Why are marine mammals so large?

The smallest toothed whale, the Vaquita porpoise, is still a hefty size compared to the smallest primate, the pygmy marmoset, which is a comfortable handful. Why is that?

This has to do with all mammals trying to maintain a body temperature of 98.6℉ (37℃), by having all cells in their bodies produce heat. Since water conducts heat much better than air, mammals living in water face much higher heat loss to the environment than land mammals. Everything else being equal, a larger body has a much smaller surface-to-volume ratio (S/V) than a smaller one. In addition, marine mammals also tend to be much more streamlined than land mammals, which further reduces the S/V ratio. This is important, since the volume - the number of body cells - is what produces body heat, while the skin surface area is where the heat is lost to the environment.

When a specific shape gets larger, the surface area will increase at a slower rate (by a factor squared) than the volume (by a factor cubed). This means that the Surface to Volume Ratio (S/V) gets smaller the larger the shape. In mammals, the volume (number of cells) produces the heat which is lost through the skin surface area.

So, the larger the body, the less heat it will lose through the skin surface relative to how much heat its cells produce. In fact, a blue whale is so large that it is struggling to dump enough heat, not to overheat.

S/V ratio can therefore explain some of the differences in both anatomy and behavior in nature. For example, humpback whale females migrate long distances from the feeding grounds in productive colder waters, such as off Alaska, to warmer waters, such as off Hawaiʻi, to give birth. The warmer waters will help the small, newborn calves to not lose too much heat to the water. The warmer waters are also where humpback whales mate. The males compete for access to females in what is called “surface-active groups”, where a receptive female is followed by a large number of males fighting over who is to escort the female and presumably mate with her. During these ‘bouts’ the animals generate a lot of extra heat that they need to get rid of, so as not to over heat. This is an extra challenge in tropical warmer waters, where heat does not dissipate as fast as in colder waters. Humpback whales have much longer pectoral flippers than other whales, just for this purpose. By directing their blood flow to their skin, just like we humans do when blushing while exercising, whales can lose heat to the water. The extra surface area of the humpback flippers makes it possible for these whales to compensate for the extra challenge posed by the warmer waters, to be able to compete aggressively in the warmer waters closer to the equator.

Another example, where variation in anatomy can be explained by S/V ratio is the dorsal fin of orcas. Here the males have much larger fins, both pectoral flippers and dorsal fins, than the females. This difference can be explained by the difference in body size between male and female orcas, males get up to 9m long, while females get up to 7.9m long. As a result, the adult males will generate more heat than adult females while performing the same task, thus necessitating the need for a larger surface area to lose the extra heat. The larger fins and flippers also help the males to maneuver their larger bodies. This difference in dorsal fin size is common among dolphin species, where adult males tend to have larger fins than the females.

What is in a name?

There is a whole group of dolphins whose common name actually includes the term “whale”. Not only that, together they are referred to as “Blackfish”. Why is that? How did that happen? The group includes the following species of oceanic dolphins: Killer whale (or Orca), False killer whale, Pygmy killer whale, Melon-headed whale and Long- and Short-finned Pilot whale.

The term blackfish originated at least twice for members of this group of species. It was initially given to the killer whale by the Tlingit and Haida tribes of western Canada. In the 1600’s pilot whales were called “blackfish” by the fishermen in the American colonies, who hunted them in a drive fishery, where the whales were driven towards shore with small boats, much like what is still going on in the Faroe islands.

This was all before animals were divided into various groups based on scientific criteria, so these animals swimming in the ocean were referred to as fish. How the other species of dolphins came to be included among the “blackfish” is not completely clear, but it probably had to do with the challenge of identifying these species in the field, or even when stranded or in the form of a skull.

All of these species are mostly black or dark gray. The killer whale has the most white on its body, with large white areas on its sides and ventral (stomach) area. For the other species the white markings are mostly seen on the ventral area and, for the smallest species, around the lips.

This coloration may partially be explained by the hunting practices of these species. All species but the orca appear to be active and feeding mostly at night on prey like squid and fish that ascend towards the surface after sunset. However, some of the species also feed on other prey at times, which may cause them to change their wake/rest cycle and be more active at other times of the day.

The pilot whales are squid specialists, and therefore are more consistently nocturnal, generally waking up around 4 or 5 in the afternoon. False killer whales and Pygmy killer whales have also been observed to catch very large fish, like mahi-mahi, wahoo and tuna, during the day. False killer whales then feed cooperatively on it - one whale holding the prey while others tear off chunks to swallow. There is even footage of false killer whales sharing their prey with divers. That this has not been described for Pygmy killer whales may be due to the limited amount of observations that have been made on this species.

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.