. It is costly to move their large bodies quickly for any distance on land. Their blubber layer is like a puffy coat that causes overheating on hot days on land. The loss of feet for flippers leaves them vulnerable to fast-moving and agile terrestrial carnivores, which partially explains their historical preferences to breed on islands devoid of bears, wolves, and coyotes. The evolutionary changes in their bodies must be a compromise between what is necessary for successful foraging at sea and breeding and molting on land. For example, elephant seals hold their breath while sleeping on land and during dives at sea.

The remarkable diving and foraging migrations of the seals have been revealed by diving instruments attached to them, which are recovered when they return to the rookery. They show that elephant seals are the deepest and longest divers of all the seals and most of the whales. They migrate twice a year to feed on round-trip voyages of up to 8,000 km. The behavioural and physiological adjustments to long, deep diving are described.

The urge to mate explains virtually everything that males do during the breeding season. They threaten, chase, and fight to establish rank in a dominance hierarchy that gives access to females. One or a few of the highest-ranking males dominate mating; the alpha male may inseminate up to 100 females in a single breeding season or more than 200 females in life. Variation in lifetime reproductive success is extreme. Most males never mate because they die before reaching breeding age or they mature, reach old age, and are prevented from mating by the power structure. Males attempt to mate with pregnant females, nursing females, females giving birth, dead females, and newly weaned pups. Females departing the harem after weaning their pups are weak, having lost 40% of their mass; they are in danger from peripheral males attempting to mate with them, and some are inadvertently killed.