The mating system of pinnipeds varies from extreme polygyny to serial monogamy.
Of the 33 species, 20 breed on land, and the remaining 13 breed on ice.
Species that breed on land are usually polygynous, as females gather in large aggregations and males are able to mate with them as well as defend them from rivals.
Polygynous species include elephant seals, grey seals and most otariids.
Land-breeding pinnipeds tend to mate on islands where there are fewer terrestrial predators.
Few islands are favorable for breeding, and those that are tend to be crowded.
Since the land they breed on is fixed, females return to the same sites for many years.
The males arrive earlier in the season and wait for them.
The males stay on land and try to mate with as many females as they can; some of them will even fast.
If a male leaves the beach to feed, he will likely lose mating opportunities and his dominance.
Polygynous species also tend to be extremely sexual dimorphic in favor of males.
This dimorphism manifests itself in larger chests and necks, longer canines and denser fur—all traits that help males in fights for females.
Increased body weight in males increases the length of time they can fast due to the ample energy reserves stored in the blubber.
Larger males also likely enjoy access to feeding grounds that smaller ones are unable to access due to their lower thermoregulatory ability and decreased energy stores.
In some instances, only the largest males are able to reach the furthest deepest foraging grounds where they enjoy maximum energetic yields that are unavailable to smaller males and females.

Other seals, like the walrus and most phocids, breed on ice with copulation usually taking place in the water (a few land-breeding species also mate in water).
Females of these species tend to aggregate less.
In addition, since ice is less stable than solid land, breeding sites change location each year, and males are unable to predict where females will stay during the breeding season.
Hence polygyny tends to be weaker in ice-breeding species.
An exception to this is the walrus, where females form dense aggregations perhaps due to their patchy food sources.
Pinnipeds that breed on fast ice tend to cluster together more than those that breed on drift ice.
Some of these species are serially monogamous, including the harp seal, crabeater seal and hooded seal.
Seals that breed on ice tend to have little or no sexual dimorphism.
In lobodontine seals, females are slightly longer than males.
Walruses and hooded seals are unique among ice-breeding species in that they have pronounced sexual dimorphism in favor of males.

Adult male pinnipeds have several strategies to ensure reproductive success.
Otariids establish territories containing resources that attract females, such as shade, tide pools or access to water.
Territorial boundaries are usually marked by natural breaks in the substrate, and some may be fully or partially underwater.
Males defend their territorial boundaries with threatening vocalizations and postures, but physical fights are usually avoided.
Individuals also return to the same territorial site each breeding season.
In certain species, like the Steller sea lion and northern fur seal, a dominant male can maintain a territory for as long as 2–3 months.
Females can usually move freely between territories and males are unable to coerce them, but in some species such as the northern fur seal, South American sea lion and Australian sea lion, males can successfully contain females in their territories and prevent them from leaving.
In some phocid species, like the harbor seal, Weddell seal and bearded seal, the males have underwater territories called "maritories" near female haul-out areas.
These are also maintained by vocalizations.
The maritories of Weddell seal males can overlap with female breathing holes in the ice.

Lek systems are known to exist among some populations of walruses.
These males cluster around females and try to attract them with elaborate courtship displays and vocalizations.
Lekking may also exist among Cali
The mating system of pinnipeds varies from extreme polygyny to serial monogamy. Of the 33 species, 20 breed on land, and the remaining 13 breed on ice. Species that breed on land are usually polygynous, as females gather in large aggregations and males are able to mate with them as well as defend them from rivals. Polygynous species include elephant seals, grey seals and most otariids. Land-breeding pinnipeds tend to mate on islands where there are fewer terrestrial predators. Few islands are favorable for breeding, and those that are tend to be crowded. Since the land they breed on is fixed, females return to the same sites for many years. The males arrive earlier in the season and wait for them. The males stay on land and try to mate with as many females as they can; some of them will even fast. If a male leaves the beach to feed, he will likely lose mating opportunities and his dominance. Polygynous species also tend to be extremely sexual dimorphic in favor of males. This dimorphism manifests itself in larger chests and necks, longer canines and denser fur—all traits that help males in fights for females. Increased body weight in males increases the length of time they can fast due to the ample energy reserves stored in the blubber. Larger males also likely enjoy access to feeding grounds that smaller ones are unable to access due to their lower thermoregulatory ability and decreased energy stores. In some instances, only the largest males are able to reach the furthest deepest foraging grounds where they enjoy maximum energetic yields that are unavailable to smaller males and females.
Other seals, like the walrus and most phocids, breed on ice with copulation usually taking place in the water (a few land-breeding species also mate in water). Females of these species tend to aggregate less. In addition, since ice is less stable than solid land, breeding sites change location each year, and males are unable to predict where females will stay during the breeding season. Hence polygyny tends to be weaker in ice-breeding species. An exception to this is the walrus, where females form dense aggregations perhaps due to their patchy food sources. Pinnipeds that breed on fast ice tend to cluster together more than those that breed on drift ice. Some of these species are serially monogamous, including the harp seal, crabeater seal and hooded seal. Seals that breed on ice tend to have little or no sexual dimorphism. In lobodontine seals, females are slightly longer than males. Walruses and hooded seals are unique among ice-breeding species in that they have pronounced sexual dimorphism in favor of males.
Adult male pinnipeds have several strategies to ensure reproductive success. Otariids establish territories containing resources that attract females, such as shade, tide pools or access to water. Territorial boundaries are usually marked by natural breaks in the substrate, and some may be fully or partially underwater. Males defend their territorial boundaries with threatening vocalizations and postures, but physical fights are usually avoided. Individuals also return to the same territorial site each breeding season. In certain species, like the Steller sea lion and northern fur seal, a dominant male can maintain a territory for as long as 2–3 months. Females can usually move freely between territories and males are unable to coerce them, but in some species such as the northern fur seal, South American sea lion and Australian sea lion, males can successfully contain females in their territories and prevent them from leaving. In some phocid species, like the harbor seal, Weddell seal and bearded seal, the males have underwater territories called "maritories" near female haul-out areas. These are also maintained by vocalizations. The maritories of Weddell seal males can overlap with female breathing holes in the ice.
Lek systems are known to exist among some populations of walruses. These males cluster around females and try to attract them with elaborate courtship displays and vocalizations. Lekking may also exist among Cali
Pinniped