Unveiling the enigmatic natural history of Mediterranean Common Dolphin (Delphinus delphis Linnaeus, 1758): Ecological and evolutionary insights for conservation challenges

The Mediterranean population of common dolphin (Delphinus delphis) presents an enigmatic natural history. Once widely prevalent, it faced a significant decline across most of the basin by the late 1960s (Bearzi et al., 2003). The precise reasons and mechanisms behind this decline remain largely unknown or poorly understood. However, a range of potential causes has been identified, encompassing deliberate culling, accidental deaths in fishing gear, reduced prey availability, and habitat degradation (Bearzi et al., 2008; Bearzi, Holcer, & Notarbartolo di Sciara, 2004; Piroddi et al., 2011). Present estimates derived from surveys and monitoring campaigns suggest either a complete absence or a near disappearance of common dolphins from areas historically known to harbor substantial populations. The International Union for Conservation of Nature has designated the Mediterranean population of common dolphins as Endangered (Bearzi et al., 2021). We reconstructed the little-known natural history of this population using Mediterranean museum skeletal specimens from the last two centuries. A multidisciplinary analytical approach, consisting of genetic analysis on ancient DNA, skull geometric morphometry, and collagen stable isotope analysis, in addition to data from strandings and historical documentation, has enabled a better understanding of the ecology of the historical Mediterranean common dolphin population. The main results are: i) the incorrect taxonomic attributions in museum collections and historical literature represent a low but significant bias in the identification of Mediterranean dolphin species and in the knowledge of their demographic and ecological history over the past decades and centuries; ii) 10% of museum specimens show a discrepancy between morphology and genetics and these could potentially represent evidence of interspecific hybridization events between the Delphinus delphis and other species more common in the basin, such as Stenella coeruleoalba and Tursiops truncatus; iii) the Mediterranean common dolphin population shows significant spatial heterogeneity in the ecological, trophic and distribution patterns.