Harp seals (Phoca groenlandicus), once present in the Baltic Sea, now stand extinct. During the Middle Neolithic period, they held significant dietary importance for the Pitted Ware Culture hunter-fisher-gatherers in Scandinavia. Because they are no longer available for ecological studies, little is known about their specific behavioural intricacies, such as diet, mobility, and reproductive strategies. Because of the seal's pivotal role in these human societies and because they provide an interesting ecological case study on post-isolation ecological adaptation to new environmental conditions, a comprehensive investigation into Baltic Sea harp seal behaviour is warranted.

In this pursuit, we employ sequential analysis of strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) in harp seal teeth sourced from archaeological contexts across three study regions. This data is harmonized with osteometric analyses from fifteen archaeological sites from the Baltic Sea area, establishing a methodological framework for comprehending the Sr isotopic pathway in a brackish water inland sea. By adopting this approach, we unveil breeding and mobility patterns of the long-extinct Baltic Sea harp seals, approximate specific breeding areas connected to each group of seals and delve into the ontogenic temporal frame governing these observed patterns. This endeavour culminates in an enriched understanding of the adaptive behaviours exhibited by Baltic Sea harp seals and equips us with the insights necessary to decipher the lifestyle of the human societies intricately intertwined with them.