Abstract
Sexual segregation in foraging occurs in some species and populations of boobies (Sulidae), but it is not a general pattern. Sexual segregation in foraging may occur to avoid competition for food, and this competition may intensify during specific stages of breeding. We examined sexual segregation in foraging in relation to breeding stage in masked boobies Sula dactylatra at Rapa Nui by tracking simultaneously incubating and chick-rearing birds using GPS recorders (n = 18) and collected a total of 11 regurgitate samples. Stable isotope analyses (δ13C and δ15N) of whole blood samples were carried out in 20 birds. There were no differences in foraging trip parameters or diet between females and males. Both sexes traveled farther and for longer while incubating than while rearing chicks. Isotopic niches (δ13C and δ15N) overlapped to some degree among all groups at all times, but the lowest overlap between sexes occurred during incubation. While preying on ephemerally distributed flying fish, vertical or horizontal competition avoidance may be almost impossible, and thus females and males share their foraging grounds. Since birds were tracked simultaneously, shorter foraging trips of chick-rearing birds must be an effect of the constraints of provisioning the chick. Differences observed in δ15N and δ13C values between sexes may be caused by subtle differences in their foraging behaviors, or by differences in physiology linked to breeding. Our findings suggest that local oceanography and its inherent food distribution are determinants for sexual segregation in foraging patterns in masked boobies and possibly also other booby species.Significance statementIn some animals, females and males forage on different areas or prey on different species to avoid competition for food resources. In boobies (Sula sp.), some studies show evidence of sexual segregation in foraging and others do not. Here, we tested if sexual segregation in foraging occurred in masked boobies on the Pacific island of Rapa Nui by studying simultaneously incubating and chick-rearing birds. We found no evidence of sexual segregation on foraging behavior or diet. We discuss that the difference between this and other studies in boobies may be an effect of the local prey availability. When the prey community is more diverse and heterogeneously distributed, each sex may access different resources and thus sexual foraging segregation will occur. In contrast, in areas like Rapa Nui where prey resources are distributed ephemerally, sexual segregation in foraging will not be useful and is thus less likely to occur.
Highlights
Sexual segregation in foraging is a widespread behavioral and ecological phenomenon among animal taxa (Wearmouth and Sims 2008)
We considered that sexual segregation in foraging would occur during periods of increased demands, whereas sexual segregation would not occur during incubation
Females appeared to forage farther and longer than males, the best model comparing foraging parameters retained breeding stage, but not sex, the interaction between breeding stage and sex, or body mass or size of birds (Tables 2 and 3, Fig. 2). Birds from both sexes traveled 59% farther and 66% longer during incubation than during chick-rearing (Table 2), but there were no significant differences in the areas used between females and males during the incubation and chick-rearing periods (50% utilization distribution (UD) Bhattacharyya coefficient (BA) 0.68–0.99, analysis of similarity (ANOSIM) R = 0.10, P = 0.15; 95% UD BA 0.68– 0.99 ANOSIM R = 0.10, P = 0.14) (Fig. 2)
Summary
Sexual segregation in foraging is a widespread behavioral and ecological phenomenon among animal taxa (Wearmouth and Sims 2008). Females and males differ in terms of their spatio-temporal distribution, at sea behavior, and feeding ecology (González-Solís et al 2000; Catry et al 2005; Forero et al 2005; Phillips et al 2011) Such differences in areas used and prey species consumed by sex have been often seen as a response to alleviate competition for food resources (Selander 1966; Phillips et al 2011). Female and male brown and blue-footed boobies (Sula nebouxii), respectively, had different δ13C and δ15N values, whereas no sexual differences in isotope levels were found in red-footed (Sula sula) or masked boobies (Sula dactylatra; Table 1) These contrasting results in terms of foraging ecology among species and colonies of boobies suggest that sexual segregation in foraging do not occur consistently and might reflect changes in competition levels and local food abundance and availability
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