Date Published: April 16, 2019
Publisher: Public Library of Science
Author(s): Rocío Espada, Liliana Olaya-Ponzone, Luisa Haasova, Estefanía Martín, José C. García-Gómez, Cheryl S. Rosenfeld.
A case of intergeneric hybridization in the wild between a female bottlenose dolphin (Tursiops truncatus) and a short-beaked common dolphin (Delphinus delphis), considered members of ‘vulnerable’ and ‘endangered’ subpopulations in the Mediterranean, respectively, by the International Union of Conservation of Nature is described in this paper. The birth of the hybrid was registered in the Bay of Algeciras (southern Spain) in August 2016, and the animal has been tracked on frequent trips aboard dolphin-watching platforms. This unique occurrence is the result of an apparent ongoing interaction (10 years) between a female bottlenose dolphin and common dolphins. The calf has a robust body with length similar to Tursiops, while its lateral striping and coloration are typical of Delphinus. It displays the common dolphin’s ‘criss-cross’ pattern. However, the thoracic patch is lighter than in D. delphis and its dorsal area is light grey, with a ‘V’ shape under the dorsal fin. This paper also provides a comprehensive mini-review of hybridizations of T. truncatus with other species.
The Bay of Algeciras, located in the south of Spain (Fig 1), hosts an important population of common dolphins (Dephinus delphis) which, since 2003, are considered ‘Endangered’ in the Mediterranean Sea according to the Red List criteria by the International Union of Conservation of Nature (IUCN)  and also ‘Vulnerable’ according to the Spanish National Catalogue of Endangered Species . This area has been considered a feeding and breeding ground for this species [3, 4]. Also, it is possible to observe, more sporadically, groups of bottlenose dolphin (Tursiops truncatus), a species which is also considered as a ‘Vulnerable’ Mediterranean subpopulation by the IUCN. Striped dolphins (Stenella coeruleoalba) are occasionally detected (‘Vulnerable’ in the Mediterranean by IUCN) mixing with common dolphin, but the groups are mainly formed by mothers, calves and immature juveniles.
The study was carried out on board opportunistic dolphin-watching platforms (14 m and 12 m in length), offering trips of 90 min with consistent daily itineraries from August 2016 until May 2017. Once the group of interest (the one including the mother/hybrid pair) was detected, standardized data were gathered (weather permitting), such as date, time, GPS position, structure and group size. Group composition data were collected using a combination of sampling methods. Individual-following protocols [37, 38], focusing on the mother/hybrid pair were applied during the sightings. The 10 m chain rule  was also applied: the pair were determined to be together if they were less than 10 m apart. Swimming positions, general behaviour and body-contact events were also gathered when they were displayed, irrespective of the time. Sea surface temperature was measured from the side of the boat, using a digital thermometer with 0.1°C graduations . The mother/hybrid pair and other dolphins were photographed (Nikon DSLR camera, Nikon 70–300 mm lens) for re-identification and also for morphological analyses of the presumed hybrid . In some cases, images were slightly retouched (descriptors: saturation, contrast, exposition, clearness and shades), with Adobe Photoshop Lightroom software, to improve the display of the morphological features described in the text. A plotter map was elaborated using ArcGIS 10.4 software, including coordinates of the mother/hybrid pair.
After the first sighting on 11 August 2016, re-sighting took place on 17 August 2016, after which they were seen on an almost daily basis mixing with ‘nursery groups’ of common dolphins (D. delphis) (Fig 3A and 3B). Data were collected between 17 August 2016 and 4 June 2017. The pair was observed 113 times (57 h 11 min of observation) in a total of 355 sightings. Of these, 104 times (53 h 55 min) the pair was found within nursery groups of common dolphins formed by females and calves , twice (1 h 23 min) in mixed nursery groups of common dolphins accompanied by mothers, calves and immature juveniles of striped dolphins (S. coeruleoalba) and in only six sightings (1 h 53 min) was the pair sighted alone, distanced from the common dolphins (minimum 500 m between groups). The pair were detected together less than 10 m apart in 112 sightings (99.1%); 1 occasion (0.83%) was Billie (female T. truncatus) separated from the hybrid by 100 m, both of them accompanied by common dolphins. Sea surface temperature (SST) during these observations was an average of 19.35°C (66.83°F) with minimum of 14°C (57.2°F) and maximum of 26°C (78.8°F). From 2 June 2017 until the end of the year, the hybrid was not sighted again, leading to the reasonable suspicion of death.
There is little information about hybrids in the wild; therefore, the significance of this potential hybridization is two-fold. First, this event between these species in the wild supports what has been observed in the non-natural conditions of captivity. Second, species such as T. truncatus and D. delphis, with spatially overlapping habitats , have rarely been recognised as interbreeding until now. Moreover, this type of intergeneric interaction occurs at a low level, as although the habitats of the two species described overlap, they rarely mix.