Term 2 Lecture 6: Marine Mammal Adaptations Flashcards
Definition of marine mammals
A functional not taxonomic grouping of organisms including whales, seals, manatees,polar bears, sea lions, otters etc.
All in the placental mammal group descendents of the first mammal morganucodon from the late Triassic c.245 MYA.
Morganucodon was haired and warm blooded with heterotrophy in teeth (molars, canines etc )
Mammal radiation occured after dinosaurs went extinct and became 3 groups: marsupials, placental mammals and monotremes.
Evolutionary challenges
All marine mammals are if terrestrial origin so they breathe air. They are homeothermic meaning they produce their own heat.
How have they adapted?
How have their mammalian origins influenced this?
Depth - pressure/cold/ darkness - a challenge to communicate in the dark
Keep warm and retain oxygen
Water - challenges movement and food finding at depth, need endurance
Birth and nursing - they give birth to live young and feed them by lactation this is also challenging in water
Marine mammals are of diverse multiple evolutionary origins
They are all adapted differently to the marine environment
Sea otters and polar bears quite recently evolved 5 MYA
Pinnipeds - seals sea lions and fur seals evolved 20-25 MYA
Sirenians - manatees and dugongs
& Cetaceans - whales and dolphins
Evolved 55 MYA
Cetaceans and sirenians are 100% marine, streamlined with a loss of hind limbs - complete adaptation to water
Polar bears and seals retain terrestrial forms as they relatively recently evolved to be marine
2 main categories of cetaceans
Mysticete cetaceans - Baleen whales have plates of Baleen adapted from hair that they use to filter out food when they take huge gulps of water containing shoals of fish or krill in order to forage
Odontocete cetaceans - toothed whales including dolphins, orcas, belugas etc
Cetaceans are most closely related to artiodactyls such as giraffes but consider other species in this clade e.g. hippos and similarities can be observed
3 main groups of pinnipeds
Phocids - true seals, very short fur, pelvic girdle does not allow rotation of hind flippers up and under body so to move on land they drag their bodies by their forelimbs. Phocids can also dive deeper and longer than otariids.
Otariids - sea lions and fur seals - can still articulate their hind limbs to stand on all fours to gallop and run on land, have longer fur, sometimes called eared seals as they have small external ear flaps
Odobenids - walruses, one or two species with some similarities to both phocids and otariids
Pinnipeds evolved from caniforms a group that includes canids, otter like species and bears (ursids)
Believed to be a monophyletic group all descended from one common bear ancestor
Morphological adaptations
Streamlining- seen in all marine mammals more so in cetaceans than otariids, but all are more streamline than terrestrial mammals.
This can be seen in skeletal structure, conformation of their internal organs and all are wrapped in blubber to create a smooth torpedo shaped body efficient at travelling through water.
Marine mammals and many aquatic animals have a localisation of all their sensory organs to the anterior end at the top of the head - so the animals need only to expose a small part of its head to breathe/observe surroundings
Evolution of cetaceans from ungulates
Change in limb confirmation - walking on all fours on land to using hind limbs to walk and paddle, then to using hind limbs to paddle and swim to finally no hind limbs at all e.g. orcas.
The forelimbs are adapted to be fin-like but retain mammalian bone structure.
Some cetaceans have vestigial bones - free- floating within the muscle tissue at the rear but no longer functional hind limbs. So hind limbs and pelvic girdle are lost/rudimentary
Skull nostril evolution in cetaceans
Has evolved to be further back up on the skull eventually becoming a blow hole above the eyes.
whale dentition (teeth)
Odontocete whales dentition reverted back to homodonty - uniform peg-like teeth to catch slippery prey like squid and fish that are swallowed whole as chewing is not possible underwater
Mysticete whales have replaced their teeth entirely with filtering baleen. Their pleated throat grooves, almost like a pelican bill, allow the throat to bulge out to take in huge volumes of water, their strong tongue pushes water out through the Baleen and forms a bolus of food to swallow
Marine mammal movement
Marine mammals undulate when they move due to their mammalian origin - compare a running cheetah with an orca
Whereas fish have a lateral side to side movement in water a sinuous motion that is retained in reptile movement but was lost in mammalian forms.
