Introduction

Marine mammals around the world have distinctive coloration patterns that vary with species, age, sex, and geographic region, and even individually (^{Perrin 2009}). The skin color in cetaceans is determined by melanin, which affords the dark color to the animals (^{Fertl et al. 2004}, ^{Koper et al. 2017}). Absence of the enzyme that produces melanin causes albinism and thereby the absence of color in the skin, eyes, and hair (^{Fertl and Rosel 2009}, ^{Hauser-Davis et al. 2020}). Another phenotype with predominantly white coloration or patches is leucism, which results from defective pigment cell differentiation, with continuous melanin production; in this case, the skin and hair can lack pigmentation, and color is retained in the eyes or as patches on the skin (^{dos Santos et al. 2016}, ^{Koper et al. 2017}). Pigmentation deficiencies have been reported for at least 25 cetacean species (^{Koper et al. 2017}), and leucism has been documented in at least 14 dolphin species (^{Hauser-Davis et al. 2020}).

Leucistic Risso’s dolphins (Grampus griseus) have occasionally been reported, and to our knowledge, published information reports only 3 alleged leucistic dolphins that were captured and transported to aquarium facilities in the Pacific (^{Funasaka et al. 2017}). Risso’s dolphins are distributed worldwide in tropical and temperate waters, occurring in multiple areas in the North Atlantic, with a resident population in the Azores (^{Pereira 2008}). Here, we present the first report of 3 leucistic Risso’s dolphins sighted in the North Atlantic.

Materials and methods

Leucistic Risso’s dolphins were sighted on 6 separate occasions north of Faial Island, Azores Archipelago (Portugal). Two of the sightings were recorded on 12 September 2020 (individuals A and B; Fig. 1) and the other 5 (individual C) on 12 June and 21 August 2021, 24 and 28 July 2022, and 3 June 2023 (38°38.717ʹ N, 28°30.379ʹ W; 38°38.748ʹ N, 28°33.040ʹ W; 38°36.975ʹ N, 28°33.104ʹ W; 38°40.183ʹ N, 28°40.726ʹ W; 38°36.51030' N, 28°36.42432ʹ W). The GPS location for the sighting of 21 August 2021 could not be obtained. Sightings were made from an 8.5-m Zodiac inflatable boat during the whale and bird watching expeditions organized by Naturalist - Science and Tourism. Data was collected opportunistically, with several photographed adult individuals but no extensive photo-identification of the group. The matching process was done visually with no Photo-ID software. Individuals were recognized by the presence of nicks, scars, notches, shape of the fin, or specific pigmentation patterns (^{Tyson et al. 2011}, ^{Sprogis et al. 2016}).

Figure 1. Photographs of 3 Risso’s dolphins (Grampus griseus) (individuals A, B, and C) with atypical pigmentation sighted north of Faial Island, Azores archipelago. Sighting event on 12 September 2020: (a) individual A, Risso’s dolphin calf with atypical pigmentation next to an adult with typical pigmentation; (b) individual B, Risso’s dolphin juvenile/subadult with atypical white body and pink coloration on the dorsal fin. Sighting event on 12 June 2021: (c-e) individual C, Martins, showing full body and fins with pale white color. Sighting event on 21 August 2021: (f-g) individual C as seen from the right and left sides; in (f) it is shown in company of the same adult that was seen throughout the years with typical body pigmentation. (h-i) Individual C photographed on 24 and 28 July 2022, respectively, accompanied by an adult. Sighting on 3 June 2023: (j-k) individual C; in (j) it is shown accompanied by a juvenile with typical coloration, and in (k) it is shown followed by the same adult as in previous years. (l) Demonstrative photograph of a Risso’s dolphin calf from another group/sighting showing the typical pigmentation of calves. Photograph in (b) was taken by Olivier Coucelos. 

Notes on social ecology and behavior and photographic imagery were collected over periods of no longer than 15 min per sighting event because of tourism activities and regulations. Photographic imagery was collected using a Canon 77D digital camera with an 18-135 mm zoom lens. Photographs were kept in the Atlantic Naturalist Database in Horta, Faial Island.

Results

Two Risso’s dolphins, individuals A (Fig. 1a) and B (Fig. 1b), were sighted separately on 12 September 2020 in a group of 100 to 120 individuals. The group was tracked throughout the day by a spotter on land, and the dolphins were sighted on 2 different occasions (11:20 AM and 2:50 PM) at a distance of ~6 km northeast of Faial Island. Individual A (Fig. 1a) was a calf with atypical full pale white body coloration and dark vertical neonatal folds; it was sighted in the morning swimming along with an adult throughout the entire sighting event. Individual B (Fig. 1b) was the size of a juvenile or subadult (^{Hartman et al. 2013}) and almost entirely white in color, with dark eyes, shades of light gray on the dorsal area posterior to the head, pink coloration on the dorsal fin, and dark coloration along the posterior edge and tip of the dorsal fin; it was sighted in the afternoon (Fig. 1b) transiting slowly within a subgroup of 5 other similarly sized individuals that were highly scared. It was not possible to confirm the sex of the dolphins in either event.

