Here are some of the sea lions in a rookery, occupied by a breeding population in the Galapagos Islands.
Photo courtesy of Molly Downey.
Social Spacing
California sea lions are a gregarious species, and are usually found terrestrially in variable sized groups during breeding and non-breeding seasons. On the islands off of California they are often found in large rookeries containing the extreme of thousands of animals during the breeding season (Riedman, 2004).
The rookery is defined as a coastal area occupied by a breeding population and a hauling round as a terrestrial site occupied by non-breeding animals (Peterson, 1967). There is a conspicuous, although not complete, sexual segregation which occurs during the non-breeding season. The adult and sub-adult males generally move northward as soon as the summer breeding season ends, and return to the rookeries in spring (Peterson, 1967). The breeding season is generally considered May to August (Riedman, 2004). The females and young either remain in the vicinity of the breeding rookeries the year round, or, as in the case of some members of the population in the California Channel Islands, apparently move a little southward in the winter (Peterson, 1967). However, little is known of the female migratory movements and the amount of space they cover.
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Here are some of the sea lions in a rookery, occupied by a breeding population in the Galapagos Islands. Photo courtesy of Molly Downey. |
Why are they gregarious?
According to Renouf (1991) there are three main hypotheses for gregariousness in mammals: The Marginal male hypothesis, ecological-marginal-male hypothesis, and the selfish-herd hypothesis. Here are the three hypotheses and how they relate to sea lions.
Marginal male hypothesis:
The sexual selection should favor female gregariousness because any female that moves to the periphery of female clusters is likely to be fertilized by a male that has been unsuccessful in establishing himself among the herd. However, evidence linking a female’s fitness to proximity to a fit male is lacking, so this hypothesis is difficult to support for the California sea lions.
Ecological-marginal-male hypothesis :
Injury and death of females and pups caused by harassment from peripheral males should lead to increased gregariousness through natural selection. However, again there is little evidence to support extreme harassment occurred from peripheral males.
Selfish-herd hypothesis :
This hypothesis explains gregariousness of animals in relation to predation pressures. Natural selection should favor an individual that moves close to other conspecifics, as in doing so increases the likelihood that its neighbor rather than itself will be taken by a predator. However, it is difficult to evaluate either importance of predation pressure on clustering of otariid females since most living otariids breed on oceanic islands, where there are no predators, although some species use both oceanic islands and coastal beaches.
Therefore, overall it is difficult to support any of these three hypotheses due to a lack of research, though any of the three may be valid.
Breeding Only Territory
California sea lions are considered nomadic with a breeding only territory. The breeding only territories are maintained for 2-6 weeks and the bulls fast during this time (Riedman, 2004). Territories only exist when and where females are present. Once territories are established, males patrol their boundaries and bark when necessary to maintain and defend them (B.E.C., 2004). The territorial boundaries are fluid and unstable, but extend from land to deep water (Riedman, 2004). The preferred territories contain the largest amount of females and are controlled by the largest breeding bulls (male sea lions) (Riedman, 2004). California sea lions tend to breed on the same section of beach year after year, and generally favor beaches on the windward side of islands (B.E.C., 2004).
The establishment of territories starts when males arrive at the rookery (Booner, 1994; Riedman, 1990). However, it is controversial as to whether males arrive at the rookery earlier in the breeding season than the females to establish territories (Renouf, 1991). Much fighting occurs the first few days the males arrive. Fights involve chest to chest pushing, lunging, and slashing at an opponent's flippers, chest, and vulnerable hindquarters (Riedman, 1990; Harrison, 1981). Once boundaries are established, agonistic displays between neighboring males become much less frequent (Riedman, 1990). Males fast during the 2-6 weeks the territory is maintained in order to avoid the risk of loosing it by leaving (Riedman, 1990). The territorial California sea lion males continually displace one another throughout the breeding season (Riedman, 1990). Non-breeding bulls and subadult males congregate in "bachelor" groups near the breeding areas (Riedman, 1990). Breeding bulls do not sequester females like the Southern sea lion, therefore they do not engage in female defense polygyny, but rather resource defense polygyny because of their defense of varying territories (Riedman, 1990). However, there it has also been suggested that male dominance, or lek, polygyny may contribute to the mating system (Renouf, 1991). (See “Mating Systems” section for more details).
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Territories can be of varying preference depending on topography of habitat. In some instances, as thermoregulation is extremely important to sea lions during the hot, dry breeding season, competition for and defense of preferred territories (those with tide pools and boulders for shade) is more intense (Riedman, 1990). For California sea lions, the location of sea lion territories on open, sandy areas may shift with the tides, air temperature, time of day, and location of females, whereas territory boundaries on rocky beaches are generally stable throughout the breeding season (Riedman, 1990). Male California sea lions may be territorial in the water as well as on land (Booner, 1994; Riedman, 1990). The average size of a territory is about 130 square meters and males are frequently observed along the beach at 1 to 15-m intervals (B.E.C, 2002; Renouf, 1991). However, the specific territorial boundaries are hard to map for several reasons: (1) typically there is no inland boundary which a bull defends, (2) the seaward edge of the territory may be underwater, hard to detect, (3) a bull only infrequently patrols the entire perimeter of his territory, and (4) intruding bulls are sometimes tolerated in territories by the resident bull for hours at a time, very probably because he remains unaware of them (Booner, 1994). Females choose which territory to haul out in and move freely in and out of a male's territory (Booner, 1994; Riedman, 1990). An average group size of females for every male is 14 females, which males make little attempt to herd (Reidman, 1990).
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