Mammalia: Egg laying Mammals and Marsupials | Zoology Optional Notes for UPSC PDF Download

Egg-Laying Mammals: Monotremes

• Mammals are vertebrates characterized by backbones, hair or fur, warm-blooded nature, and four-chambered hearts.
• They nurture their young with milk and typically give birth to live offspring.

Monotremes: Mammals that Lay Eggs

1. Common Characteristics:

   • Endothermic with a high metabolic rate.
   • Hair on their bodies.
   • Produce milk through mammary glands.
   • Single bone in the lower jaw.
   • Three middle-ear bones.

2. Distinctive Features:

   • Lack the corpus callosum (connective structure) found in placental mammals.
   • Anterior commissure carries all neocortex commissural fibers.
   • Extant monotremes (spiny anteater and platypus) lack teeth as adults.

3. Reproduction and Development:

   • Spiny Anteater (Echidna) lays a leathery-shelled egg in her pouch, where it hatches after 10-11 days.
   • Platypus lays eggs in an underground nest and feeds tiny babies with milk.
   • Echidna and Platypus are unique with one body opening for wastes and eggs.

4. Distribution:

   • Monotremes (platypus and spiny anteater) are native to Australia, Tasmania, and New Guinea.
   • Notable for lower body temperature compared to most warm-blooded animals.

Tooth Structure and Evolutionary Insights

• Extant monotremes lack teeth as adults.
• Fossil forms and platypus young exhibit "tribosphenic" molars, suggesting independent evolution.

Venomous Characteristics

• Platypus is one of the few venomous mammals.
• Male platypus has a hind foot spur delivering a painful venom, lethal to small animals.

Marsupial Mammals

Introduction to Marsupials

• Marsupials belong to the mammalian infraclass Marsupialia, endemic to Australasia and the Americas.
• A defining feature is the pouch in which most young are carried during early development.

Reproductive Characteristics

1. Birth and Development:

   • Birth to relatively undeveloped young.
   • Young reside in the pouch on mothers' abdomens.
   • Marsupials give birth to tiny babies that crawl into the pouch.

2. Pouch Nourishment:

   • Babies drink milk from the mother's pouch.
   • Development continues inside the pouch.

3. Distribution:

   • About 70% of the 334 extant species occur in Australia, Tasmania, New Guinea, and nearby islands.
   • Remaining 30% found in the Americas, primarily in South America, Central America, and North America.

Well-Known Marsupials

1. Kangaroos and Wallabies:

   • Iconic hopping marsupials.
   • Develop and nourish young in pouches.

2. Koalas:

   • Arboreal marsupials.
   • Babies crawl into pouches, where they are fed and continue to develop.

3. Opossums:

   • Only marsupial in North America.
   • Give birth to multiple babies but only the first thirteen to climb into the pouch survive.

Marsupials in North America

• The opossum is the sole representative of marsupials in North America.
• Can give birth to as many as twenty-one babies, but only thirteen can be sustained due to the limited number of nipples.

Conclusion

• Monotremes and marsupials showcase the diverse reproductive strategies among mammals.
• Monotremes are characterized by egg-laying, while marsupials demonstrate unique pouch-based nurturing of underdeveloped young.
• The distribution of these mammals highlights the evolutionary and geographical diversity within these two groups.