240
of skin and bone samples from individuals collected in Brazil and Uruguay (Drago
et al. 2015). The species seems to perform a wide variety of feeding strategies
throughout the year, though feeding is reduced during the pre-breeding. Also, during this period there seems to be no overlap in the isotopic niche between sexes. The
dietary shift is probably a strategy for maximizing energy intake to prepare for a
period of high-energy demand (Costa 2008). The difference between sexes in the
pre-breeding period can be attributed to the higher forage capacity of males compared to females. The larger body size and related physiological attributes of males,
such as energy conservancy, allow them to have easier access to larger and more
energetically preys in deeper waters than females (Page et al. 2005).
Assuming that the diet of females’ otariids can be inferred by the diet of their
pups, Drago et al. (2010) and Franco-Trecu et al. (2012) used serum and skin samples of pups and concluded that there is no trophic overlaps between females of
South American fur seal (Arctocephalus australis) and South American sea lion.
The higher δ
13
C and δ
15
N values for sea lion pups indicate that the lactating females
of this species consumed nearshore resources, while fur seals probably foraged further offshore and included anchovy and squids in their diet. As the isotopic values
are related to the tissues’ turnover rates (Tieszen et al. 1983), authors related the
serum isotope composition to the prepartum period of females’ diet and the skin, to
the postpartum period. Both species showed a shift between these periods, but for
different reasons. For fur seals, it was attributed to lower variation in trophic levels
of the preys, whereas for sea lions it was related to a decrease in the diversity of
foraging areas. The authors have also raised the possibility of some females foraging at different latitudes to explain the large variation in their δ
13
C values. These
movements could benefit the species by minimizing intraspecific competition,
mainly during the reproductive period.
With the aim of understanding the reasons for the different population trends of
these sympatric species in Uruguay, Franco-Trecu et al. (2014) assessed their forage
ecology through SIA of individuals’ whiskers. A minimum overlap between the
isotopic niche both at individual and population levels was observed. At the intraspecific level, in spite of using different foraging areas, a large overlap between
sexes was observed for both species. Nevertheless, the niche was wider and the
individuals were considered more generalists in fur seals than in sea lions individuals, which presented a considerable specialization. The narrower niche and the high
degree of individual specialization, among others, were considered by the authors
possible explanations for the population declining of the latter species.
The overlap between the diet of these two species was also evaluated in areas to
the south (northern and southern Argentina) and to the north (southern Brazil) of the
La Plata river estuary (Saporiti et al. 2016). In this study only adults were considered. The diet of female sea lion overlapped with larger male sea lions, but not with
male fur seals. Morphological similarity was significantly correlated with the isotope niche represented by the ellipses generated by δ
13
C and δ
15
N of each group in
the La Plata river estuary.
The long-term diet of these species was assessed by the isotope composition of
individuals’ bones (Vales et al. 2013; Zenteno et al. 2014). The tested hypothesis
E. Seyboth et al.
of skin and bone samples from individuals collected in Brazil and Uruguay (Drago
et al. 2015). The species seems to perform a wide variety of feeding strategies
throughout the year, though feeding is reduced during the pre-breeding. Also, during this period there seems to be no overlap in the isotopic niche between sexes. The
dietary shift is probably a strategy for maximizing energy intake to prepare for a
period of high-energy demand (Costa 2008). The difference between sexes in the
pre-breeding period can be attributed to the higher forage capacity of males compared to females. The larger body size and related physiological attributes of males,
such as energy conservancy, allow them to have easier access to larger and more
energetically preys in deeper waters than females (Page et al. 2005).
Assuming that the diet of females’ otariids can be inferred by the diet of their
pups, Drago et al. (2010) and Franco-Trecu et al. (2012) used serum and skin samples of pups and concluded that there is no trophic overlaps between females of
South American fur seal (Arctocephalus australis) and South American sea lion.
The higher δ
13
C and δ
15
N values for sea lion pups indicate that the lactating females
of this species consumed nearshore resources, while fur seals probably foraged further offshore and included anchovy and squids in their diet. As the isotopic values
are related to the tissues’ turnover rates (Tieszen et al. 1983), authors related the
serum isotope composition to the prepartum period of females’ diet and the skin, to
the postpartum period. Both species showed a shift between these periods, but for
different reasons. For fur seals, it was attributed to lower variation in trophic levels
of the preys, whereas for sea lions it was related to a decrease in the diversity of
foraging areas. The authors have also raised the possibility of some females foraging at different latitudes to explain the large variation in their δ
13
C values. These
movements could benefit the species by minimizing intraspecific competition,
mainly during the reproductive period.
With the aim of understanding the reasons for the different population trends of
these sympatric species in Uruguay, Franco-Trecu et al. (2014) assessed their forage
ecology through SIA of individuals’ whiskers. A minimum overlap between the
isotopic niche both at individual and population levels was observed. At the intraspecific level, in spite of using different foraging areas, a large overlap between
sexes was observed for both species. Nevertheless, the niche was wider and the
individuals were considered more generalists in fur seals than in sea lions individuals, which presented a considerable specialization. The narrower niche and the high
degree of individual specialization, among others, were considered by the authors
possible explanations for the population declining of the latter species.
The overlap between the diet of these two species was also evaluated in areas to
the south (northern and southern Argentina) and to the north (southern Brazil) of the
La Plata river estuary (Saporiti et al. 2016). In this study only adults were considered. The diet of female sea lion overlapped with larger male sea lions, but not with
male fur seals. Morphological similarity was significantly correlated with the isotope niche represented by the ellipses generated by δ
13
C and δ
15
N of each group in
the La Plata river estuary.
The long-term diet of these species was assessed by the isotope composition of
individuals’ bones (Vales et al. 2013; Zenteno et al. 2014). The tested hypothesis
E. Seyboth et al.