1 The origins of aquatic mammals¶
As an introduction to aquatic mammals, join Hermione Cockburn as she watches the bottlenose dolphins in the Moray Firth, Scotland in this video.
Video 1 Bottlenose dolphins in the Moray Firth
HERMIONE COCKBURN: The Moray Firth is in the northeast of Scotland, and its beautiful and unspoiled coastline stretches for more than 800 km. It’s also home to one of just three populations of bottlenose dolphins that live around the UK. But unless they’re protected, these amazing creatures would struggle to survive. Today I’m in stunning Spey Bay to find out why.; Spey Bay lies on the south side of the Moray Firth, just to the east of where the River Spey flows into the sea, and it’s a haven for the many birds and mammals that live along the shoreline.; There are 130 dolphins in the Moray Firth, but sadly research has shown that the population is declining by 6% per year. These intelligent animals are increasingly vulnerable to threats like overfishing, getting tangled in nets, and pollution.; But if you don’t have an expensive camera or specialist training, don’t despair. Monitoring dolphins is something that anyone can do. Here at Chanonry Point, we’re a little bit further along the Moray Firth, towards Inverness, and the dolphins come in so close, they’re practically right up to the beach. One of reasons for that is that there’s very strong tidal flow here, which acts a bit like a food conveyor belt, and the dolphins just hang around and wait for the fish to come past, which they can then ambush.; |
1.1 Introduction to aquatic mammals¶
Aquatic mammals have clearly come a long way from their terrestrial ancestors and now they are superbly adapted to a fully aquatic life. The versatility of mammals will be obvious to you, but surely no environment has tested that versatility as much as the rivers and oceans of the world. As a first step, how many other aquatic mammals can you list?
Activity 1 Grouping aquatic mammals¶
Timing: Allow about 5 minutes
Question¶
Using your general knowledge of mammals, make a list of aquatic mammals that you can recall, dividing them into two groups: aquatic and semi-aquatic.
Discussion¶
Aquatic mammals | Semi-aquatic mammals |
|---|---|
whale | hippopotamus |
seal | otter |
sea lion | beaver |
walrus | coypu |
manatee | muskrat |
dugong | water shrew |
duck-billed platypus |
Mammals are often considered to be a successful group, based on the fact that they are very widely distributed and flourish in seemingly hostile environments. Another useful measure of success might be numbers of species – though in this case, the 4700 or so species of mammal are dwarfed by the known number of insect species in the world, which some estimate to be as many as 10 million. Other expressions of the success of mammals that are worth considering include the degree of physiological sophistication of their systems (which can be difficult to assess) and the number of individuals of a particular species or broader group that exist in total.
1.2 The origins of aquatic mammals¶
Mammals are essentially a terrestrial group of animals: living mammals are descended from shrew-sized ancestors that evolved on land, and most mammals are still land-based animals.
Figure 2 An early mammal, Megazostrodon
A shrew-like fossil of an early mammal, named Megazostrodon, is 200 million years old. Rather than shuffling along, with splayed-out limbs in the manner of many reptiles, this animal had limbs that were more erect and aligned under the body. Fossil evidence shows that the skulls of very early mammals have a distinctive lower jaw structure and sites on the skull for the attachment of chewing muscles. We can be confident that between 225 and 195 million years ago, mammal-like reptiles evolved into true mammals, though for the next 100 million years or so these unobtrusive animals, none larger than a pet cat, co-existed with the dinosaurs.
For the first two-thirds of the evolutionary history of the mammals, from about 210 to 65 million years ago, dinosaurs dominated the plains and forests. But the dinosaurs conquered more than just the land: the seas and estuaries were home to other large aquatic reptiles, including ichthyosaurs and plesiosaurs. These animals disappeared at the same time as their terrestrial cousins – and their demise opened up new habitats and new sources of food that the mammals were quick to exploit.
Question 1¶
Timing: Allow about 5 minutes
Question¶
Using your general knowledge of mammals, try describing up to three biological features that mammals commonly possess.
Discussion¶
Mammals are distinguished by:
the production of milk
the possession of hair or similar structures, such as bristles
they have the ability to regulate their body temperature, usually keeping it above the temperature of their surroundings, an ability that they share with birds. They are often described as being warm-blooded.
1.3 The pinnipeds, sirenians and cetaceans¶
Three major groups of mammals have independently adopted an aquatic way of life: the suborder Pinnipedia (part of the order Carnivora), the order Sirenia, and the order Cetacea. These groups will be the focus of your attention in this course. The smaller lineages that have become aquatic to some extent, such as the river and sea otters, won’t be covered in any detail.
