Establishing growth chronologies from marine mammal teeth: a method applicable across species

Multidecadal datasets are important for investigating the effects of a changing climate on top predators, particularly if short-term variations are to be differentiated from long-term trends. Annual increments (growth layer groups: GLGs) formed in the teeth of marine mammals have the potential to provide multidecadal proxy records or chronologies of energy budgets associated with growth, allowing for the investigation of potential environmental drivers of interannual variability and longer-term changes in growth. To date, methodology universally applicable across marine mammal species for developing such chronologies has not been established. Methodologies developed are often “bespoke” being developed specifically for individual species and datasets. This thereby limits the applicability of such methodologies to other species and regions and introduces difficulties in the replication of methods.By modifying dendrochronology (tree-ring dating) techniques, we provide a method for developing chronologies from GLG widths using sperm whales (Physeter macrocephalus) and long-finned pilot whales (Globicephala melas) as examples. The method firstly utilizes statistical crossdating to identify and correct potential errors in GLG identification ensuring assignment of GLGs to the correct calendar year. Common dendrochronology “detrending” methods were then tested for applicability and the most appropriate applied to remove age-related trends and variability specific to each individual in the example dataset. Finally, individual chronologies comprised of a standardized growth index were calculated and then averaged into a master chronology for each dataset, maximizing common patterns in growth across individuals and reducing noise in the data due to individual variability.The described approach to chronology development provides a number of advantages over others previously used on marine mammals; first, it has been formed on the basis of well-established and tested techniques and second provides a step-by-step process that is readily repeatable, thereby allowing direct comparisons between similarly developed chronologies from different species or regions. Once developed, chronologies can be used in modeling studies and compared with annually resolved climate indices to explore sensitivities in tooth growth and associated energetic budgets to environmental conditions