Delphinids: Right Whale Dolphins

Northern right whale dolphin

Dolphins are familiar animals. We see them at aquaria and boat trips to see them in the wild are relatively common. In recent decades, there has been a rise in 'swimming with dolphins' tourist experiences, which studies have shown to be good in the short term for humans, but less so in the long term for the dolphins. Either way, we know a fair amount about them, both culturally and scientifically and, depending on the part of the world you live in, there may be many different species that you can see.

Some dolphin species, however, are less well-known than others. I've covered some already, but perhaps the most obscure are the right whale dolphins. 

Delphinids: Newest and Largest Dolphins

Fraser's dolphin

One of the largest features at the British Natural History Museum is a full-scale model of a blue whale, occupying a large chunk of one of the mammal halls. This was installed in 1938 by Francis Fraser, a Scottish zoologist with a lifelong interest in cetaceans. Eighteen years later, still working at the museum, he was put in charge of reorganising their collection of cetacean skeletons and came across one that hadn't been closely examined since it had arrived in 1895.

It had been donated by Charles Hose, a colonial administrator and amateur naturalist who had found the skeleton on a beach near a river mouth in Sarawak (then a British Protectorate). Hose hadn't been quite sure what it was, and simply labelled it "white porpoise ? Lagenorhynchus sp." before sending it on. When Fraser examined it, however, he soon realised that it couldn't possibly be what Hose had guessed and that it was, instead, an animal previously unknown to science.

Delphinids: Dolphins in the Irrawaddy

Irrawaddy dolphin

While we normally think of dolphins as being sea-dwelling animals, there are no fewer than eight species referred to as such that are commonly found in rivers. Six of these, however, are not true members of the "dolphin family", or Delphinidae, their ancestors having split off from that group even before those of some of our modern whales did. Of the two exceptions, one is entirely freshwater, and lives in South America. The other is more varied in its habitat and lives in Asia.

The Irrawaddy dolphin (Orcaella brevirostris) was first described in 1866, from a specimen caught, not in a river, but off the northeast coast of India. We now know that this is at the far western edge of its range, and that it is also found all along the coast from northeast India, around the Malaysian Peninsula, to as far east as southern Vietnam. It is also found further south, around Borneo and along the north coasts of Sumatra and Java. In 1999, a very small population was discovered in the Philippines, living in a couple of isolated bays very far from the remainder of the animal's range, presumably the result of some having been swept away in a storm decades or even centuries before.

Delphinids: The Freshwater Dolphins of Brazil

Tucuxi

In this series so far, I have generally been referring to the Delphinidae as the "dolphin family". That's a literal translation of the name and serves to distinguish it from, say, the porpoise family. However, as I mentioned in the first post, not all animals commonly referred to as "dolphins" belong in the family. Thus, when zoologists want to distinguish the family from those other animals, but want to avoid saying "delphinids", the more common term is "Oceanic dolphins". Oceans are, after all, where they are found.

With, it turns out, one exception.

The tucuxi (Sotalia fluviatilis) is unique among Oceanic dolphins is being an exclusively freshwater animal. It lives in the Amazon River and its major tributaries, mostly in Brazil, but also further upstream into Colombia, Peru, and Ecuador. Indeed, it was first formally described, by Paul Gervais in 1853, from an animal sighted in Peru, about 2,500 km (1,500 miles) from the mouth of the Amazon... and they are known to get further upriver than that, until they are stopped by features such as waterfalls.

Delphinids: Dolphins of the Deep Seas

Rough-toothed dolphin

To our human eyes, it's easy to distinguish the major habitat types on land. There are pine forests, tropical jungles, open prairies, deserts, mountains, and so on. When it comes to the sea, however, it's less obvious. Most maps show the sea as a solid mass of blue which is, of course, what most of it looks like from the surface. But there are different environments and habitats within it, even if they aren't necessarily arranged in quite the same way.

While for some dolphin species the only real limitation is pack ice preventing them from surfacing to breathe, most have more specific requirements. Temperature is the most obvious, with some species preferring tropical or subarctic seas, but the depth of the underlying water is also significant. Species such as common and bottlenose dolphins are most comfortable over the continental shelves, where nutrients well up from the sea bed to feed the fish and squid on which they prey. Rather more species prefer shallow waters, close to the coast. Here, the water is shallow enough for light to reach the bottom, allowing seaweed or coral to grow, which benefits a different kind of fish than those further out.

