What is a species when describing dolphins?

There is something basic to humans to try to categorize and organize things into clear categories. Once there is a system and a category to place each single thing then it seems easier to make sense of new and different items as they are discovered. This kind of activity is fundamental not only to science but it is even more deeply human because it underlies all kinds of human activities such as cooking, mechanics and gardening. There is a drive to make sense of chaotic confusion by creating a system and organizing the chaos into distinct categories. So, a cook will label and organize spices and seasonings so they are easy to find and use and a mechanic will arrange tools on well organized hooks or shelves. This drive to categorize and organize based on important characteristics is what underlies the scientific field of taxonomy. Biological taxonomy is the human attempt to make sense of the overwhelming diversity of life on Earth.


According to the Cambridge dictionary, taxonomy is “a system for naming and organizing things, especially plants and animals, into groups that share similar qualities”. The system that we finally settled on was developed by Carl von Linne’ (born Carolus Linnaeus but knighted by the Swedish King), also known as the “Father of Taxonomy”. This system is familiar to anyone who has used a field guide to try to figure out what kind of bird they just saw or what kind of wild flower they just picked while out on a walk. It is the familiar naming system that people have learned by using the famous mnemonic “King Philip Came Over For Good Spaghetti” (Kingdom, Phylum, Class, Order, Family, Genus, Species).


In its strictest form, a species is defined as a group of similar organisms that can interbreed with each other and form fertile offspring. They should not be able to interbreed with other species. However, in reality new species are formed on a regular basis and it is a process that takes time (thousands to millions of years) to complete. So different groups of organisms are at different stages in this process.


One example would be a geographic barrier that prevents species from even interacting with each other. In this case, species that are divided into two isolated groups by the formation of a river or a mountain range (clearly something that would take some time) will no longer be able to meet and mate. At this point the two groups could start to diverge from each other, in terms of appearance, diet, etc. The speciation process would likely not be completed, however, unless members of the two groups came back into contact again. At that point there could be some disadvantages to hybrids between the two groups, such as hybrids having a mix of the parents' appearances or not quite suited for either parent's diet. At that point, it would be possible for a reproductive barrier to evolve, since it would benefit parents from both groups, as they would be more likely to provide successful offspring, if breeding within their groups. Examples of reproductive barriers are many, including mating at different times of day or different seasons, different mating behaviors, or the development of genetic barriers. For example, the number of chromosomes can change in one group and become a feature that prevents fertile hybrids. This is why breeding a horse with a donkey will result in an infertile mule. The horse and the donkey can mate and produce a living offspring but the mule will be a dead end and no further offspring will result from the breeding.


Marine mammals present a number of interesting challenges to our concept of speciation. There is only one ocean so in many instances geographic barriers are much less likely to create new species. While there are only a few species of marine mammals in groups such as manatees and right whales there are many species of dolphins in the world. For example, there are about 37 species of oceanic dolphins in the world, from the largest member, Orca or killer whales which range from 9 m-7.9 m (30 ft to 26ft) , to the smallest, Hector’s dolphin, only found in New Zealand waters which range from 1.4 m-1.5 m (4.6 ft to 5 ft). In addition to massive size differences, oceanic dolphins have a wide variety of anatomic features, diets, behaviors and ecologies.


And yet, although there are such major anatomical and ecological differences between different dolphin species, they can interbreed and have offspring that are fully fertile. An example is an individual known as the “wholphin”, born in Sea Life Park, Hawaiʻi in 1985. Kekaimalu meaning " from the peaceful ocean" is female and has a false killer whale father and an Atlantic bottlenose dolphin mother. This was a remarkable event, as the male false killer whale was over twice as long (14 vs. 6 ft) and 5 times as heavy (2,000 vs. 400 lbs) as the female bottlenose dolphin. The wholphin had features halfway between its parents, such as number of teeth (66 vs. 44 and 88) and shape of forehead and front of the face. The wholphin is a female that later gave birth to three calves whose father was a bottlenose dolphin. These calves interestingly had 77 teeth, again half-way between their two parents.



There are also several examples of individual dolphins seen in the wild which look like intermediates between two different dolphin species. For example, one field study in Brazil found a school of spinner dolphins where two females had calves that appeared to be hybrids of spinners with two different species of dolphins. (One calf appeared to be a hybrid of spinner and spotted dolphin, and the other appeared to be a hybrid of spinner and Clymene dolphins). One of these calves was seen nursing from the spinner dolphin mother and was followed for 2 years. So, while these may be rare occurrences there are clearly examples of successful breeding with surviving offspring.



A spinner dolphin female (above) with a Spinner/Spotted dolphin hybrid (below). From Silva Jr. et al. Two Presumed Interspecific Hybrids in the Genus Stenella (Delphinidae) in the Tropical West Atlantic. Aquatic Mammals 2005, 31(4), 467-471.



This brings up the question of how useful is the term “species” in describing dolphins if there are living exceptions to this basic definition? No category system is perfect and there will always be exceptions to the rule. The key to continuing to use a classification system is whether or not it works the majority of the time. And, while there are exceptions to the species definition for some dolphins, they are rare instances and the majority of the animals in this group do fall into distinct species that can be categorized by clearly defined characteristics. The short answer to this question is that the system works most of the time.


But, when it is wrong the problem lies with the categories not with the natural world so a revision is needed to some or all of the ways of describing and defining relationships between species. The mismatch between the reality and complexity of our living world and the systems of categories created by taxonomists provides a constantly furtile arena for debate. The current efforts to integrate the new findings provided by genetic studies of dolphins into the Linnean taxonomic system has resulted in several different maps of relationships between species of oceanic dolphins.


Added to the struggle to fit species into neat categories and clear relationship to each other is that from an evolutionary perspective all these species are in a constant state of flux and change as they evolve and adapt to their environment. Species of dolphins are evolving into separate niches and while they may currently be able to reproduce with another species, through time as they continue to have divergent feeding habits and ecological niches they will become more and more separated until eventually, they are reproductively isolated and no longer able to cross-breed.


And recent genetic studies have shown that things in the realm of dolphin species and relationships are even more interesting than we thought, it seems that certain dolphin species could be completely made up of hybrids between two other species. For example, genetic analysis suggests that one species, the Clymene dolphin, is the result of hybridization between striped and spinner dolphins. Link.