Breeding Sites (marine mammals)

Giving birth to young and the subsequent nursing of those young present unique problems to marine mammals. The strategies that marine mammals employ to deal with these problems can be divided into two groups: (a) those animals that need to leave the water to breed (seals and polar bears, Ursus maritimus) and those that remain at sea to breed (cetaceans, sirenians, and sea otters, Enhydra lutris). Whichever strategy is used, a crucial component of the reproductive process is the site used for breeding (here, the term breeding is restricted to parturition and suckling and does not include mating). Breeding sites used by marine mammals are quite diverse, both in terms of geography and physical characteristics. ranging from polar to equatorial regions and from sandy beaches to deep ocean basins. Which sites are used for breeding by a particular species is determined by a complicated mixture of factors, including evolutionary history, requirements of the young, requirements of the adults, biological characteristics (such as the proximity of prey), and physical characteristics (such as water temperature or beach substrate).

I. Species That Leave the Sea to Breed

All species of seals and polar bears leave the sea in order to give birth, and most also need to be ashore to suckle their young. Some species (predominantly the phocids) remain ashore for the entire lactation period, whereas in others (the otariids), the females regularly return to sea to forage during lactation. Of the phocids, only harbor seals (Phoca vitulina) and walruses (Odobenus rosinams) have been reported to suckle young at sea. The sites used by these animals for breeding activities can be divided into two groups: those that breed on land and those that breed on ice.

A. Marine Mammals That Breed on Land

Pregnant polar bears leave the arctic pack ice with the onset of spring to breed on land. The deteriorating summer pack ice is too unstable a substrate for the bears, which have a relatively long period of cub dependence. Females excavate caves in the side of riverbanks or hillsides, thereby getting close to the permafrost and providing an environment with a stable temperature. As summer progresses and snowfall increases, these caves get snowed in and the females need to maintain a cavity with sufficient wall thickness to provide insulation, but also thin enough to allow the passage of air.

About twenty species of seals breed on land. This includes all of the species of otariids (fur seals and sea lions) and six of the phocids (true seals) (Table I). None of these species use any kind of shelter and all give birth and suckle on the beach exposed to wind, rain (or snow), and waves. Land-breeding seals often occur in very large aggregations during the breeding season, which may offer some protection for young seals from the weather, but this also puts them at risk of being damaged in fights between adult seals. Land-based breeding sites occur at all but the most extreme polar latitudes.

Some species utilize both land- and ice-breeding sites. Gray seals (Halichoerus gnjpus) breed on beaches in northern Europe and America, but on pack ice in Northern Canada. Although primarily a land-breeding species, harbor seals also breed in pack ice in northern Canada.

Most land-breeding seals use islands as their principal breeding sites, with only a few species utilizing mainland beaches. This is likely to be an attempt to avoid large, mainland predators, including humans. Generally, species have quite specific requirements of their island-breeding sites, and so suitable island-breeding sites are often limited. Consequently, those sites that are suitable tend to hold very large numbers of seals, which provides an ideal condition for the evolution of polygeny.

TABLE 1

Land-Breeding Marine Mammals. Including the Primary Geographic Type of Breeding Site

Different species have different substrate requirements. Most species use gradually sloping sandy beaches, such as those used by northern elephant seals. Fur seals, which are generally more agile than phocids, can also breed on rocky substrates, but again this is quite species specific. For example, at Madquarie Island, where both Antarctic (Arctocephalus gazella) and sub-Antarctic (A. tropicalis) fur seals breed, A. gazella use open beaches, whereas A. tropicalis breed on nearby rocky headlands. The reasons for these different preferences are unknown, but may arise from resource partitioning.

Proximity to a food source may be another factor that determines the location of breeding sites for some land-breeding seals. For example, female otariids need to return to sea to replenish their energy reserves regularly during the lactation period. As their pups are fasting during these trips to sea, faster growth and heavier weaning masses can be achieved if the foraging trips are kept as short as possible. Breeding sites are often located close to the continental shelf break or other oceanographic features that tend to have enhanced primary productivity. In cases where this is not possible, such as the sub-Antarctic fur seal breeding site on Amsterdam Island, female foraging trips are much longer than in other species (or populations) and the pups have correspondingly lower growth rates.

Land-breeding phocid seals generally have no requirement for feeding during lactation, and therefore the breeding sites can be located considerable distances from the foraging sites. Southern elephant seals (Mirounga leonina), for example, tend to feed in high-latitude waters, but breed thousands of kilometers away on sub-Antarctic islands. In this case the primary requirement of a breeding site is suitable beach structure and perhaps a moderate climate to help the pup in its early life.

B. Marine Mammals That Breed on Ice

Thirteen species of pinnipeds breed on ice, either floating pack ice or fast ice attached to land (Table II). Ice-breeding seals tend to be monogamous, and this is likely to be a consequence of the breeding habitat. Unlike suitable beaches for land-breeding seals, ice is not a limited resource, and females can haul out anywhere to breed, so aggregations of females tend not to occur. This limits the opportunities that males have to monopolize access to several females, and the best strategy for them is to find a female and remain with her until estrus. Weddell seals (Leptonychotes weddellii) are an exception to this rule, as they breed on fast ice, with limited access to open water. Female Weddell seals therefore tend to aggregate around tide cracks and other sources of permanent open water.

