Page 402 - (DK) Ocean - The Definitive Visual Guide

400 ANIMAL LIFE

Mammals

ONLY A SMALL MINORITY OF THE WORLD’S mammals live in seawater,
DOMAIN Eucarya
but taken together, they show an extraordinary range of shapes, sizes, and
KINGDOM Animalia
lifestyles. They include cetaceans (whales and dolphins), sirenians (manatees
PHYLUM Chordata
and dugongs), and carnivores, particularly the pinniped carnivores (seals,
CLASS Mammalia
sea lions, and walruses). All marine mammals breathe air, like their terrestrial
ORDERS 27
counterparts, and they give birth to live young, either in the sea or onshore.
SPECIES About 5,500
Many species are migratory, with a sophisticated navigational sense.

Anatomy and Physiology heart beats rapidly
after surfacing
Marine mammals have many adaptations for life at sea, DIVING MAMMAL 180
not only in their anatomy, but also in their physiology, When a Harbour Seal 160
regulating how their bodies work. Cetaceans and dives, its heart rate 140
falls below 10 beats a
sirenians have lost all visible traces of hind limbs; instead, minute. Blood diverted 120 heart rate drops
they propel themselves with their tail flippers or flukes, from its muscles and 100 as seal dives
which beat up and down. Fur seals and sea lions swim digestive system flows HEART RATE (BPM) 80
with their front flippers, while true seals use their rear to its heart and brain. 60
flippers, bringing them together like a pair of hands. Despite needing to rate remains low
40 throughout dive
breathe air, many marine mammals are superb divers. Some, such as the
20
elephant seal, can reach depths of over 3,300 ft (1,000 m) and stay underwater
for up to two hours. When they dive, their heart rate drops, and blood flow 0 2 4 6
is modified so that vital organs receive enough oxygen until they resurface. TIME (minutes)
Instead of breathing in before they dive, the deepest divers often exhale.
This helps them to avoid
decompression sickness, humerus
or the “bends.”
phalange radius
SHARED PATTERNS
A sea lion’s front
flipper has the same
arrangement of bones
as a human arm. The
“arm” bones are short and sturdy, ulna scapula
helping to bear the animal’s bulk
metacarpal
on land. Long finger bones make sonic lips
up the flipper’s blade. blowhole (source
of sound)
INSULATING BLUBBER outgoing clicks
Compared to air, seawater (to prey)
drains much more heat from
FLIPPERS AND FLUKES mammals’ bodies. To keep
A humpback whale’s warm, many polar species,
flippers contain bones, such as this walrus, have melon
and beat like a pair of a thick layer of incoming
wings. Its flukes, or tail insulating fat, ear drum (reflected)
clicks
fins, are made of rubbery called blubber,
tissue, and contain no under the skin. sound channel USING ECHOLOCATION
bones at all. in jaw Dolphins and toothed whales use pulses
of high-pitched sound to locate prey. The
forehead contains an oil-filled organ called
VARIED DIET the melon, which is thought to function as
Penguins are just one item an “acoustic lens” to focus outgoing sound.
on the leopard seal’s menu.
Despite its reputation for Feeding
ferocity, at least half of
its diet consists of krill, Apart from plant-eating manatees and dugongs, most
which it filters with marine mammals are exclusively carnivorous. In open
its cheek teeth.
water, many pursue individual prey, tracking it by sight
or by echolocation. Some seals have a twin strategy.
They catch prey individually, but they can also filter
out planktonic animals in bulk, using complex
cheek teeth that interlock to form a sieve. This
OCEAN LIFE baleen whales, which cruise through shoals of fish
efficient feeding method reaches extremes in the
or krill, often swallowing over 220 lb (100 kg)
of food at a time. Not all marine mammals catch
moving prey. Sea otters dive to collect clams,
mussels, and sea urchins, while walruses and gray
whales suck mollusks out of seabed sediment.