Isotopic (δ¹⁵N and δ¹³C) profiles in dentine indicate sex differences and individual variability in resource use among narwhals (Monodon monoceros)

Abstract

While the impact of ecological changes on Arctic marine mammals remains largely unknown, quantifying variability in resource use among conspecifics can serve as an indication of adaptive plasticity and help assess species vulnerability to climate-induced changes. We measured withintooth δ 15 N and δ 13 C profiles of narwhals (Monodon monoceros) sampled 35 years apart from Baffin Bay, Nunavut, Canada, to evaluate intrapopulation variation in narwhal resource use. Narwhal dentine δ 15 N and δ 13 C were lower post-1983 (n = 7) by 0.83‰ and 0.52‰, respectively, compared to pre-1983 (n = 13), but isotopic niche breadth increased from 0.95‰ 2 to 1.12‰ 2 . Between sexes, the isotopic niche of males expanded from pre-1983 to post-1983 while that of females decreased, which may indicate increased flexibility in resource use for males and the opposite for females. Additional isotopic variation among narwhals (n = 23) suggests individual differences in diet composition and/or foraging location during both periods.Individual isotopic niches ranged between 0.08‰ 2 and 2.27‰ 2 , where a larger isotopic niche likely indicates more variable foraging strategies over time. Narwhal as a species is considered vulnerable to ecosystem change; however, the variability in resource use between sexes and among

Insights on long-term ecosystem changes from stable isotopes in historical squid beaks

“…However, all of these are long-living taxa, in which it can be difficult to disentangle ontogenetic signals from changes in the ecosystems, since the respective isotopic signal changes over time [ 33 , 35 ]. In general, most of the long-term bulk SIA studies of marine mammals from the Arctic and North Atlantic published to date do not account for the Suess effect and baseline variation [ 22 , 31 , 34 , 37 , 39 – 41 ]. The use of compound-specific SIA (CSIA) does not require baseline corrections, since it uses differences in isotopic signatures of trophic and source amino acids to overcome baseline-related issues [ 42 ].…”

“…However, challenges of this method include unknown taxon-specific trophic discrimination factor and β values for many taxa, as well as the requirement for additional data collection for fingerprinting of δ 13 C and δ 15 N [ 42 ]. Overall, both bulk and CSIA retrospective studies on Arctic and North Atlantic marine mammals present either that δ 13 C and δ 15 N increase, decrease or stay the same of over time in the different areas and across species [ 22 , 31 , 34 , 36 , 37 , 39 – 41 , 43 , 44 ] (Tables S1 & S2 ). This suggests that top predators’ trophic ecologies respond to ecosystem changes in diverse and inconsistent ways.…”