Keeping warm
Hair or fur (pelage) is a diagnostic mammalian characteristic, uniquely mammalian and found on all mammals at some point. Hairs are made up of dead epidermal cells strengthened by keratin.
Fur is only useful for insulation if it is a complete coat so fur probably evolved via sensing and signalling uses such as whiskers.
Hair for thermal regulation can be seen in the dense oily fur that sea otters fluff up for extra insulation
The same can be seen in polar bears and otariids - the trapped air provides a barrier in the water - to a certain depth
If you dive very deep then the water pressure expels the air that was trapped between the follicles - insulatory capacity is lost and the hair lies flat.
Hence deep diving marine mammals (cetaceans) have lost all their hair and phocids (deeper diving pinnipeds) have only a thin fur layer more useful for streamlining - with no real insulatory capacity.
Instead they have a thick layer of blubber (subcutaneous fat) to maintain optimal body temperature even when deep diving or resting on ice. Blubber also acts as an energy store.
Size for diving
Largeness also provides beneficial volume to surface area ratio. Larger volume also means more blood capacity and therefore higher potential to store oxygen for deep diving
Reproduction and nursing young
Pinnipeds have duality in life, hunting in the water, resting and raising young on land.
They are vulnerable to predation on land being incapable of swift movement - especially pups.
So they breed in placed relatively predator free e.g. offshore islands and coves bound by cliffs - behavioural adaptation to life at sea.
Cetaceans are born able to swim independently immediately and must surface to take their first breath immediately after birth.
Lactation: milk is produced by modified sweat glands (apocrine glands) in all mammals and this is the reason for maternal care and polygamy in mammals as only mother’s produce milk.
Nursing is easier for pinnipeds and more complex for cetaceans
Cetacean mammary glands are hidden in mammary slits near the sexual organs and are kept closed to protect the nipple until the calf nudges against the slit causing the mammary gland to evert. The calf uses its tongue to form a tube like a straw that it wraps around the mammary gland to drink.
Usually calves feed in many short bursts rather than a long period of suckling (as seen in terrestrial mammals and phocids) this is due to the fact the calf must surface regularly to take breaths
Phocids and otariids have different strategies to cope with spatial and temporal separation of breeding and feeding
Phocids are capital breeders, they lactate for a short period after the calf is born. They nurse with fat rich milk ftom about one week to one month nonstop losing a lot of body mass in the process by transferring it to the pup.
Otariids are income breeders, every now and then after their pup is born they leave it in a ‘creche’ with other pups to forage at sea to fatten up. So otariids breed near reliable food sources alternating feeding and nursing.
Otariids can nurse for 100 days up to a year and a half.
The phocid approach is very different, they stock up on fat storing it as blubber so that when they give birth they can stay with the pup essentially fasting and pumping very energy rich milk into their pup - a much faster process.
This way elephant sealpups are weaned after one month.
Capital breeding grey seal milk is 60% fat , 10% protein and 30% water.
Comparatively human milk is just 4% fat, 3% protein and 93% water.
A grey seal pup can be weaned in 20 days from a birth weight of 16-20kg up to 60kg at weaning. In the proces the mother loses 1/3 to 1/2 of her body weight.
Hooded seals achieve the same weaning weight in just 4 days. This is an adaptation to the unstable environment as they birth on ice floes that exist for limited time periods
Adaptations of cetaceans and pinnipeds for deep diving
Marine mammals relying on fur to trap air for insulation are shallow divers.
Elephant seals can dive over 1500m and stay at depth for over 1hr a sperm whale can dive up to 2000m though for not as long as an elephant seal
