Ecological niche theory predicts the coexistence of closely related species is promoted by resource partitioning and leads to the use of different ecological niches. Australian snubfin (Orcaella heinsohni) and humpback (Sousa sahulensis) dolphins live in sympatry throughout most of their range in northern Australia. We compared stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) in their skin to investigate resource partitioning between these ecologically similar species. Skin samples were collected from live Australian snubfin (n = 31) and humpback dolphins (n = 23) along the east coast of Queensland in 2014-2015. Both species had similar δ¹³C and δ¹⁵N values and high (>50%) isotopic niche space overlap, suggesting that they feed at similar trophic levels, have substantial dietary overlap, and both rely on similar basal food resources. Despite similarities, snubfin dolphins were more likely to have a larger δ¹⁵N than humpback dolphins, indicating they may forage on a wider diversity of prey. Humpback dolphins were more likely to have a larger δ¹³C range suggesting they may forage on a wider range of habitats. Overall results suggest that subtle differences in habitat use, and prey selection are likely the principal resource partitioning mechanisms enabling co-existence of Australian snubfin and humpback dolphins.

Isotopic ratios were transformed into parts per thousand (‰) using delta notation (δ):

where δX is δ¹³C or δ¹⁵N, R sample is the ratio of stable isotopes in the sample and R standard is the ratio of stable isotopes in the standard reference materials (atmospheric nitrogen gas and carbon from Pee Dee Belemnite, a limestone from South Carolina).

Statistical analysis

We tested δ15N and δ13C data for each species for homogeneity of variance (non-parametric Levene’s test) and normality (Shapiro-Wilks Test). Tests revealed homogeneity of variance for species, but assumptions of normality were not met for δ¹³C for humpback dolphins (P = 0.04). We, therefore, used a one-sided randomisation test with 10,000 permutations at 0.05 significance level to investigate differences in isotopic values between species. This test compares the difference of the mean δ¹⁵N and δ¹³C per species with the difference obtained by randomly allocating the observed isotopic values among the two species (Manly 2018).

We used six metrics proposed by Layman et al. (2007a) to compare the isotopic niches of Australian snubfin and humpback dolphins:

1. δ¹⁵N range, which is the difference between the highest and lowest δ15N values of each species. δ15N range provides information on the range of trophic levels at which each species has been feeding.

2. δ¹³C range is a measure of the difference between the highest and lowest δ13C values of each species. δ13C range provides an estimate of the variability of trophic sources of each species.

3. Total area (TA), which is a measure of the total amount of niche space occupied by a species in ‰2. TA was calculated from a convex hull drawn around the most extreme data points on an isotope δ¹³C–δ¹⁵N bi-plot. As TA is sensitive to differences in sample size because the area can only increase as new data points are added; we used the corrected version of the standard ellipse area (SEAC) as a measure of the mean core area (40%) of each species isotopic niche (A. L. Jackson, Inger, Parnell, & Bearhop, 2011).

4. Mean distance to centroid (CD) is the mean Euclidean distance of each individual of a population to the δ15N–δ13C centroid, where the centroid is the mean δ¹⁵N–δ¹³C value for all species in the food web. CD provides an estimate of overall dietary diversity.

5. Mean nearest neighbour distance (MNND) is the average nearest-neighbour euclidean distance between an isotopic coordinate relative to all other coordinates within a species. MNND provides an estimate of species packing and shows how similar or dissimilar the members of a population are to one another.

6. Standard deviation of nearest neighbour distance (SDNND). A measure of the evenness of spatial density and packing of individuals. Low SDNND values indicate a more even distribution of trophic niches.

We bootstrapped all Layman metrics with replacement (n = 10,000, indicated with a subscript ‘_boot’) based on the smallest sample size in the data set (n = 23) to enable statistical comparison between dolphin species (M. C. Jackson et al., 2012; Manly, 2007). To further assess niche widths and isotopic niche overlap between species, we followed a Bayesian approach using multivariate ellipse-based metrics (A. L. Jackson et al., 2011). This method is particularly useful when comparing groups with small sample sizes, as it corrects for the influence of outliers. We calculated standard ellipse areas (SEAs), which are the bivariate equivalent to standard deviation in univariate analyses. We also calculated SEA corrected (SEA_C) to minimize bias introduced by small sample sizes. In addition, we calculated SEAB (Bayesian SEA) using 1000 posterior draws to statistically compare niche width between species. We used SEA_B to calculate the niche overlap between Australian snubfin and humpback dolphins, calculated as the proportion of the total SEAB for each species respectively. We calculated all metrics using the R package SIBER (Stable Isotope Bayesian Ellipses in R (A. L. Jackson et al., 2011; R Core Team, 2021).