Stable carbon and nitrogen isotope ratio profiling of sperm whale teeth reveals ontogenetic movements and trophic ecology

Teeth from male sperm whales (Physeter macrocephalus) stranded in the North-eastern Atlantic were used to determine whether chronological profiles of stable isotope ratios of C (δ¹³C) and N (δ¹⁵N) across dentine growth layers could be used to detect known ontogenetic benchmarks in movements and trophic ecology. Profiles showed a general decrease in δ¹³C (median = 1.91‰) and an increase in δ¹⁵N (median = 2.42‰) with age. A marked decline in δ¹³C occurred for all 11 teeth around 9–10 years and again for six individuals around 20 years. After the early twenties the δ¹³C continued to decline with age for all teeth. These results are consistent with males segregating from natal groups in low latitudes with the onset of puberty between 4 and 15 years and gradually dispersing pole-ward into ¹³C-depleted temperate waters. Penetration into further depleted, productive high latitudes after the age of 20 might facilitate the spurt of accelerated growth rate observed around this age. Breeding migrations back to lower latitudes were not reflected in the δ¹³C profiles possibly due to being short compared to the time spent feeding in high latitudes. The timings of marked isotopic change in the δ¹⁵N profiles reflect those of the δ¹³C profiles, suggesting a link between dietary changes and movements. The observed increase in δ¹⁵N with age is likely to be caused by a trophic level increase as males grow in size, probably feeding on larger prey. An additional explanation could be that, in the higher latitudes of the North Atlantic, the main prey source is the high trophic level squid Gonatus fabricii. Also, the lower latitudes from where males disperse are depleted in basal ¹⁵N. Profiles of δ¹³C and δ¹⁵N in sperm whale teeth gathered from different regions, sexes, and periods in time, could provide a unique way to understand the ecology of this species across different oceans.