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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

  Diving

• 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

  Walrus

• 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

  Cetaceans

• 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

  Cetaceans

• 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

  Echolocation

• 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