You have just met the scientific names of some mammalian groups called orders. The class Mammalia is sub-divided into orders. Many scientific names have Latin or Greek roots and with a good dictionary (and maybe a web search), you may be able to unravel some of these words for yourself. For example, the Weddell seal which you will meet later in this course has the scientific name Leptonychotes weddelli. Scientific names are always printed in italics; if you write them by hand, you should underline the name. In this instance, the Greek word leptos means ‘slender’ or ‘fine’, and onyx is a Greek word meaning ‘nail’ or ‘claw’. So the name of the genus Leptonychotes refers to the narrow claws on the flippers of the seal (actually, the hind flippers in this case). The species name weddelli is given in honour of Captain James Weddell, who described and illustrated the seal in his writings about an Antarctic voyage to catch seals in the 1820s. Weddell will forever be associated with that seal, as specific names cannot be changed once formally given to a species.
Activity 2 Interpreting scientific names¶
Timing: Allow about 10 minutes
Question¶
You might like to try unraveling the derivation of a scientific name for yourself. Try the Pacific white-sided dolphin Lagenorhynchus obliquidens. Note down any thoughts or findings below.
(Hint: use a comprehensive dictionary to look up ‘lagena’, ‘rhyncho-’ and ‘oblique’.)
Discussion¶
The generic name Lagenorhynchus should have been fairly easy to interpret, but the specific name of obliquidens might well have been more challenging. Lagenorhynchus is derived from the Greek ‘lagenos’ meaning ‘bottle’, and ‘rhynchus’ meaning ‘beak’. You might think that the bottlenose dolphin in Figure 1 would belong to this genus, but it does not – a quirk of the naming process. The species name obliquidens means ‘oblique’ or ‘slanting’ tooth, referring to the fact that the teeth are slightly curved.
1.3.1 The suborder Pinnipedia The name of this suborder comes from a Latin word meaning ‘wing-footed’, which refers to the modification of limbs into flippers. Most pinnipeds move onto the land from time to time to rest and bask in the sun, and in the breeding season they gather, often in large numbers, on beaches or flat areas of sea-ice to give birth. There are three main families: the eared seals, which include fur seals and sea lions; the earless or true seals; and the walrus. Remember that these mammals are carnivores; in view of their continuing links to land, they are sometimes aptly called ‘amphibious carnivores’.
The other two groups, the sirenians and cetaceans, are fully committed to a life in the water.
1.3.2 The order Sirenia
Figure 3 (a) Manatee, Trichechus manatus; (b) Drawing to show the possible appearance in life of the extinct Steller’s sea cow
There are four living members of the order Sirenia: three species of manatees and the dugong. These animals are descended from ancestors that also gave rise to the elephants, and they are the only herbivorous, completely aquatic mammals. The order is named after the Sirens of classical mythology, sea-nymphs who lured sailors to their deaths. Until relatively recently, there was a fifth species: Steller’s sea cow. This animal weighed around 10 metric tons, was 9–10 m in length and lived mainly on kelp. It had no teeth, though it did have rough plates on its palate. Hunting and the decline of the kelp beds caused a drastic fall in its numbers, and it holds the dubious record of the shortest period of time between discovery by western science (1741) and extinction (1768).
Manatees are amongst the earliest mammals to move into water. Fossils have been found that date from 50 million years ago. They are extremely well adapted to live in water and are found both in freshwater and the sea. One species lives in the Amazon. Smaller than the extinct Steller’s sea cow, manatees can reach 4 m in length.
1.3.3 The order Cetacea The cetaceans get their name from the Greek word ‘ketos’ meaning ‘whale’. The order is divided into two suborders: the Odontoceti, or toothed whales, which include the sperm whale, killer whale, dolphins and porpoises; and the Mysticeti, or baleen whales, which include the blue whale, right whale and humpback whale, and are named for the brush-like plates that they use to filter krill from the water.
Video 2 Humpback whale filtering krill
1.4 Moving to the water¶
In moving to the water, aquatic mammals have had to survive, feed and reproduce using a set of biological characteristics that evolved in association with life on land. This course will explore how these characteristics have provided challenges – as well as opportunities – for mammals that spend some or all of their time in the water. You will examine some of the important differences between a life in air and a life in water, and learn how the bodies and behaviour of aquatic mammals have become better suited to living and hunting in an aquatic environment. In Section 3, you will explore the so-called ‘diving response’, and some of the problems involved in finding out how the animals behave on their dives. Section 4 looks at what is now known about the evolution of the mammals that have made the most complete transition from the land to the water – the whales. And finally, Section 5 will examine how human activity impacts on these ecosystems.