Abstract
In pelagic species inhabiting large oceans, genetic differentiation tends to be mild and populations devoid of structure. However, large cetaceans have provided many examples of structuring. Here we investigate whether the sperm whale, a pelagic species with large population sizes and reputedly highly mobile, shows indication of structuring in the eastern North Atlantic, an ocean basin in which a single population is believed to occur. To do so, we examined stable isotope values in sequential growth layer groups of teeth from individuals sampled in Denmark and NW Spain. In each layer we measured oxygen- isotope ratios (δ18O) in the inorganic component (hydroxyapatite), and nitrogen and carbon isotope ratios (δ15N: δ13C) in the organic component (primarily collagenous). We found significant differences between Denmark and NW Spain in δ15N and δ18O values in the layer deposited at age 3, considered to be the one best representing the baseline of the breeding ground, in δ15N, δ13C and δ18O values in the period up to age 20, and in the ontogenetic variation of δ15N and δ18O values. These differences evidence that diet composition, use of habitat and/or migratory destinations are dissimilar between whales from the two regions and suggest that the North Atlantic population of sperm whales is more structured than traditionally accepted.
Highlights
In the main oceans of the world, the absence of geographical barriers permits pelagic species to occupy vast geographic ranges inside which they often engage in long migratory movements
To exclude the effect of potential variation in isotopic baselines with time, we examined the relationship between δ15N values and the year of formation of the 3rd year dentinal growth layer group (GLG) in sperm whales from both regions and found absence of correlation (Figure 3a, Denmark: Pearson's r=-0.27, p=0.73; Spain: Pearson's r=-0.50, p=0.39)
Trends of δ13C with age for each individual are depicted in Figures 4c and 4d, while AIC and p values resulting from the application of the general linear model (GLM) and general additive model (GAM) models to each individual set of data are shown in Table 3. δ13C showed a significant negative linear correlation with age in two (H and I) out of the four males from Denmark, while none of the other males from Denmark showed any significant trend (F and G) (Figure 4d)
Summary
In the main oceans of the world, the absence of geographical barriers permits pelagic species to occupy vast geographic ranges inside which they often engage in long migratory movements This result in high population sizes which, together with the high mobility of many of the species, favours mild genetic differentiation and consequent absence of structuring [1,2]. When reaching an age between 4 and 21 yr males leave the breeding units to join the bachelor schools, structures which are formed by males in their teens and twenties that are not considered to be socially mature. From their mid-twenties, males may start migrating from low to high latitudes. The frequency and duration of the large-scale latitudinal migrations a well as the extent of the geographical segregation are still poorly understood
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