Delphinids: Common and Bottlenose Dolphins

Common dolphin

When the genus Delphis was first named in 1758 at the dawn of taxonomy, it included three species of small to medium-sized toothed cetacean. By the time the dolphin family, Delphinidae, was named in 1821, one of those species (the porpoise) had been moved elsewhere, but five new ones had been added. Many more followed, but, from the mid-18th century onwards, naturalists began to notice subtle differences between animals that were probably more mysterious to them than land-dwelling mammals, and many species of dolphin began to be separated out into newly created genera.

So much so in fact, that by the time we reach the 21st century, only one species remains in the genus originally created to contain all dolphins and dolphin-like animals. That species is, of course, the one that we believe Linnaeus happened to be thinking of when he named the genus, and therefore is the defining (or 'type') species not only for its genus but for the dolphin family as a whole. This is the aptly named common dolphin (Delphinus delphis).

Delphinid Dolphins

Of all mammal groups, those that have undergone the most dramatic change from their original form are perhaps the cetaceans. The most thoroughly adapted of all mammals to life in the water, there is little else they could possibly be mistaken for. The cetaceans have been considered a distinct order of mammals since the dawn of taxonomy in 1758, and today are divided into fourteen living families. The largest of these families, in terms of the number of species, is the dolphin family.

Technically known as the Delphinidae, this was named by John Edward Gray in 1821 as part of one of the earliest formal lists of mammal families. Gray's original definition basically included all toothed cetaceans other than narwhals and sperm whales, encompassing four genera, only one of which is still placed in the family today - Delphinus, from which it takes its name. He didn't list how many species he thought that genus contained, but it would certainly have included the two named by Linnaeus in 1758, and probably at least three others that had been described in the interim. Over the following decades, the number expanded considerably, with Gray himself identifying several of them, notably when he catalogued the observations and specimens collected by the Ross Expedition of 1839-43.

Friendship and Fission-Fusion

Mammal species have a wide range od different social systems into which they organise themselves. Many are essentially solitary outside the breeding season, others form long-lasting pair bonds, and others live in larger associations which may themselves have varying different structures. Among the latter, one common pattern is that of the fission-fusion society.

Here, rather than having a long-lasting association, perhaps bonded by ties of family, the animals live in groups but the membership of that group is not constant. New animals are constantly wandering in, while others break off and leave for other groups. On a larger scale, it may be that the individual fission-fusion groups - the actual bands of animals you would see travelling together - are themselves gathered into larger social networks that may have a relatively consistent structure. That is, the new animals joining the group aren't random; they're individuals already known to the group, and rival social networks may exist nearby that do not mix their members.

Miocene (Pt 37): The Miocene Oceans

Xiphiacetus

Given their extreme adaptations to a life at sea, it should not be surprising that whales and dolphins have a long evolutionary history. Even at the dawn of the Miocene, when seals were at best, only very recent arrivals, and may still have been semi-aquatic, like otters, the cetaceans were already well-established. A time-travelling whale-watching expedition at almost any point during the Miocene would likely have been just as rewarding as one today and, especially so far as one would have been able to see them above the waves, the animals you would have been spotting would not have looked all that different.

At a more detailed anatomical scale, that would have been less true and many of the creatures would not have been as closely related to the modern forms as one might have supposed. Today, the largest number of cetacean species belong to the "true" or "oceanic" dolphin family, including not only all of the sea-going dolphins but some larger animals such as killer whales. But in the Miocene, true dolphins seem to have been comparatively rare and we know of very few fossils predating the subsequent, Pliocene, epoch.

Prehistoric Mammal Discoveries 2022

New reconstruction of the sabretooth cat
Homotherium, showing that the teeth
would not have been as visible as popularly
supposed

2022 approaches its conclusion and I have to say that, while far from perfect, it was an improvement on the previous two years. There were a couple of weeks without posts this year, one for positive reasons, and one less so, but overall it has been good. Next year, you can look forward to something other than monkeys in my "family" series of posts and also to the conclusion of the long-running Miocene series, which should be as early as February. Until then, here is the now-traditional year-end look back at the paleontological discoveries that didn't quite make it into the blog.

