9.3 Marine mammals: cetaceans, pinnipeds, and sirenians

Marine mammals, including whales, seals, and manatees, evolved from land-dwelling ancestors millions of years ago. They've developed incredible adaptations for life in the water, like streamlined bodies, blubber for insulation, and specialized respiratory systems for deep diving.

These animals play crucial roles in ocean ecosystems as top predators and nutrient cyclers. However, they face threats from human activities like pollution and fishing. Conservation efforts aim to protect these fascinating creatures and the vital ecological functions they perform.

Evolutionary Origins and Adaptations of Marine Mammals

Groups of marine mammals

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• Cetaceans include whales, dolphins, and porpoises
  • Evolved from terrestrial artiodactyls, even-toed ungulates, around 50 million years ago
  • Closest living relatives are hippopotamuses (hippos)
• Pinnipeds include seals, sea lions, and walruses
  • Evolved from terrestrial carnivores around 25-30 million years ago
  • Divided into three families:
    • Phocidae - true seals
    • Otariidae - eared seals (fur seals and sea lions)
    • Odobenidae - walruses
• Sirenians include manatees and dugongs
  • Evolved from terrestrial herbivores around 50 million years ago
  • Closest living relatives are elephants and hyraxes (small, rodent-like mammals)

Adaptations to aquatic life

• Streamlined body shape enables efficient swimming through water
• Reduced or absent external ears and hind limbs minimize drag while swimming
• Thick layer of blubber provides insulation in cold waters and stores energy reserves
• Specialized respiratory system allows for extended breath-holding during dives
  • Lungs and ribcage collapse under pressure at depth
  • Efficient extraction of oxygen and storage in muscles and blood
• Enhanced senses are adapted for underwater navigation and communication
  1. Cetaceans have acute hearing and use echolocation
  2. Pinnipeds and sirenians have sensitive whiskers (vibrissae) to detect vibrations and movement
• Diving adaptations enable prolonged, deep dives to forage and escape predators
  • Heart rate slows and blood is shunted away from non-essential organs to conserve oxygen

Social Structures, Communication, and Reproduction

Social behavior of marine mammals

• Cetaceans often live in complex social groups called pods with strong familial bonds
  • Killer whales (orcas) stay with mothers for life in matrilineal pods
  • Bottlenose dolphins form fission-fusion societies that change composition
• Pinnipeds vary from solitary to highly social depending on the species
  • Elephant seals are solitary at sea but aggregate in breeding colonies
  • Sea lions form large rookeries with harems defended by males
• Sirenians are generally solitary or live in small, loosely associated groups
  • West Indian manatees may congregate seasonally near warm water springs
• Marine mammals communicate using a variety of vocalizations and non-vocal cues
  • Cetaceans produce clicks, whistles, and complex songs (humpback whales)
    • Also use echolocation for navigation and prey detection
  • Pinnipeds vocalize, use body postures, and scent mark territories
  • Sirenians use vocalizations and gentle touch between individuals
• Reproductive strategies are adapted for marine life but vary by species
  • Delayed sexual maturity and long gestation periods are common
  • Low reproductive rates and high parental investment ensure offspring survival
  • Mating systems include monogamy, polygyny (elephant seals), and promiscuity

Conservation and Ecological Roles

Conservation of marine mammals

• Marine mammal populations face numerous anthropogenic threats
  • Habitat loss and degradation from coastal development and pollution
  • Chemical pollution (oil spills), noise pollution, and marine debris entanglement
  • Bycatch and entanglement in commercial fishing gear
  • Ship strikes from vessel collisions, especially in high-traffic areas
  • Climate change impacts like rising sea levels and ocean acidification
• Management strategies aim to protect and restore marine mammal populations
  • Establishment of marine protected areas (MPAs) and reserves
  • International agreements regulate whaling, trade, and other threats
    • International Whaling Commission, Marine Mammal Protection Act
  • Monitoring programs track population trends and identify emerging threats
  • Research studies provide data to inform conservation decisions
  • Public outreach and education promote awareness and support for conservation

Ecological roles in ocean ecosystems

• Marine mammals are top predators that structure marine food webs
  • Cetaceans consume fish, squid, and other prey at various trophic levels
  • Pinnipeds feed on fish, squid, and crustaceans in benthic and pelagic habitats
  • Sirenians are primarily herbivores that graze on seagrasses and algae
• Nutrient cycling and distribution are influenced by marine mammal behavior
  • Whale pump effect: release of nutrient-rich fecal matter at the surface stimulates phytoplankton growth
  • Benthic-pelagic coupling: transfer of nutrients between the seafloor and water column during foraging dives
• Some marine mammals act as ecosystem engineers that modify habitats
  • Sirenian grazing helps maintain seagrass meadow structure and diversity
  • Sea otter predation on sea urchins promotes kelp forest growth and stability