We estimate that loggerhead scute samples may contain up to 12 years of resource use history, providing a lengthy record from which to investigate patterns in a long-lived species. To our knowledge, our study reports the longest record of resource use history obtained from living individuals.
Comparison of long-term scute records () with isotopic scenarios presented in reveals that this generalist population is composed of individual specialists. Although all of these loggerheads were sampled at the same nesting beach and an entire ocean basin is potentially available to the population, individuals use only a limited fraction of the available isotopic niche space (figure S2 in the electronic supplementary material).
In our study, specialization is not limited to a diet consisting of a single prey item, but the observed specialization results from a consistent mixture of prey, habitat and geographical location, which we are unable to separate with our sampling regime. Consumption of a prey mixture is likely, as individual loggerhead stomach contents often contain several prey species (Bjorndal 1997
). While some of the variation among individuals may be owing to individual variation in isotopic discrimination or physiology rather than differences in foraging (Barnes et al. 2008
), it is unlikely that this would result in the wide isotopic range observed.
The large population range in δ15
N values (9.0‰) could be indicative of a population that is feeding over several trophic levels if the baseline nitrogen is stable in all of the foraging locations of these individuals (Post 2002
). However, if baseline nitrogen signatures change with foraging location, isotopic differences will be more reflective of habitat or location than of trophic level feeding differences because the same prey species will have different isotope signatures among these areas. We believe locational differences are more likely than trophic level differences, as the similarly large range of δ13
C values (10.5‰) indicates that loggerheads have geographically separated foraging areas and/or are incorporated in food webs with enriched or depleted δ13
The gap in δ13
C values between −12.5‰ and −14.5‰ (b
) represents the division between two foraging groups identified by Reich et al. (2010)
. The groups represent two general habitat use patterns that could result from food webs with different δ13
C baselines owing to an isotopic gradient (e.g. oceanic/neritic, pelagic/benthic, latitudinal). Only one turtle crossed between groups, indicating that individuals have high fidelity to foraging sites and/or habitat type. This foraging fidelity is consistent with the observations of eight adult female loggerheads tracked from North Carolina, USA; two different movement types were observed, but all individuals exhibited interannual fidelity to discrete foraging sites (Hawkes et al. 2007
Intrapopulation variation in resource use can have ecological, evolutionary and conservation consequences. Resource use heterogeneity, indicated by the broad population isotopic niche width and narrow individual niche widths, reduces intraspecific competition and may alter selective pressures (Bolnick et al. 2003
). Reduction in intraspecific competition appears to be substantial in adult loggerheads, given the small proportion of variance in our study attributed to within-individual variation (less than 7%, ). In comparison, a recent study of diet specialization in sea otters, based on vibrissae isotope signatures, estimated that 28 per cent of the variance was attributed to within-individual variation (Newsome et al. 2009
Examining the degree of individual specialization within a population provides a better understanding of its ecology, behaviour and population dynamics. Our approach to resource use has broad application for species that possess consistently growing, inert tissues that can be serially sampled. Because diet and habitat are confounded in this study, loggerheads should be sampled at a series of foraging grounds to distinguish the effects of diet, habitat and geographical location on isotopic signatures.