Most ice-breeding seals give birth and suckle their young on the ice. Exceptions to this rule are ringed seals (Pusa hispida) and Baikal seals (P. sibirica), which can use ice lairs under ice ridges. These lairs afford some protection from the elements and perhaps from predators, although polar bears are adept at locating and breaking into these lairs.

TABLE 11

Ice-Breeding Marine Mammals. Including

Although pack ice is the breeding substrate, many species have preferred geographic regions to which they move for the breeding season. Harp seals (Pagophilus groenlandicus), for example, migrate to the southerly edge of the pack ice and occupy only a small part of their overall range during the breeding season. The biggest areas are around Newfoundland and off western Greenland. Hooded seals (Cystophora cristata) show a similar pattern of migration, moving from widespread northerly feeding areas to more proscribed breeding areas, which appear to be associated with the continental shelf at the southern extent of the summer pack ice. This may provide the adults with access to the shelf for feeding immediately after weaning their pups.

II. Species That Stay at Sea to Breed

Three groups of marine mammals are sufficiently adapted to a marine existence that do not need to leave the water to give birth or suckle their young. These are the cetaceans, the sirenians, and the sea otters. By giving birth at sea, these animals are no longer constrained to use what is essentially a limiting resource: land. Nonetheless, many species still have quite specific requirements of their breeding sites and migrate thousands of kilometers to reach their breeding grounds. Because such profound separation of feeding and breeding sites incurs large energetic costs, the specific characteristics of these areas must be of considerable importance.

Dugongs (Dugong dugon) and manatees (Trichechus spp.) have no specialized breeding site requirements, although newborns may be kept close to shore or in protected bays or inlets. Young are born and remain with their mother while she forages on sea grass beds. Likewise, sea otters tend to give birth at sea (often among kelp beds) and the cubs remain with their mothers. Neither group appears to migrate to specific regions for breeding and remain within their foraging areas.

Cetaceans are the only group of ocean-breeding marine mammals in which some species have clear separation of feeding areas and breeding areas. Within the cetaceans, breeding sites can be loosely categorized as (i) coastal, (ii) open ocean, or (iii) nonspecific. Species with nonspecific-breeding sites are those that show no evidence of requiring different environments for breeding. This group contains most of the odontocetes and several of the mysticetes. Many of the smaller odontocete species do make seasonal migrations, but these are not clearly linked to breeding and seem more related to changes in prey distribution.

Several species of beaked whales appear to have year-round high-latitude distributions and do not migrate for breeding, giving birth and suckling their young in polar waters. This behavior is also seen in two mysticete species; the common minke (Balacnoptera aaitorostrata) and the bowhead (Balaena imjs-ticetus) whales. Bowhead whales are never far from pack ice, including during the breeding season.

A. Cetaceans with Coastal Breeding Sites

Humpback whales (Megaptera novaeangliae) have several recognized breeding grounds and all are associated with coastal regions. The Southern Hemisphere populations use either the coast of the major southern continents (South Africa, Australia, or South America) or smaller oceanic islands such as Tonga and Fiji, with the preferred sites generally north of 30°. The Northern Hemisphere humpbacks move to the Caribbean, Hawaii, or Cape Verde Islands.

Gray whales (Eschrichtus robustus) move south from arctic feeding grounds to breed in Baja California and. as with humpbacks. the breeding sites are close inshore. In fact, they are so inshore that they are close enough to be seen from land and form the basis of a tourist industry.

Of the inshore breeding cetaceans, the right whales (Eubal-aena spp.) are least migratory. Neither the southern (E. australis) or the northern right whale species (E. glacialis and E. japonica) make long migrations to their breeding sites, but all three species favor coastal sites and sheltered bays for giving birth. Individual southern right whales show strong breeding site fidelity and use the same bays on several consecutive breeding events.

Only one odontocete species, the beluga (Delphinaptenis leucas). seems to have an inshore migration, and often breeds in shallow inshore waters. Why these species seek out inshore waters in not really known. Aside from the thermal advantages common to all migrating whales (see later), the specific advantages associated with inshore breeding are likely to be related to environmental conditions and predator avoidance.

B. Cetaceans with Oceanic Breeding Sites

Less is known about the characteristics of the breeding sites for these species due to their lack of coastal aggregations, so much of what is known coines from early tagging studies and whaling records.

Fin whale (Balacnoptera phtjsalus) breeding sites are widely spread over oceanic waters in temperate and subtropical waters. Some fin whales move toward land and even form loose aggregations, but they do not cross the continental shelf, remaining in deep water. The little information available for blue whales suggests that they use similar breeding sites to fin whales. This may also be the case for sei whales (B. borealis). although they may not penetrate tropical waters. Brydes whales (B. edeni) do not appear to migrate to breed, presumably this is a reflection of this species’ largely tropical habitat all year round.

There is ongoing debate regarding the reasons for the use of temperate or equatorial breeding sites by mysticete whales. Thermal and energetic advantages to newborn calves is one possibility, but if so, why do some species such as the minke and bowhead whales, as well as many smaller odontocetes, remain in high-latitude waters to breed? An alternative view is that the abundant and predictable food supply of food at high latitudes declines over the dark winter months, but only species with a large body size (and lower mass specific metabolic rates) are able to make the long migrations that allow them to take advantage oi warmer waters.