ẟ15N and ẟ13C cycles in narwhal (Monodon monoceros) embedded teeth reveal seasonal variation in ecology and/or physiology.

All narwhals were harvested near Pond Inlet, Nunavut, Canada, as a part of subsistence hunts in 1982 (male n = 5, female n = 5), 1983 (female n = 3), and 2017 (male n = 4) during summer (June–August), and in a humane harvest during an ice entrapment in December 2015 (male n = 1, female n = 9). Narwhal embedded canines were collected by Inuit and provided to Fisheries and Oceans Canada through a voluntary collaborative sampling program with the Pond Inlet Hunters and Trappers Association. The embedded canines were subsequently processed at DFO in Winnipeg for δ15N and δ13C analyses. Specifically, embedded teeth were longitudinally bisected, and the half exposing a greater portion of the midline was sanded and acid etched to reveal GLGs. Prenatal dentine in embedded teeth is well preserved and separated from postnatal dentine by a thin dark neonatal line. A dentine GLG in a narwhal embedded tooth consists of a broad and a narrow layer with several thin bands in each layer, which were referred to as accessory layers. Narwhal age was estimated from counts of dentine GLGs assuming they are deposited annually. However, dentine GLGs in narwhal embedded teeth become compressed and eventually occluded by cementum (often at GLG 8–16), which makes estimating absolute age and calendar year of formation impossible for older whales. Dentine was collected from individual accessory layers of longitudinally sectioned teeth using a micromill fitted with a 300 μm drill bit. The accessory layers were micromilled up to GLG 11, beyond which compressed growth prevented discrete sampling of accessory layers. A total of 839 dentine accessory layer samples were analyzed at Fisheries and Oceans Canada, Winnipeg, MB, using an Elemental Combustion System CHNS-O (ECS 4010, Costech Analytical Technologies Inc., USA) connected to a DELTA-V mass spectrometer (ThermoFisher Scientific, Bremen, Germany). Isotope values are reported as heavy to light isotopic ratios relative to international standards.