Marine Mammals fall into 3 orders:
The entire lifecycle is aquatic. There are two sub-orders based on their feeding mechanism - Mysticeti and Odonticeti
Baleen whales - including the largest animals - Blue Whales. They use brush-like triangular plates (baleen) which hang from roof of mouth - maybe 480 plates on each side of jaw - continuously renewed. The plates filter out zooplankton, small fish or squid and the tongue moves this food into the throat.
These whales have mechanisms to increase feeding success. Fin Whales scare their prey into schools while Humpback whales use a bubble-net to amass their prey
Toothed whales, porpoises, dolphins. First teeth are retained throughout life - grasp and swallow whole prey. Opportunistic feeders - fish, squid, crustaceans. Killer whales eat warm-blooded prey - cetaceans, seals, penguins. Improve feeding by co-operative hunting. Killer whales will surf onto a beach to capture seals, or tip up ice flows to disturb prey.
River and coastal sp. have small home ranges while offshore sp. forage widely and have broad home ranges. Some live in warm equatorial waters, others are found in all oceans eg Killer Whale and Bottle-nosed Dolphins. Large baleen whales migrate from winter breeding in tropics (no feeding) to summer feeding at high latitudes (40% wt gain in 4-5mths). Odontocetes seasonal response is movement on-off shore
The shear size of many whales means that their death represents a major recycling opportunity. Carcasses scattered on the deep sea bed are thought to act as hot spots for biological activity.
Propulsion is achieved with the broad, flat, boneless tail flukes while stability is introduced by the fibrous dorsal fin (if present) and the bony pectoral flippers. In order to reduce friction, cetaceans exude oil and shed skin up to as much as 12 times/d. Many dolphins are surface active and bowride - developed to move in bow wave of large whales without using energy.
Breaching (rising out of water and flopping back onto it) is most regular in Humpback whales.
These mamals inhale, and then dive with the lungs full. Efficient lungs transfer O2 to the blood which is high in haemoglobin. Muscles are rich in myoglobin which gives the meat a dark appearance. Networks of blood vessels (retia mirabilia) are found but the function is not known - they may be involved in regulating blood pressure or temperature, or as an N trap. The lungs become compressed as they dive and collapse below 100m. Cetaceans exchange 80-90% of lung volume as they breath in and out, compared to 15% in humans and exhale only 1.5% O2 after a dive so ventilation and respiration are extremely efficient.
Sperm Whales routinely dive to >1000m but have been recorded at 2800m on a dive of over 2 hrs. Smaller dolphins/whales dive to approx 100m and are submerged for 8 mins. The heart rate drops and blood is restricted to the brain and heart - vital organs. To counteract the low temperatures at depth, cetaceans have a fatty layer called blubber for insulation which has minimal blood content. A counter current heat exchange mechanism in the circulatory system also reduces heat loss.
Of the 5 mammalian senses, cetaceans have a poor sense of smell but use taste to detect chemicals on the water currents and touch for bonding or to maintain order within the pod. Sight is quite good with many capable of binocular vision. Different areas of the retina are adapted for air & water vision. Most cetaceans both produce and detect sound. The middle & inner ear are isolated from the skull which allows detection of the direction of sound. Odontocetes have anatomical adaptations to produce/detect ultrasonic clicks for echolocation.
Odontocetes are gregarious with social structure whereas the mysticetes do not form fixed groups. The average life of the cetaceans is 20-40yrs with a range of 10-80yrs. Rates of reproduction are low with adults reaching sexual maturity by 2 - 20 yrs (depending on the species) and pregnancy varies 8-16 months. Birth weight is relatively high and the young grow rapidly on rich milk with 16-46% fat (3-5% human). This means that the SA/V ratio is reduced as quickly as possible.
Cetacean prey consumption may exceed the catch of commercial fishing eg they take 2.3x as many squid as fishermen in the Mediterranean. It is estimated that baleen whales in the Antarctic may have consumed 190x106 tonnes krill/yr before whaling: this is 2x the world catch of all marine species.
