MAMMALS 2/17

Overview of Marine Mammal Biology

Introduction

  • Discussion on marine mammal anatomy and physiology focusing on circulatory and reproductive systems.

  • Comparison between marine mammals and terrestrial mammals.

Circulatory System

Heart

  • The heart is the central feature of the circulatory system.

  • Anatomical similarity to terrestrial mammals with some differences.

  • Size and structure:

    • About 1% of body mass in pipettes (small marine mammals) and 0.5% in larger species like whales.

    • Typical mammalian heart structure with adaptations for marine life.

Structurally Adapted Circulatory Features

  1. Glycogen Stores

    • Seals exhibit enlarged glycogen reserves allowing for greater anaerobic metabolism which improves oxygen storage during dives.

    • Enhances endurance in low-oxygen conditions.

  2. Hepatic Sinus

    • Blood is stored in the hepatic sinus which is located posterior to the diaphragm.

    • Blood passes through the caval sphincter allowing control of blood flow to the heart, preventing overload during rapid movement.

    • Elastic properties of the hepatic sinus modify the size for blood storage.

  3. Superior Posterior Vena Cava

    • Well-developed with the ability to fill with more blood compared to typical structures.

    • Functions similarly to terrestrial blood vessels but is tailored to marine mammals' needs.

Advanced Blood Vessel Structures

  1. Media Neurobilia

    • Latin for "wonderful nets"; describes a complex network of blood vessels, allowing increased blood storage in small spaces.

    • Primarily arteries with thin-walled veins.

    • Functions include:

      • Regulating blood flow.

      • Oxygen conservation.

      • Shunting blood during dives to prioritizing vital organs (brain, heart).

      • Assisting in pressure changes experienced in deep dives.

  2. Countercurrent Heat Exchange

    • Mechanism where warm blood from arteries flows parallel to cooler blood in veins, minimizing heat loss and regulating core body temperature.

    • Blood warmers as it returns to the body’s core, preserving warmth in cold aquatic environments.

    • Important for thermoregulation during dives and heat dissipation through extremities (fins, flippers) in overheating scenarios.

  3. Arteriovenous Anastomosis (AVA)

    • Specialized blood vessels bypassing capillary networks, allowing direct flow regulation and temperature management.

    • Important for thermal regulation and supporting oxygen flow during diving events.

    • Found in most marine mammals but absent in sea otters and some pinnipeds.

Blood and Oxygen Storage

  • Oxygen Transport:

    • Utilizes hemoglobin efficiently to bind oxygen in larger volumes.

    • Adaptations confer greater anaerobic capacity than terrestrial mammals, crucial for underwater life.

Digestive System

Differences in Stomach Structure

  1. Complex Stomach Anatomy in Cetaceans

    • Cetaceans have multi-chambered stomachs (often described as four chambers or two to three distinct stomachs).

    • Stomach function:

      • Fore stomach: Non-glandular with muscular movements.

      • Fundic stomach: Main area for chemical breakdown with gastric glands.

      • Connected stomach: Considered by some as an additional chamber, facilitating transit between stomachs.

      • Pyloric stomach: Final digestion before entry into intestines.

    • Highly efficient for nutrient absorption from whole prey items.

    • Estimated that small intestines can be as long as 8-25 times the body length depending on species.

  2. Monogastric Stomach in Pinnipeds and Other Species

    • Simpler structure resembling terrestrial carnivores with significant adaptations related to their diets.

    • Digestive capabilities include chewing (in pinnipeds) and the presence of extensive compartmentalization for digestion.

Importance of the Liver and Kidneys

  1. Liver Functionality

    • Larger volume and greater density of red blood cells in marine mammals lead to well-developed livers capable of recycling blood cells efficiently.

    • Cetacean livers consist of five lobes and include functional redundancy without a gallbladder.

  2. Kidney Adaptations

    • Marine mammals possess specialized hydroosmotic kidneys adapted to filtering seawater and concentrating urine.

    • Advanced structures in kidneys (multilobed system) provide high filtration capacity necessary for their aquatic lifestyle.

    • Example: Dolphin kidneys can have hundreds of renal tubules to optimize filtration and concentration of urine.

Water Acquisition Mechanisms

  • Marine mammals primarily obtain water from the food they consume, such as fish or squid, which possess high water content (60-80%).

  • Metabolic processes also contribute to water production, particularly through fat metabolism, yielding 1.7 grams of water per gram of fat.

Reproductive System

Female Reproductive Anatomy

  1. Similarities to Terrestrial Mammals

    • Basic anatomy includes two ovaries and a branched uterine structure.

    • Unique features such as pseudocervices facilitate sperm storage and protection from seawater.

  2. Ovary Changes and Functionality

    • Examination of ovary structure and changes through reproduction cycles provides insights into age and reproductive history.

    • The left ovary typically dominates in egg production, while the right serves as a backup.

Male Reproductive Characteristics

  1. Larger Testes

    • Male cetaceans have significantly enlarged testes (7-25 times larger than terrestrial mammals) associated with their mating strategies.

    • Functionality focused on maximizing reproductive success in aquatic environments.

Conclusion

  • The anatomy and physiology of marine mammals exhibit specialized adaptations to their aquatic lifestyle, encompassing circulatory, digestive, and reproductive systems that allow them to thrive in the oceanic environment.