Large Herbivores

Probably the most distinctive thing about deer is that the males have antlers; branching bony head ornaments that are shed and regrown each year. This naturally raises the question of how this evolved, since no other animal has quite the same thing. Acteocemas was an Early Miocene deer, but despite living very early in the group's evolutionary history, it already had antlers that split into two near the tip - which the horns of animals such as cows and true antelopes never do. A Spanish fossil of the antlers described earlier this year showed that it was already shed and regrown, but microscopic analysis indicated that it appeared to have been present for over a year, suggesting longer a more irregular pattern of shedding that must have changed to the seasonal pattern we are familiar with more recently, perhaps in the Middle Miocene.

How Big is a Small Whale?

The ongoing expansion of humanity has placed several animal species under threat of population decline or extinction, whether due to direct effects such as loss of wild spaces or more indirect ones such as climate change. (And a great many of these species are non-mammalian, of course, despite the focus of this blog). On the other hand, there have also been many conservation efforts that aim to restore, or at least preserve, species at risk. As I've previously described, for example, both black-footed ferrets and European bison went completely extinct in the wild during the 20th century, but are now back living in their original native habitats, if only in small numbers.

Not all such restoration efforts have been successful, and there can be many different reasons for this, not all of which are necessarily biological. Better then, of course, to try and prevent species from becoming endangered in the first place. For this reason, many species are protected even if they are not currently at risk. The more we can understand about these species, the better we will be able to keep them that way. 

The Dolphins of Switzerland

Kentriodon

The Mediterranean is very nearly an inland sea. Its only natural connection to the world's wider oceans is through the Strait of Gibraltar between Spain and Morocco, a passage just 13 km (8 miles) wide at its narrowest point and in places just 300 metres (1,000 feet) deep; pretty shallow as such things go. It's probably because of this that some of the larger whales that inhabit the Atlantic (blue whales, humpback whales, etc.) are rarely if ever seen venturing into its waters.

Dolphins are a different matter, with the majority of North Atlantic dolphin species also being commonly seen in the Mediterranean, albeit in some cases only in its most westerly waters. This includes some of the really big dolphins that we'd normally call "whales", such as the killer whale, and there are three species of genuine whale that live there, too. The connection between the Mediterranean and the Atlantic has not always been there, however; for a long time during the Late Miocene, the two bodies of water were separated by a land bridge between Spain and Morocco, entirely cutting the Mediterranean off until it ended in the cataclysmic Zanclean Flood. 

How Dolphins Got Their Large Brains

As a rule, larger animals tend to have larger brains, just as they have larger hearts, livers, and other internal organs. But the relationship is not a linear one, partly because, unlike the liver, there's a minimum size below which the brain doesn't have room for the basic housekeeping functions that keep the body ticking over, so that small animals have proportionately larger brains than large ones. If we want to know whether a particular creature has a larger, or smaller, brain than we'd expect then, as I've described before, we use a mathematical formula called the encephalisation quotient to determine what the size of the brain "should be". 

By definition, an average mammal should have an EQ of 1. The formula breaks if we try to apply it to non-mammalian animals, such as birds, probably because their brain architecture is different from ours at quite a fundamental level. Even for mammals, there is some debate as to exactly what formula we should be using; most older studies have determined that brain size typically rises as the 2/3 power of body size (that is, as the cube root of the square) but a 2019 study argued that it's perhaps more accurate to base it on the 3/4 power and this seems to be a growing consensus.

Prehistoric Mammal Discoveries of 2020

The largest known Oligocene whale was
moved into the new genus Ankylorhiza
this year

Well, that year was... different. Fortunately, it didn't stop me posting and, indeed, a cancelled summer break meant that you got one more post than anticipated you lucky people, you. But as 2020 finally passes into the history books where it can bloody well stay, it's time for the end-of-year roundup of paleontological discoveries that didn't make the regular blog. As always, no theme, just a random assortment of journal papers that may, for all I know, be thoroughly contradicted by this time next year. But this is a collection of things that some scientist, somewhere, thinks that they demonstrated in 2020:

Porpoisely Quiet?

Way back in the early 19th century, the naturalist Karl Ernst von Baer, better known for his work on embryology, made an interesting discovery that seems fairly obvious to us in hindsight: the blowhole of a porpoise is, in fact, its nostril. Other anatomists followed up on this, leading to the further discovery that a number of air sacs appear to be connected to the nasal cavity. They can hold a fair amount of air when fully inflated, but nothing like as much as the lungs, so exactly what they were for wasn't clear.

