OCB3034 Main Page

FIU Home

FIU Marine Biology Home

Dept. Biology Home

Frank Jochem Home


Marine Mammals [Heithaus lecture]
Lecture Outline

    Return to the Oceans

  • Mammals have returned to the oceans multiple times
  • All marine mammals are viviparous
  • Newborns are fed by milk from the mother’s mammary glands
  • Have needed to adapt to giving birth and suckling young in the marine environment
  • Staying warm has been a major challenge
  • We’ll take a look at marine mammals from the least to the most adapted to the marine environment


  • For marine mammals (and many of the marine birds and reptiles we talked about), diving is a critical part of life
  • Many activities take place underwater
    • Foraging (the reason to return to the sea)
    • Reproduction
    • Resting
    • Social Behavior
  • Diving abilities vary widely among marine mammals

    Adaptations for diving

  • Exchange a large amount of air on each breath
    • Up to 90% in each breath (humans exchange about 20%)
  • Blood with more oxygen carrying capacity
  • Heart rate slows during diving to reduce oxygen use
  • Blood flow restricted to non-essential body parts
  • Collapsing lungs
    • Dive with no air in contact with blood vessels to avoid problems of nitrogen being forced in

    Polar bears

  • Polar bears are the least adapted to the marine lifestyle
    • Land animals that are adapted to the cold
    • Considered marine mammals because they feed almost exclusively on marine organisms
    • Very good swimmers but can’t dive below surface well
    • Hunt seals and walruses, occasionally cetaceans

    Sea otters

  • Well adapted for life in the ocean
  • Spend much time rafting at surface but somewhat accomplished divers
  • Live in the kelp forests of the North Pacific
  • Densest fur of any mammal helps keep them warm
    • Must be groomed often to stay clean
    • Almost led to their extinction by fur trappers
  • Can give birth at sea – very clumsy on land
  • Eat benthic invertebrates
    • Love slow-moving fish when they are around
  • Use tools to crush shells

    Seals, sea lions, and walruses

  • The “pinnipeds”
  • Descended from bear or raccoon-like ancestor
  • Extremely well adapted to life in the ocean, but they must return to land to give birth
  • Most live in cold waters and stay warm with a layer of fat (blubber) under their skin
    • Fat provides warmth and an energy reserve
  • Some species also have hair that keeps them warm

    True seals

  • Largest group of pinnipeds (19 species)
  • No external ears, cannot push up on foreflippers
  • Use their rear flippers to swim, but have small foreflippers that are not very good for getting around on land
  • Include the best divers of the pinnipeds
    • Elephant seals routinely dive over 1000m and may spend months at sea 1000s of km from their haul out
  • Includes the largest pinniped: elephant seal
  • The only tropical pinnipeds are the monk seals
    • The Caribbean monk seal was hunted to extinction

    Eared seals

  • Includes sea lions (5 sp) and fur seals (9 sp)
  • Have external ear flaps and long foreflippers used in swimming and getting around on land
  • Very agile swimmers but not the greatest breath-holders
  • Adult males tend to be much bigger than females
  • Sea lions are often thought of as a nuisance in fisheries and fur seals were hunted for their fur


  • One remaining species
  • Swim like a true seal but can push up on land
  • Have tusks which help them hold onto the ice when they get up or used in defense
  • Feed on bivalves and other molluscs using suction

    Pinniped Reproduction

  • All species return to land to give birth, usually every year
  • True seals nurse their pups for fairly short periods (days to weeks) and do not leave the beach while they nurse
    • Pups end up fat so fast that they have to hang out on the beach while development finishes and mothers return to the sea
  • Eared seals give birth each year but nurse their pups for months before weaning them
    • Mothers make many trips to sea while still nursing
    • Can recognize their pup by calls and smell
    • By the time they are weaned, pups can swim and dive
  • Walrus pups can swim well and accompany their mother on trips

    Pinniped Mating

  • True seal females mate after they have weaned their pup
    • Many species mate in the water
      • Males display to attract females
      • Bit of a scramble for mates
    • Some mate on land and a small number of males are successful at mating
      • Elephant seal males defend territories were females are resting
      • Gray seal males defend small groups of females that move around
    • Male contests are common
      • Rarely become serious fights
  • Eared seals mate just after they give birth
    • Males form territories where many females gather to give birth
      • Very few males can monopolize mating opportunities
    • Some territories may guard other resources for females
      • Cool-down spots

    Pinniped Foraging

  • True seals tend to feed on slower prey like cephalopods, often at great depths
    • Not super-fast and maneuverable swimmers
    • Crabeater seals feed almost exclusively on krill
  • Eared seals are feed on fast-moving fish but not that deep
  • Trade-off: speed for diving abilities
  • Some species of seals and sea lions forage far offshore and tend to forage at night when prey comes close to the surface
  • Prey taken varies greatly with general foraging habitat but fish and cephalopods are the most commonly taken prey
  • Most pinnipeds show amazing flexibility in prey choice, but will switch among prey items based on availability and quality
    • Harp seals always prefer capelin, only take Arctic cod in nearshore waters
    • harbor seals off Scotland feed on most abundant prey but prefer fish 10-16cm
    • Even the seal-eating leopard seal switches to krill when it is abundant

