Question

Describe the features shared by all mammals. Then, differentiate between the reproductive strategies of Monotremes, Marsupials, and Placentals. Do you think the life history of any of these groups is superior to the others? Why/why not?

Answer 1

Answer:

Among the large variety of life on the earth Mammals constitute around 6000 different species. Class Mammalia has following common features:

1. They are all Vertebrates.
2. They differ from other Vertebrates by virtue of presence of hair on the body / fur.
3. They are warm blooded animals.
4. They have mammary gland that produce milk, feeding on which their babies grow in their initial life.
5. They have three bones in the inner ear.

The class Mammalia is further divided into sub classes Monotremes; Marsupials and Placentals. This subdivision is hugely based on their reproductive strategies and how their Babies grow. The major differences among these classes are explained under:

Monotremes:

• Monotremes are only mammals that lay eggs.
• There are only 5 monotremes surviving today - 4 species of echidna or spiny ant eater and one of Platypus.
• These mammals differ a lot from other mammals. They have the lowest body temperature among the warm blooded animals (a feature they share with reptiles).
• They pass their body wastes and their eggs through the same opening, hence the name monotremes.
• Echidnas look like small balls of spikes as they have sharp spines scattered throughout their fur. They lay eggs in the pouch on the mother's belly. Baby hatches in the pouch and stays there for up to 3 weeks nursing on mother's milk. Then baby starts to develop spines and mothers scratches her out of the pouch.
• Platypus have duck like bills but their bills don not open into mouth. They have very soft fur on their body. They live on water and mother lays her eggs in nest made in the muddy banks. Here baby's hatch and feed by licking milk that oozes out of fur on the mother's belly.

Marsupials:

• These mammals give birth to Babies.
• At birth the Babies are not fully developed.
• After birth these Babies climb into an external pouch on the mother's belly, where in mammary glands (milk glands) are located.
• Babies attach themselves to these glands and nurse until they get fully developed.
• In a way Marsupials do not rely on placental nourishment of their Babies.
• Examples - Koalas, Kangaroos, Opossums and Wallabies

Placentals:

• Most mammals fall into this group, it is most diverse subclass having around 4000 known species.
• The name comes from the fact that Babies are attached to the mothers through placenta in the womb. All the nourishment of baby is done through this placenta. Whatever waste baby excretes is passed on to mother and she eliminates through her body functions.
• The babies are born only after their body and body systems are fully developed.
• After birth baby's are nursed by their mothers, feeding on milk expressed through mammary glands for varying amount of time depending on species.
• Humans belong to this group.

Life story of any of the animal is superior or inferior is very difficult to say or judge. It depends on what we classify as superior. Each living animal has developed its own way to survive and have done it so far. Among all animal life on planet earth, humans have diversified most by providing for their needs under all climatic conditions due to their superior developed brain. The intelligence and ability to think makes humans superior to other life forms but at the same time it humbles us to know through our knowledge that other animals also have developed and survived so far. The same capacity to think also puts onus on us to know that all life on earth is interrelated and we must learn to live with all kinds of life forms around us because we need them as much as they need us.