Fast forward to 1956, and scientific research confirmed another interesting fact about such animals: they can echolocate by sending out sonar pings. In 1968, the two facts were put together when it was demonstrated how the unusual structure of the porpoise's head allows it to transmit the necessary sound pulses, which are initially generated somewhere in the nasal passages.

Chorus of the Dolphins

It's well-known that cetaceans - dolphins, porpoises, and whales - make calls to communicate with one another, in addition to the sonar pings used to navigate. The complexity of these calls varies significantly between species, although even sperm whales, for example, producing little more than a regular pattern of clicks, while humpback whales produce what appear to be sophisticated 'songs'.

Among the most studied of such calls are those of bottlenose dolphins (Tursiops spp.) which are the type most commonly seen in aquaria, making them relatively easy to observe. Having said which, it was only in 1998 that scientists confirmed that there was more than one species of bottlenose, leading to some confusion as to which one lived where and, by extension, which one any given prior study referred to. And, as I blogged about when the announcement was made back in 2011, we now know of a third one as well.

Requiem for a Dolphin

Dolphins are social animals, living in pods and engaging in what seems to be fairly sophisticated communication. It's likely that such behaviour is part of the reason for their success, with well over 30 species spread across the world's oceans... and that's excluding porpoises, and some other "dolphin-like" animals outside the dolphin family proper.

Sociability involves a number of different traits and behaviours, but one that's known to exist in cetaceans and relatively little else, other than primates, is what's technically known as epimeletic behaviour. This is, in essence, the act of helping other members of your species when they are in trouble, typically due to an injury of some kind. (For what it's worth, this compares with etepimeletic behaviour, which is acting in such a way as to make it easier for others to care for you).

When Dolphins Emigrate

While most mammals tend to have a "home range" in which they spend most of their adult lives, there are many reasons why they might wish to move elsewhere. Perhaps most obviously, there is the movement of juvenile individuals leaving home for the first time, and so travelling out of their parent(s) home range to establish their own territory. There's also the issue, mostly with males, of travelling about in the hope of encountering fertile members of the opposite sex. On a larger scale, there can be regular migration events, including short-range movements such as goats moving down mountain slopes in winter to avoid the worst of the weather.

But large-scale movements on a one-off basis are relatively rare. In social animals, individuals may move from one herd (or other group) to another for all sorts of reasons, although, in many species, even that can carry risks. Incidents in which a large portion of a herd ups sticks and moves to a place already occupied by another herd, before trying to integrate with the locals, are relatively rare. Which also makes them difficult to study, since it largely relies on the luck of happening to already be looking at a given group when it happens to occur. It's perhaps particularly hard to do so for cetaceans, which live underwater, and can be difficult to track.

First and Last of the South Asian River Dolphins

South Asian river dolphin skeleton -
note the strange shape of the skull

Although the whales are undeniably spectacular, the majority of cetacean species are much smaller; the sort of animals we generally refer to as "dolphins". The great majority of these belong to the family Dephinidae, variously termed the "oceanic dolphins", "pelagic dolphins", or even "true dolphins". This is a large family, with nearly forty species, including killer whales and pilot whales alongside their smaller kin.

Whatever we call this family, it, in turn, belongs to the larger group of the "delphinoid cetaceans", which also includes the six species of the porpoise family and two other whales - the narwhal and beluga. Taken together, these animals and their extinct relatives have dominated the count of cetacean species across the world for millions of years, forming a key part of the ocean ecology. Of all the other cetacean groups, only the mysterious deep-sea beaked whales come close in terms of the number of species, and even they don't appear to be as varied.

Pliocene (Pt 16): The Pliocene Oceans

Bohaskaia

Over the last two and a half years, I have talked about a range of mammal species found across the Pliocene world. So far, I have only looked at the continents, and the various animals that lived on land. But the Pliocene seas were, of course, equally teeming with animal life. Much of this naturally consisted of either fish (most famously the giant "megalodon" shark) or invertebrates of various kinds. Sea turtles also existed, including some quite large ones, but, in keeping with the scope of this blog, I'm going  to focus on the mammals.

While we often tend to think that prehistoric animals tended to be larger than those alive today, this, was however, rather less true of whales, which have grown more or less steadily in size over the course of their evolution, perhaps in part to make it increasingly difficult for anything else to eat them. So, for example, the Pliocene killer whale (Orcinus citoniensis) was around 4 metres (12 feet) in length, barely more than half that of the modern species - although still quite impressive on a human scale.