    Dugongs and Manatees

  • Most complete transition to marine life along with whales and dolphins
  • Related to the elephant, but common ancestor didn’t look like either of them
  • Once many more species around
  • Swim with up and down movements of the tail
  • Large layer of blubber
  • Origin of the mermaid myth
  • Manatees have paddle-like tails and frequent freshwater
  • Dugongs are exclusively marine and have a dolphin-like tail
  • Herbivores – almost exclusively eat plants
    • Manatees tend to crop and grab with prehensile lips
    • Dugongs tend to dig seagrass rhizomes
  • Nostrils on top of snout have valves to keep water out
  • Both species have one calf at a time
  • Tend to have a single calf every 3 years
  • Manatees are larger than dugongs
    • Few predators of manatees but dugongs at risk from tiger sharks


  • Most complete transition to marine life along with dugongs and manatees
  • Largest group of marine mammals
  • Streamlined bodies for slipping through the water easily
  • Still have a front pair of flippers, but hind limb has been lost (still present in embryos)
    • Bones of front flipper resemble our hand

    Structure of Cetaceans

  • Tail ends in a fluke that is moved up and down to swim
    • Sometimes leap through the air to minimize drag at high speeds
  • Breathe through a blowhole
    • Blow shape and direction can be used to determine species
  • Many species have a dorsal fin
  • A layer of blubber keeps them war


  • Whales, dolphins, porpoises
  • Two major groups now living
    • Baleen whales (Mysticetes)
    • Toothed whales (Odontocetes)

    The origin of cetaceans

  • The first recognizable whales come from over 50 million years ago
    • Pakecetus – earliest whale; India and Pakistan
    • Ear morphology gives them away as cetaceans
    • Lived in an arid environment with ephemeral streams and floodplains
    • Mainly land animals that waded in streams (poor swimmers)
  • Ambulocetids
    • Better swimmers than packicetids
    • Likely slow on land
    • Elongated hind feet and tail that would aid in locomotion
    • Probably ambush hunter like modern crocodiles
  • Basilosaurids
    • Starting to look like a whale!
    • Very reduced hind limbs – fully aquatic
      • Basilosaurus grew to 25m
    • Lived in the tropics and subtropics
    • Ate fish
  • Dorudontids
    • Dolphin-like and smaller than basilosaurids
    • Had a fluke and swam like a modern cetacean
    • Likely ancestors of modern whales and dolphins

    Modern cetaceans: baleen whales

  • Largest of the marine mammals
    • Blue whale might be the largest animal to ever live (possibly up to 22m, 110’ long)
    • Three major types of baleen whales
      • Right whales and bowheads have large, arched mouths and are very rotund
      • Rorqual whales (blue, humpbacks) have throat grooves and are streamlined for speed
      • Gray whales are somewhat intermediate
  • Have no teeth, but use baleen to capture prey
    • Triangular plates with frayed trailing edges that form a sieve
    • Hang from the upper jaw
    • Made of keratin (same as fingernails and hair)
    • Size and roughness of trailing edge relate to prey (bigger strainer for bigger prey)

    Baleen whale feeding: batch feeders

  • Three types of batch feeding: Skimming
    • Mainly right and bowhead whales (sometimes gray whales)
    • Gap between left and right baleen rows
    • Feed at surface and throughout water column
    • Tongue sweeps prey from inside of plates
    • Baleen may be over 4m tall!
  • Three types of filter feeding: Gulping
    • Engulf patches of prey and water
    • Throat pouch inflates to increase volume

    The rorqual jaw

  • Jaw unlocks allowing 30-90º opening
    • Water rapidly fills mouth because of high swimming speed
    • Timing of opening critical – too soon and bow wave pushes prey away
  • Front of jaw disarticulates
  • Lower jaw disarticulates
  • Water dumped from the back corners of the mouth

    Baleen whale feeding

  • Three types of filter feeding: Gulping
    • Humpback whales use bubbles to help concentrate prey
    • Sometimes feed in large groups and use loud sounds to scare herring
  • Three types of filter feeding: Suction Feeding
    • Gray whales suck huge amounts of fine sediment and invertebrates then filter out sediment and water to keep food in
  • Baleen whales are not great divers compared to true seals and toothed whales
  • Diet of baleen whales
    • Rorquals tend to eat krill, especially in the Southern Ocean, but some species have a large portion of fish in their diets
    • Right whales filter finer prey like copepods
    • Gray whales feed on bottom invertebrates including amphipods and ploychaetes
  • In general, filter feeding allows feeding near base of food chain
    • Allowed high abundance and wide distributions of whales pre-exploitation
      • 90% of energy lost with each step in the food chain

