Foraging segregation in tropical and polar seabirds: Testing the Intersexual Competition Hypothesis

Highlights

• Intersexual Competition Hypothesis (ICH) was tested in tropical and polar seabirds.

• ICH postulate that differences in body size play a role in niche segregation.

• No correlation was found between total dimorphism index and differences in stable isotope values.

• Sexual size dimorphism seems to facilitate niche segregation in non-tropical seabirds.

Abstract

The Intersexual Competition Hypothesis (ICH) predicts that sexual size dimorphism (SSD) in seabirds may reduce intraspecific food competition through the exploitation of different trophic niches by each sex. We tested the ICH using stable isotopes (δ¹⁵N and δ¹³C) from whole blood and muscle from six tropical and five polar seabird species sampled at breeding sites. We expected that greater morphological differences between sexes would be related to larger differences in δ¹⁵N and δ¹³C values, reflecting potential diet and spatial segregation between males and females. We also compared trophic segregation in non-tropical and tropical seabirds to determine if there was more intense feeding competition during the generally shorter breeding season in non-tropical areas, leading to more pronounced segregation mechanisms; alternatively, more abundant food resources during breeding at temperate and polar areas, in contrast to oligotrophic tropical areas, could lead to a relaxing of segregation. No significant differences in δ¹⁵N or δ¹³C were found between sexes in seabird species from tropical or polar regions. In addition, there was no correlation between total dimorphism index and differences in mean δ¹⁵N or δ¹³C values of females and males for each species. Analysis of data from the literature, accounting for phylogeny, indicated that size-dimorphic seabird species from temperate and polar regions tend to show trophic (δ¹⁵N) or spatial (δ¹³C) segregation (71%; 30 out of 42 study cases) more often than tropical dimorphic species (19%; 3 out of 16 study cases). Overall, SSD seems to facilitate trophic or spatial segregation in non-tropical seabirds, but not in tropical species. Further investigations are necessary to confirm the lack of this pattern in tropical seabirds.

Introduction

Sexual size dimorphism (SSD), where one sex is larger than the other, has been reported for many taxa, including seabirds (Fairbairn and Shine, 1993, Fairbairn et al., 2007, Shine, 1989). Many hypotheses explain SSD as the result of sexual selection or niche segregation (Andersson and Norberg, 1981, Krüger, 2005, Shine, 1989). Sexual size dimorphism may also be related to the different parental roles of each sex — larger females can store more energy for reproduction and produce larger broods or chicks, care for more young or better defend territories (Andersson, 1994, Shine, 1989, Weimerskirch et al., 2009). Conversely, male-biased SSD is expected to result from sexual selection, as males' larger size gives them advantages in intra-sexual contests for mates and in attracting females (Andersson, 1994) or territory defense (Nelson, 2005). The Intersexual Competition Hypothesis (ICH) predicts that SSD in birds may reduce intraspecific food competition, as males and females exploit different trophic or dietary niches (Selander, 1966). In addition, this hypothesis predicts that differences in morphology (e.g., body size) results in the segregation of males and females in relation to diet composition (Awkerman et al., 2007, Forero et al., 2002, Forero et al., 2005, González-Solís and Croxall, 2005, Selander, 1966). Such segregation may also be spatial, where males and females exploit different foraging areas or depths (Cook et al., 2013, González-Solís and Croxall, 2005, Weimerskirch et al., 1993, Weimerskirch et al., 2006) or at different time of the day (Paredes et al., 2008). The ICH can be tested using colonial nesting seabirds where there is a high density of individuals competing for limited prey resources, and they are constrained by their nest-bound chick to central-place foraging, which limits adult foraging behavior (Ainley et al., 2004, Forero et al., 2002). Seabirds represent a variety of points along the SSD continuum and show varying degrees of dietary segregation (González-Solís et al., 2000, Stauss et al., 2012, Weimerskirch et al., 2009). Furthermore, Phillips et al. (2011) hypothesized that the extent of sexual segregation in foraging ecology was a function of sexual size dimorphism, which varies considerably among species.

