Calf-Initiated Behaviors and Their Influence on Group Dynamics of Common Dolphins (Delphinus delphis) along the South Coast of Portugal

Antony Pieters, Francisco O. Borges, Alicia Quirin , André Cid, Heidi C. Pearson, Joana Castro

Abstract of the oral presentation

Antony Pieters ^(1,2), Francisco O. Borges ^(1,3,4), Alicia Quirin ⁽¹⁾, André Cid ⁽¹⁾, Heidi C. Pearson ⁽⁵⁾, Joana Castro ^(1,3)

⁽¹⁾ AIMM – Associação para a Investigação do Meio Marinho, Rua Maestro Fred. Freitas N 15-1, 1500-399 Lisboa, Portugal

⁽²⁾ IMBRSea – Marine Biology Research Group, Ghent University, Krijgslaan 281, S8, Ghent, B-9000, Belgium

⁽³⁾ MARE – Marine and Environmental Sciences Centre/ARNET – Aquatic Research Network, Faculdade de Ciências da Universidade de Lisboa, 1749-016 Lisboa,  Portugal

⁽⁴⁾ Department of Biology, Faculdade de Ciências da Universidade de Lisboa, 1749-016 Lisboa, Portugal

⁽⁵⁾ University of Alaska Southeast, Juneau, Alaska, United States of America

Cetacean nursery groups play a critical role in the successful rearing of calves, offering social learning and integration into complex social networks. While these groups are well-documented in species with stable, matrilineal social structures, little is known about how calf social development is supported in species with more fluid social dynamics. To answer this question, we investigated common dolphin (Delphinus delphis) calf rearing groups along Portugal’s southern coast, a known nursing and calving area for this species. Here, we present a novel calf-centered approach to examine how calf-initiated behavior (CIB) might influence the group’s natural behavior. CIBs provide a clear, observable point in time from which to track potential subsequent responses to allow us to better understand the mechanisms underlying social responsiveness and group-level coordination. Systematic behavioral logging of aerial observations obtained using unmanned aerial vehicles was employed to analyze shifts within the group’s dynamic in response to behavioral initiations by calves versus adults. In total, 290 thirty-second observation intervals from 20 focal follows were analyzed using Generalized Additive Mixed Models. Our models’ results indicated that although calves were significantly less likely to initiate directional changes than adults, maternal responsiveness increased with these directional misalignments by their calf. Besides mothers, non-mother individuals within the group responded significantly more often to calf-initiated behaviors than to those initiated by adult dolphins, suggesting a form of alloparental alertness. Additionally, this responsiveness increased as the proportion of calves in the group increased, indicating threshold-based social response. Our findings provide insight into adaptive social strategies showing that calves are not just passive recipients of care but can actively influence group dynamics. This study also offers a foundation for future efforts to establish thresholds for defining nursery groups in common dolphins.