A second Risso’s dolphin calf (individual C; Fig. 1e-ff) was sighted on 12 June 2021 in a group of 30 to 50 individuals. This calf showed atypical coloration, with a pale, white body with some light gray marks along the edge of the dorsal fin and on the top of the head, dark vertical lines on the side (neonatal folds), and dark eyes. During this sighting event, it spent most of the time close to an adult with typical body pigmentation. The same calf was resighted in the afternoon along with the accompanying adult. Given the good weather conditions on that day, it was possible to observe the calf nursing.

Individual C was resighted (Fig. 1g-h) on 21 August 2021 in a small group of at least 15 individuals. It was resighted on 24 July 2022 in a group of at least 12 individuals, and on 28 July of that same year, it was seen together with a darker, older juvenile/subadult; 10 adults from the 28 July event were photo-identified. In all instances, the calf was accompanied by the same adult as the one in the first sighting (12 June 2021). Calf identification was made possible because of the distinctive pale coloration and the notch at the base of the dorsal fin.

On 3 June 2023 individual C was resighted north of Faial Island. It was accompannied by an adult, totaling 10-12 individuals in the pod, including 3 juveniles. The group was resting. Only some of the animals were photo-identified. Individual C was named Martins in honor of the land spotter that has overseen the ocean in the area north of Faial Island for many years and has been resighting the species.

Discussion

Risso’s dolphins exhibit ontogenetic changes in their external coloration, becoming lighter in color as they age. Risso’s dolphin calves are light brown to silvery gray, subadults are dark brown/black, and adults are often white because of the lightening of the skin and accumulation of scars. The alteration of body pigmentation is more evident because of the cumulative scarring from social interactions, providing enough phenotypic differences for age estimation (^{Hartman et al. 2013}, ²⁰¹⁶). This is a rare characteristic among delphinids, but it is a diagnostic characteristic in Risso’s dolphins. Thus, cases of leucism in Risso’s dolphins may be more easily distinguished when the animals are still in their early life stages than when they are older.

The dolphins reported in this study (2 calves [individuals A and C] and 1 juvenile/subadult [individual B]; Fig. 1 a, e, b, respectively) were the sizes of and behaved like young individuals, but they had evident abnormal coloration, as juvenile/subadult individuals typically have dark pigmentation and noticeable scarring. This was further demonstrated by the comparison of individuals with similar age and typical coloration (Fig. 1 a, k, l) and by the size difference with respect to adults shown in Figure 1. Unlike individual B, the 2 leucistic Risso’s dolphins presenting neonatal folds were swimming close to adults. Furthermore, the nursing behavior of individual C and the presence of the same adult with typical coloration in the 2021, 2022, and 2023 sightings suggest that one of the progenitors was not leucistic. Considering the dates of the nursing behavior, we estimated the birth date to be between late 2020 and early 2021. The fact that 3 Risso’s dolphins with atypical coloration were observed in a relatively short period of time is noteworthy. Group size seems to be around 12 individuals. The consistency of observations north of Faial Island indicates that the pod visits the area at least during the spring-summer months, although not staying extensive periods, according to the daily observations made by a whale spotter with binoculars.

The presence of dark areas on the skin of the 3 individuals reported here is indicative of leucism and discards the possibility of the individuals being albinos (^{Fertl et al. 1999}). Leucistic delphinids have been reported in the Macaronesian region, for example, the Atlantic spotted dolphins (Stenella frontalis) in the Azores Archipelago (^{dos Santos et al. 2016}) and off Madeira Island (^{Alves et al. 2017}). Observations of other species with leucism have gone unreported. Despite the extensive research on cetaceans in the Azores (e.g., ^{Pereira and Gonçalves 2009}, ^{Silva et al. 2014}) and in other regions of the North Atlantic (^{Gaspari et al. 2007}, ^{de Boer et al. 2013}, ^{Stevens 2014}), particularly research on Risso’s dolphin social ecology (e.g., Pereira 2008; ^{Hartman et al. 2008}, ²⁰¹³, ²⁰¹⁴), the 3 individuals reported here are the first Risso’s dolphins with atypical hypopigmentation (generalized light patching on the skin) to be reported for the Atlantic. This report may be of value in population studies and monitoring surveys in the Azores, as these abnormally white dolphins are easy to track. The absence of records may be partially explained by the fact that unless pigmentation is totally absent, as with albinism, and the iris can be observed, atypical hypopigmentation in Risso’s dolphins can be easily detected only during early life stages.

Leucism is the result of mutation in a few genes that may be heritable (e.g., ^{van Grouw 2013}). Since large dolphin groups are expected to comprise several different stable pods (^{Hartman et al. 2008}, ^{Pereira 2008}) and considering the size difference between individuals A and B, both individuals could have descended from the same adult. The group was not exhaustively photographed, and no adult with atypical coloration was sighted in this group. Additionally, the process of individuals progressively turning lighter in color until becoming fully white with advanced age has not been fully understood at the molecular level. In the case of individual C, Martins, the pod size was relatively stable and signs of hypopigmentation in the other individuals were not evident. Future works should address the dispersion of leucism within the population, fitness impacts, or early coloration change in this particular species.