Table 4. Results from linear mixed models on the effects of habitat (coastal or inland) and age of animal (adult or juvenile) on three attributes of individual variation in δ¹³C and δ¹⁵N.

Fixed effect	DF	F	P
Within individual difference between collagen and muscle
Habitat	1, 35	4.12	0.050
Age	1, 35	3.95	0.054
Habitat×Age	1, 4	1.21	0.334
Euclidean distances to group centroids in difference between collagen and muscle
Habitat	1, 35	12.1	0.001
Age	1, 35	12.9	0.001
Habitat×Age	1, 4	2.24	0.209
Individual specialization index
Habitat	1, 35	10.25	0.003
Age	1, 35	39.6	<0.001
Habitat×Age	1, 4	11.2	0.029

Within individual isotope niche breadth was estimated as the Euclidean distance between collagen and muscle within individuals, between individual variation in individual isotope niche breadth was calculated as the Euclidean distance of each individual difference between muscle and collagen to group centroids in a 2 dimensional isotope space, and an individual specialization index that relates intra individual variation to the total isotope niche breadth of each sample group, calculated as the ratio of the average Euclidean distances of muscle and collagen samples to within individual centroids and the average Euclidean distances to group centroids. Groups were in all cases defined as age classes within each habitat.