    Baleen whale reproduction

  • Reproduction in most baleen whales involves long migrations between feeding and breeding areas
  • Reproduction tends to occur on a two year cycle
  • During Year 1 females conceive on the breeding grounds then migrate toward feeding area
  • Calf born after 11-12 months near breeding area
  • Nursed for 6-10 mo during trip to and from feeding ground
  • During migrations, young gray whales and females with their calves stay close to shore to avoid running into killer whales

    Toothed whales

  • Includes a wide range of species from the sperm whale to small porpoises
    • The book is wrong! Sperm whales are more closely related to other toothed whales than the baleen whales!
  • Wide variety of diving abilities from species that dive for a couple minutes to shallow depths to sperm and beaked whales diving for over an hour to more than 1000-1500 m


  • Toothed whales can use sound to “see” their environment
  • Produce clicks that travel out, hit objects and reflect back
  • Produced by a structure in the airway called the “monkey lips”
  • Sound received through the lower jaw
  • Low frequency clicks travel further but can only be used for big objects
  • High frequency clicks can discriminate small objects but don’t travel as far

    The big bang hypothesis

  • Suggestions that toothed whales can stun prey with sound
  • Currently no experimental support for hypothesis but it is unlikely
    • Total energy low
    • Disorientation of fish in schools more likely due to reduced oxygen level
    • Why chase a fish if you can just stun it?
    • Suction feeding is a better explanation of many observations that supposedly provide “support”

    Other sounds

  • Many species of dolphins produce complex whistles that are used in communication
    • In some species, each individual has its own signature whistle
    • We still don’t know how much information is conveyed in these whistles

    Some types of toothed whales

  • Sperm whales
    • Tend to be found in deep ocean waters
    • Dive to great depths to hunt squid
    • Females live in stable groups with relatives
    • Males are found in bachelor groups (when smaller) and then alone as large adults
    • Males are much larger than females (15 m vs 11 m)
  • Beaked whales
    • Poorly known group of large toothed whales usually found far offshore in oceanic waters
    • Thought to dive deep to eat squid and bony fish
    • Some species are known only from one or two individuals that have washed up on beaches
    • Teeth are lost in many species – use suction to catch prey
      • Some have teeth for display or battles
  • Beluga whales and narwhal
    • Found in northern waters
    • Male narwhal have a tooth modified into a tusk used in male battles and/or to impress females
    • May use water jets to excavate benthic invertebrates
  • Dolphins
    • Most common group of toothed whales
    • Some species live close to shore in small groups while others are found in groups of thousands in deep waters
      • Bigger groups are probably to provide protection from predators like sharks
    • Includes some “whales” like killer whales and pilot whales
  • Porpoises
    • Six species of small toothed whales
    • Have spade-shaped teeth rather than peglike teeth of dolphins
    • Do not have a beak like dolphins do

    Toothed whale feeding

  • Use a wide variety of tactics to capture prey from small fishes and squids to the largest marine mammals
    • A number of species herd schools of fish together and then pick them off by swimming through or whacking them with their flukes
    • Some will catch prey on land or use the land to make prey capture easier
  • Some cetaceans use tools
    • Sponges to protect the snout
    • Bubbles to capture prey
  • Many species will forage only during tides or times of the day when prey is easy to catch
    • Spinner dolphins rest nearshore during the day then move into oceanic waters to feed on migrating prey at night – may capture 1000 prey items a night!

    Toothed whale reproduction

  • Reproduction is not usually as highly seasonal as it is in baleen whales
  • Calves are able to swim at birth and nurse from their mother for a long period of time
    • 8-24 months in many species but 3-6 years in bottlenose dolphins of Shark Bay
    • Records of 13 year old sperm whale suckling
  • While one calf is nursing, female will not have another one
  • Females may go 3+ years between calves!

    Marine Mammal communities

  • Important parts of many communities and interactions among species may be complex
    • In Antarctica Removal of cetacean biomass (45 million tons to 9 million tons) released ~150 million tons of krill
    • Led to huge increase in crabeater and Antarctic fur seals as well as penguins and possibly minke whales

    The role of marine mammals

  • Because marine mammals are warm blooded and need lots of food to support their high metabolisms, they have the potential to have a major impact on marine communities
  • Sea otters are a keystone species in kelp forests
    • When they are removed the whole community changes – even the kelp disappears
    • Occurs because otters control urchin populations that graze on kelp
  • In Arctic ecosystems, benthic foraging by walrus and gray whales may increase productivity of benthic habitats
    • Gray whales may turn over 9-27% of northern Bering sea substrate annually
  • Dugong grazing can alter the biomass and species composition of seagrass beds
    • Grazing helps fast-growing species

    Marine mammals and fisheries

  • Generally thought of as competitors with fisheries and are sometimes culled
  • May not always harm fisheries
  • Collapse of Atlantic cod was largely unforeseen, perhaps because of cod behavior and their responses to seals
  • As cod collapsed, may have formed bigger schools to stay safe from predators like harp seals