Whaling for oil, bone & meat grew from 1700s but a major affect on cetacean abundance was seem in the 1900s with increased mechanisation. The International Whaling Commission was set up in 1946 but catches continued to increase. Pre and post-whaling estimates of population numbers indicate that virtually all whale sp were greatly reduced and many spp. are officially endangered. Only the Minke Whale increased probably because it is relatively small, was not heavily exploited and benefited from the reduction in large baleen whale numbers. In 1986 the IWC placed a ban on commercial whaling but some countries eg Norway, Iceland and Japan continue to catch whales for 'scientific research'.
Nowadays it is the smaller cetaceans which are in most danger from fishing gear (dolphin friendly tuna!) and coastal habitat destruction but all cetaceans are susceptible to marine pollution which may reduce their immunity and leave them open to infection. They act to bioaccumulate toxic chemicals.
Part of the lifecycle is terrestrial - ie they move onto ice/land to breed and rest<. Pinnipedia means 'Feather-footed' since they have 4 webbed flippers. Thay are distributed in all oceans but mainly found at high latitudes and live and travel in herds, sometimes carrying out long migrations.
Heat conservation -
Pinnipeds achieve a low SA/V ratio & develop insulation using blubber and hair. Fur seals may have as many as 57000/cm2 falling into 2 types, guard hairs and underfur fibres. They all moult, usually in summer after breeding and some must stay out of water until it is completed.
Respiration -
They exhale prior to deep dives (opposite to whales) but again have relatively high concentrations of haemoglobin and myoglobin.
Diving -
Again the heart rate drops during deep dives and the blood is restricted to the brain & heart. The Weddell Seal can dive to 700m, travel 12km from it's breathing hole & stay under for 82mins while the Southern Elephant Seal has been recorded at 1700m during a 2 hr dive.
Pinnipedes feed mainly on fish and squid with crustaceans, molluscs and zooplankton. The walrus uses tusks to dig for benthic animals. Predators include sharks (eg Great White), Killer Whales and Polar Bears (Arctic). Breathing holes allow hunting far from open water but they attract predators so it is best to have a network of holes in order to avoid predation. If the holes close up then moving between oxygen and food supply is prohibited so the teeth are used to keep holes open but this activity can wear the teeth down so sharing the hole is a compromise between the need to keep it open and the extra competition for food.
Mating occurs annually just after they give birth but pregnancy actually lasts less then one year - development is suspended for a long period after fertilisation. This means that the adults are all together at one period and the females don't come to land separately for mating and to give birth. Pups again grow rapidly on the rich milk.
Pinnipedes were exploited for fur, oil and ivory but most have recovered well (esp in the S.O.) but Monk seals are endangered by fishermen angered by the competition for fish.
Toxins accumulate in the blubber and may reduce immunity resulting in viral infection etc.
Fishing nets and lines are also a potential problem.
The entire lifecycle occurs in water. They are the only herbivorous aquatic mammals
Historical manuscripts indicate that sirenians had widespread distribution but were hunted for meat, oil and hides. They were particularly vulnerable being placid relatively slow moving animals in inshore waters. Nowadays, danger stems mainly from powerboats and habitat loss. They are thought to have been highly social but now tend to be solitary or in family groups.
Dugongs occur in the Indian and Pacific Oceans - strictly marine. They have thin blubber and no heat exchangers in skin.
Manatees are found in tropical Atlantic coastal waters & river systems.
The flat tail produces propulsion with steerage achieved using the paddles. They can move backwards or 'walk' along the bottom in shallow waters. Movement is generally slow but bursts of up to 13knots can be produced.
Sireneans inhale before diving (same as whales). Manatees can dive for 20min and Dugongs for 10min. Like other marine mammals they exchange approx 90% of lung volume when they breath.
Manatees have large lungs running much of the body length which may be used to control buoyancy and the absence of marrow in the long bones makes them very heavy which is postulated to function as ballast.
Bristles around the snout are used to detect food and muscular lips serve to manipulate the plant material. Horny pads at the tips of the jaws break off the food. Like all herbivores they have the problem of breaking down plant cell walls. The stomach is multi-chambered (like cows) but they are actually hind-gut ruminants like horses & elephants (digestion in the large colon). Digestion takes 1 week to complete.
Manatees can see well underwater and smell and taste are retained. They touch when greeting each other and produce chirps especially between mother and calf.
Dugongs live for 70yrs but fecundity is low. They do not reach puberty until 10 yrs old and only produce 1 calf every 5 yrs so their population's ability to recover from hunting or environmental stress is very limited.
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