South American Fur Seals are sexually dimorphic. Adult males are approximately 1.3 times longer and 3.3 times heavier than adult females. Adult males can reach 2 m and 90–160 kg, and possibly 200 kg; females do not exceed 1.5 m long and weigh on average 60 kg. Males of the Peruvian subspecies are smaller than the South American subspecies and females are slightly larger (Oliveira et al. 2005, 2008a). Newborns are 50-65 cm and 3-7.5 kg (Vaz-Ferreira 1982, Punta San Juan Program unpublished data), with initial mass being significantly greater for male pups (Baladán Corbo 2012). Females reach their maximum reproductive value at 3-5 years old (Lima and Páez 1995, 1997). Their reproductive cycle has a duration of 11 months, with a 3-4 month embryonic diapause (Vaz Ferreira et al. 1982, Katz et al. 2013).
Breeding takes place from late October through mid January (Majluf 1987a, Franco-Trecu 2005). Pupping peaks in mid November to mid December, and mating occurs 1-6 days after the female gives birth (Franco-Trecu 2005, Pavés and Schlatter 2008). Following birth, the mother suckles her pup and fasts on shore for almost 11 days (Franco-Trecu 2010). Then the female begins to make foraging trips punctuated by time attending the pup ashore, spending up to 4-5 days foraging at sea and 1-2 days feeding offspring at rookeries (Bastida and Rodriguez 2003). During the first three months of maternal care, duration of foraging trips by females is highly variable, which affects the survival of offspring since longer trips increase pup mortality (Franco-Trecu et al. 2010b).
Time spent on feeding trips and attending offspring likely varies with location, changes in marine productivity, and age of offspring, or a combination of these factors. Trip distances, trip durations and lengths of visits increase throughout the season. During the early lactation females perform short nocturnal foraging trips. Trip duration starts increasing during mid-lactation, but the majority of foraging effort still concentrates close to breeding sites. Later in the season, when pups are capable of withstanding longer fasts and metabolic demands of pregnancy and lactation are higher, females may stay at sea longer and forage in more distant prey patches (Thompson et al. 2003). In the Atlantic, the effects of El Niño-La Niña are less strong. Within the usual variability, the differences between years are relatively minor and the effects of the El Niño Southern Oscillation (ENSO) are relatively moderate. In the absence of drastic changes in the ecosystem, attendance patterns are not affected (Franco-Trecu 2010).
Colonies are generally found along rocky coasts, on ledges above the shoreline or in boulder strewn areas. Most areas utilized have some source of shade such as at the base of cliffs or under boulders, and easy access to the ocean or tidal pools (Stevens and Boness 2003). Around midday, Fur Seals make daily movements from high and dry levels of the rookeries to cool off in low and wet areas or in the sea (Cassini 2001, Vaz-Ferreira and Ponce de León 1984). During the reproductive season, these movements can often cause female aggression towards conspecifics, mostly at other females, in order to protect their new-born pups and maximize their offspring’s survival (Cassini 2001).
Males are polygynous and territorial, and fighting can result in serious wounds and scars (Cappozzo 1995). The number of breeding females associated with a territorial male varies between 3-6 in southern Chile (Pavés and Schlatter 2008) and 6-20 at Punta San Juan (Majluf 1987a). The highest number of territorial males at the colonies can be found in late December, decreasing in January (Franco-Trecu 2005, Franco-Trecu et al. 2014). Individual bulls can occupy territories for up to 60 days (Cappozzo 1995) until most of the females are mated, and then they leave their territories to start foraging at sea (Pavés and Schlatter 2008). Only a few adult males out of the total population achieve mating, and a large proportion is excluded to peripheral or male exclusive areas. In Uruguay, the breeding pattern is a “lek” system where females have extensive home ranges that overlap with the small territories of many males. Females move freely in the colony and males do not monopolize access to females (Franco-Trecu et al. 2014).
Most pup mortality take place at the beginning of the breeding season, during the peak of birthing (Franco-Trecu 2010). In general, the principal causes of death of South American Fur Seal pups are enteritis with microscopic lesions of bacteremia (associated with the presence of hookworms) (28%), and starvation, trauma, and drowning (63%) (Seguel et al. 2013). Locally, pup mortality is also caused by predation by adult male South American Sea Lions that can be significant at some colonies (Harcourt 1993). Maternal aggression was also recognized as an important source of mortality for Peruvian Fur Seals before the 1997-98 ENSO (Harcourt 1991,1992; Majluf 1992).
Based on satellite tracking data, South American Fur Seals apparently forage between 50 and 600 m with no clear water-depth preference, and mean duration of the female foraging trips during the breeding season is 126 hours (Thompson et al. 2003). Lactating females in Uruguay forage 41-185 km from the breeding sites (Vaz-Ferreira 1976, York et al. 1998). For the Peruvian Fur Seal, ENSO years have a negative impact on animals and during those years females must spend much more time attempting to forage, which affects offspring growth and survival (Majluf 1987a, Gentry and Kooyman 1987). During the 1983 ENSO, adult female Peruvian Fur Seals showed mean dives to 29 m, with a maximum of 170 m and mean duration of 2.5 minutes and maximum of 7 minutes (Majluf 1987b). During the 1997 ENSO, adult females had to spend longer periods at sea foraging (10-20 days) causing their pups to die of starvation.
The location of foraging grounds almost certainly depends on the distribution of preferred prey (Laptikhovsky 2009). The foraging area of the South American Fur Seals southwest of the Falkland Islands coincides with the region of the highest abundance of the Lobster Krill (Munida spp.), the most important food resource of Fur Seals off the Falklands (Laptikhovsky 2009). Seasonal variations in intensity and position of both the Falkland Current and Argentine Drift could also be a reason for seasonal changes in female foraging grounds (Thompson et al. 2003).
The Southern Fur Seal diet varies according to prey availability. Although they are trophic generalists with the potential to prey upon many species, a few species dominate their diet. Pelagic and demersal fishes and cephalopods are the most common prey (Vásquez 1995; Zavalaga et al. 1998, Arias-Schreiber 2000, 2003; Oliveira et al. 2008b, Vallejos 2010, Franco-Trecu et al. 2012, 2013, 2014).
Available data on South American Fur Seal abundance were compiled and reviewed in the 2016 IUCN Red List assessments for the two recognized subspecies. Estimates of the number of mature individuals, and population trend, for each of those subspecies were as follows:
- South American Fur Seal–99,000, trend increasing weakly;
- Peruvian Fur Seal–10,500, trend unknown.
The figures indicate the total number of mature Southern Fur Seals is approximately 109,500, and 219,000 is a reasonable estimate of the total population size. Since the much more abundant South American subspecies is known to be increasing it is likely the species as a whole is increasing also.