Phenotypic Changes Across a Geographic Gradient: The Case of Three Sympatric Dolphin Species

By: Kalyani Twyman, Stella Martinelli, Devin Kiley, and Joe Monteiro (Stonehill College, BIO323: Evolution, Spring 2020)

This study aimed to investigate the morphological differences, or the differences in the form and structure of an organism, in three closely related dolphin species within the context of geography. The three species that were investigated were Delphinus delphis,the short-beaked common dolphin, Stenella coeruleoalba, the striped dolphin, and Tursiops truncatus, the bottlenose dolphin. These dolphins all belong to the same dolphin subfamily, Delphininae, and are all widely distributed. They can be found in both the tropical and temperate waters of the Pacific, Atlantic, and Indian oceans and in most of the world’s seas, including the Mediterranean. This study investigated the morphological differences in the dolphin’s mandible, or its jaw, across three species and a geographic gradient. This geographic gradient was from the North Atlantic to the Mediterranean. These two bodies of water are very different: the North Atlantic has dense, cold water that is characterized by low salinity, and the Mediterranean has warm water that is characterized by high salinity and variability in both ocean productivity and seabed structure. Previous studies have looked at both genetic and morphological differences in the three species within this area and have found significant differences when comparing Atlantic to Mediterranean dolphins within their species. This study aims to test two main hypotheses: (1) the three dolphin species show significant differences in the structure of the mandible when comparing organisms from the Mediterranean versus the Atlantic, and (2) these morphological variations follow a similar pattern across the discrete geographic areas in the three species. The scientists hypothesized that the three species of dolphins have experienced the same pressures from the outside environment in each of the distinct geographic locations and have evolved similarly. 

Figure 1. Labeled diagram of the short-beaked common dolphin (Delphinus delphis), striped dolphin (Stenella coeruleoalba), & bottlenose dolphin (Tursiops truncatus), and a close-up view of the mandibular shape configurations used for comparison.

To investigate the variation in mandibular morphology, a total of 220 adult dolphins were sampled: 64 D. delphis, 51 S. coeruleoalba, & 104 T. truncates. Each species was sampled from the Mediterranean Sea (MS), the Northeast Atlantic Ocean (AO), and the North Sea (NS). Mandibles were photographed in the lateral and medial view with a digital camera. Only the right mandible was analyzed in this study. To provide sufficient coverage of mandibular morphology, eighteen landmarks, or distinct places on the bone, were identified for comparison. The landmarks’ locations were recorded and digitized using a computer program called TpsDig2. The scientists then created two new sets of variables out of the landmarks: a shape variable and a set of size values. Mandibular shape and size differences within (intraspecific) and across (interspecific) species were investigated using comparative statistical analyses, primarily ANOVAs. Sexual dimorphism, or physical differences between sexes between the same species, and allometry, or body proportion, analyses were conducted to assess intraspecific variation, or variation within a species. Delphinus delphis samples were excluded from sexual dimorphism analyses due to the lack of female specimens.

In the intraspecific experiments, significant differences were found in size and shape morphology among populations. In the interspecific experiments, the bottlenose dolphin was found to be the only statistically different species compared to the other two species when comparing mandibular size. In terms of mandibular shape, ANOVA tests highlighted significant differences among all three species. The researchers also compared the trajectory of these physical changes across the three species on a geographical gradient from the Mediterranean to the North Sea and found that the three dolphin species displayed similar changes in mandibular size and shape in different regions. Specifically, the Mediterranean common dolphins had a longer corpus and smaller ramus, two parts of the mandible, when compared to the Atlantic common dolphin. The striped dolphin’s Mediterranean population exhibits a more anterior angular process and robust ramus compared to its Atlantic population. Lastly, the bottlenose dolphin’s Mediterranean population, similarly to the common dolphin, has a longer corpus and narrower ramus than its Atlantic population.

Altogether, the geographical shape differences among the three dolphin species reflect the various evolutionary mechanisms that enable multiple species to inhabit the same region. In an effort to avoid direct competition, species living in the same area develop adaptations that enable them to consume different types of prey and employ different feeding strategies. These adaptations result in rapid changes in physical appearances, such as changes in jaw structure. For example, in the Mediterranean, the common dolphin is observed in association with the striped dolphin, but they don’t compete for food because the striped dolphin has a taller and more robust jaw that enables it to eat different prey than the common dolphin. Furthermore, species can coexist in the same geographical region by partitioning the habitat. In the Atlantic and the North Sea, the common and striped dolphins have very similar jaw structures, but there is relatively little competition because the common dolphin lives near the shore while the striped dolphin inhabits the open ocean. The bottlenose dolphin has a very different pattern of jaw structure compared to the smaller dolphins, largely because it is an opportunistic feeder, and thus its jaw is adapted to feed on the varying prey found in each region.

Despite the differences in the bottlenose dolphin jaw structure compared to the other two dolphins, there are clear physical patterns in the jaw structures of the three species. First, for all three dolphin species, the Mediterranean populations are clearly different from the Atlantic populations, and secondly, the amount of physical change is similar among the species. These results are consistent with the fact that the Mediterranean Sea is semi-enclosed and thus forces species to develop more adaptations to avoid direct competition. These conclusions all help to illuminate the evolutionary mechanisms that allow multiple similar species to live in the same environment and to partition resources.  In a future experiment, researchers should test other sympatric marine mammals to determine concrete mechanisms that enable organisms to live together.

Figure 2. Geographical distribution of the common dolphin (Delphinus delphis).
Figure 3. Geographical distribution of the striped dolphin (Stenella coeruleoalba).
Figure 4. Geographical distribution of the bottlenose dolphin (Tursiops truncatus).

Article: Guidarelli, G., P. Colangelo, M.C. de Francesco, P. Nicolosi, C. Meloro, and A. Loy. 2018. Phenotypic changes across a geographic gradient: the case of three sympatric dolphin species. Evolutionary Biology, 45: 113–125.

All figured adapted from: International Whaling Commission. 2020. Online Whale Watching Handbook. Downloaded on 19 April 2020.

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