Trophic niche segregation is often assessed using differences in diet, and it is the most frequently studied component of ecological niche segregation, but there are limitations associated with conventional dietary analysis (Barrett et al., 2007, Bearhop et al., 2004). Traditionally, quantifying trophic segregation has been based on analysis of stomach contents, pellets or regurgitations (Barrett et al., 2007, Duffy and Jackson, 1986, Karnovsky et al., 2012). However, these methods reflect only the most recent food consumed, and prey species differ in their digestion rates, which affects estimates of diet composition. Attached position-logging devices have increased our ability to track birds and quantify their spatial niche (Wilson and Vandenabeele, 2012, Wilson et al., 2002), but such methods are usually expensive and are not feasible for tracking movements of smaller species (Hobson, 1999, Wikelski et al., 2007). Stable-nitrogen and carbon isotopes have been applied extensively in the past two decades to understand trophic niche segregation (Bearhop et al., 2002, Hobson, 2011, Hobson et al., 1994). Isotope values in seabird tissue are derived from the values of their prey and may indicate the origin and type of prey consumed (Hobson and Clark, 1992, Kelly, 2000). The stable-carbon isotope ratio (¹³C/¹²C, expressed as δ¹³C) is higher in inshore as compared to offshore feeding animals (Cherel et al., 2008, Hobson and Welch, 1992, Hobson et al., 1994), making it a useful tool for the study of spatial segregation in foraging areas. On the other hand, stable-nitrogen isotope values (¹⁵N/¹⁴N, or δ¹⁵N) increase with trophic position in a stepwise manner (Post, 2002). Using stable isotopes to investigate trophic niches is facilitated by the large variety of isotopic values within and among food webs (Newsome et al., 2007). The isotopic niche can be represented as an area in 2-dimensional space with isotopic values as coordinates, where axes represent relative proportions of isotopically distinct resources incorporated into an animal's tissues (Bearhop et al., 2004, Newsome et al., 2007).

Tropical seabirds live in an oceanic environment with lower productivity and less seasonality where prey are distributed more erratically than in temperate and polar oceans (Ballance et al., 1997, Longhurst and Pauly, 1987). Most tropical seabirds must travel extensively to their feeding ground, where they feed mainly on fish and squid within the first few meters of the water column, by aerial feeding and plunge diving (Ashmole and Ashmole, 1967, Diamond, 1983, Harrison et al., 1983, Nelson, 2005, Weimerskirch et al., 2006). Non-tropical (i.e., polar and temperate) seabirds include more species that can exploit the ocean through pursuit dives, including planktivores, piscivores, squid-eaters and apex predator-scavengers (Ainley and Boekelheide, 1983, Hobson et al., 1994, Ridoux, 1994, Sydeman et al., 1997). However, tropical seabirds feed on a broad variety of prey and frequently diet diversity is much greater than for non-tropical species (Harrison et al., 1983). These differences in seabird foraging ecology are related to the differences in productivity of the world's oceans (Ballance and Pitman, 1999). Examining the ICH in seabirds from tropical and polar areas can be used for both seabird management and fisheries management.

In this study we tested the ICH using 11 seabird species from tropical and Arctic breeding sites and hypothesized that size-dimorphic species would show greater trophic segregation than monomorphic species. Specifically, we expected that greater morphological differences between sexes would be related to greater differences in tissue δ¹⁵N and δ¹³C values, reflecting trophic segregation between males and females. We also compared trophic segregation between tropical and non-tropical seabirds to determine if differences in trophic segregation were potentially due to more intense feeding competition during the shorter breeding season in non-tropical areas, leading to more pronounced segregation mechanisms; alternatively, more abundant food resources during breeding in non-tropical areas, in contrast to oligotrophic tropical areas, could lead to a relaxing of segregation. In order to study the intraspecific trophic segregation in tropical and non-tropical seabirds, our objectives were: 1. to classify the degree of dimorphism between sexes using a dimorphism index, based on morphological characteristics; 2. to identify trophic and spatial segregation between males and females of each species using stable isotope analysis; and 3. to perform a literature review to determine if dimorphism and trophic segregation were more common in non-tropical than in tropical seabirds.