Breeding in seals and social organitzation

Pinniped species, commonly known as seals, breed on land or ice. Depending on the place they breed on, they present a social organization or another. In this post, we will review both the breeding systems in seals and their social organization. Do you know that in the Mediterranean Sea live seals? 

MATING AND BREEDING IN PINNIPEDS

Pinnipeds have different mating systems: while most of the species are polygynous, it is that males mate with several females; others are monogamous and males mate with just one female during a breeding season. In the first case, males are much larger than females, while in the monogamous system there are almost no differences between sexes.

Like in the rest of species, females are more valuable than males because they produce the ovule, carry and nourish the young, produce milk after birth and provide all parental care. On the other side, it is much better for males to copulate with as many females as possible to increase their reproductive success. Therefore, maternal nurturing plays a key role in organising their societies. 

Pinnipeds use different habitats for their breeding:

1. Land
2. Ice: both pack ice (floating ice) and fast ice (ice attached to land).

In the following sections, we will explain the mating and breeding systems in each type of seal, in addition to their social organization.

LAND-BREEDING SEALS

20 of the 33 pinniped species breed on land, specially on islands because are more favourable than mainland beaches and sandbars (where they can also breed on).

South American sea lions (Otaria bryonia) breed on land (Picture: Steven Hazlowski, Arkive).

However, there are neither many favorable islands to seals nor many suitable breeding sites in these islands and, thus, females and pups tend to congregate in colonies, where males either compete for breeding territories (in otariids, it is sea lions and fur seals) or establish dominance hierarchies (in elephant seals). 

South African and Australian fur seals (Arctocephalus pusillus) live in large colonies (Picture: Pete Oxford, Arkive)

These aggregations let males copulate with a great number of females (after an intense competition among males).

Among the species that breed in large colonies, there are a marked variability in the social organization. Some species form annual breeding aggregations at traditional locations called rookeries. Rookeries are formed by all otariids, elephant seals and gray seals. During this period, females and pups live in zones controlled by alpha males, while juvenile and subdominant males live in bachelor groups.

Even during the nonbreeding season, they  usually live in association with other animals because it gives some advantages:

• Thermoregulatory effects of huddling together during cold weather.
• Protection from predators.

ICE-BREEDING SEALS

Different from the land-breeding seals, ice-breeding species on pack ice are not obliged to form aggregations due to the vast available ice and, therefore, males cannot mate with so many females, just one or a few females.

Ross seal (Ommatophoca rossii) chiefly live on dense consolidated pack ice. They are usually solitary or live in small groups (Picture: NOAA, Creative Commons).

So, it is common in seals that breed on pack ice to be monogamous or slightly polygynous. 

On the other side, seals can breed on fast ice (ice which is attached to land), usually in cracks and open holes. Therefore, they live in small to moderate-sized groups where a male can mate with only some females close to these particular points.

Fast-ice breeding seals, such as ringed seals (Pusa hispida) live in small to moderate-sized groups (Picture: Shawn Dahle, Creative Commons).

In general, ice-breeding seals of both sexes have a similar size, with the exception of the hooded seal (Cystophora cristata) and the walrus (Odobenus rosmarus), in which males are bigger than females; and of the Antarctic Weddell seal (Leptonychotes weddellii), in which females are bigger than males. The reason is that males maintain aquatic territories beneath the ice near holes and cracks and being smaller makes easier to protect territories and mate with females.

In Weddell seals (Leptonychotes weddellii), females are bigger than males (Picture: Samuel Blanc, Creative Commons).

CONCLUSION

In conclusion, when the available space is limited, female seals congregate in large colonies, where males can mate with several females; while when the space is dispersed, females are isolated and males can mate with just one female and colonies are not formed.

REFERENCES

• Acevedo-Gutiérrez, A (2009). Group Behaviour. In Perrin, W; Würsig, B & Thewissen, JGM (ed.). Encyclopedia of Marine Mammals. Academic Press (2 ed).
• Antonelis, GA (2009). Rookeries. In Perrin, W; Würsig, B & Thewissen, JGM (ed.). Encyclopedia of Marine Mammals. Academic Press (2 ed).
• Berta, A (2009). Pinnipedia, Overview. In Perrin, W; Würsig, B & Thewissen, JGM (ed.). Encyclopedia of Marine Mammals. Academic Press (2 ed).
• Mesnick, S. & Ralls, K (2009). Mating Systems. In Perrin, W; Würsig, B & Thewissen, JGM (ed.). Encyclopedia of Marine Mammals. Academic Press (2 ed).
• Riedman, M (1990). The Pinnipeds. Seals, sea lions and walruses. University of California Press.
• Shirihai, H. & Jarrett, B (2006). Whales, Dolphins and Seals. A field guide to the marine mammals of the world. Bloomsbury.
• Main Picture: Ecotrust