Monotremes are one of the most unique and ancient groups of mammals, distinguished by their ability to lay eggs instead of giving live birth. Found primarily in Australia and New Guinea, monotremes include just a few species, notably the platypus and the echidna. These odd creatures represent a rare evolutionary link between reptiles and mammals, having retained some reptilian characteristics while also exhibiting traits typical of mammals, such as lactation.

Characteristics of Monotremes

Monotremes are mammals that belong to the order Monotremata. Unlike most mammals, which give birth to live young, monotremes lay eggs, a feature that aligns them with their reptilian ancestors. However, monotremes still share several mammalian traits, including:

• Like other mammals, monotremes regulate their body temperature internally.
• Monotremes are covered in hair or fur, a key feature of mammals.
• Female monotremes produce milk to feed their young, but unlike other mammals, they do not have nipples. Instead, the milk is secreted through patches of skin and the young lick it off.
• The term “monotreme” comes from the Greek words “mono” (one) and “trema” (hole), referring to the cloaca—a single opening for excretion and reproduction, which is a characteristic shared with birds and reptiles.

Their Evolution and How They Differ from Other Mammals

Monotremes are considered some of the most primitive living mammals, having split from other mammalian lineages approximately 150 million years ago during the Jurassic period. Their evolutionary path diverged significantly from marsupials and placental mammals, resulting in the retention of certain reptilian features, such as egg-laying.

However, monotremes also developed key mammalian traits, such as the ability to produce milk, though they do so without the use of nipples, unlike marsupials and placentals. They also have unique reproductive strategies. For instance, monotreme eggs are leathery, similar to those of reptiles, and the young hatch in a relatively underdeveloped state, requiring extensive care.

Monotremes are also unique in their sensory abilities. For example, the platypus has a remarkable adaptation called electroreception, which allows it to detect electric fields generated by the movements of prey in water. This trait is not found in other mammals, showcasing the distinct evolutionary path of monotremes.

The Unique Biology of Monotremes

Monotremes, including the platypus and echidna, exhibit some of the most unusual biological traits among mammals. From their egg-laying reproductive process to their specialised adaptations, these creatures stand out in the mammalian world for blending both ancient and modern features.

Egg-Laying Process

Unlike most mammals, monotremes are oviparous, meaning they lay eggs rather than giving birth to live young. This trait is more commonly associated with reptiles, highlighting the monotreme’s ancient evolutionary roots. The process is unique among mammals and involves several distinct stages:

• Female monotremes lay small, leathery-shelled eggs. Platypuses lay between 1 to 3 eggs, while echidnas lay a single egg. The female platypus incubates her eggs in a burrow, while echidnas carry their eggs in a temporary pouch on their abdomen.
• After being laid, the eggs are incubated for around 10 to 11 days (in platypuses) or 10 days (in echidnas). During this period, the female keeps the eggs warm by curling around them in a nest or pouch.
• Once hatched, the young are extremely underdeveloped, similar to marsupials. They rely entirely on their mother for warmth and nourishment.

Special Adaptations

Monotremes are equipped with several remarkable adaptations that help them thrive in their environments.

1. Electroreception in Platypus

One of the most fascinating adaptations of the platypus is its electroreception ability. Electroreceptors located in the platypus’s bill allow it to detect the electric fields generated by the movements of prey, such as shrimp and small fish, underwater. This sensory system gives the platypus a significant advantage when foraging in murky or dark waters, as it can locate food without relying on sight or sound.

• As the platypus swims with its eyes and ears closed, it sweeps its bill through the water. The electroreceptors pick up on tiny electrical impulses produced by the muscle contractions of its prey, allowing the platypus to pinpoint their location with incredible precision.
• This makes the platypus an expert hunter in low-visibility environments, such as muddy riverbeds or in areas with fast-moving water.

2. Spines of Echidnas

The spiny exterior of the short-beaked echidna provides it with excellent protection from predators. These spines, made of hardened keratin (the same material as human fingernails), cover the back and sides of the echidna, deterring predators like dingoes and birds of prey.

• When threatened, the echidna can curl into a ball, exposing only its sharp spines, making it difficult for predators to attack. Echidnas can also burrow quickly into the ground, using their spines and strong claws to protect themselves further.
• In addition to providing defence, echidna spines and their strong forelimbs are adapted for digging into ant and termite nests, their primary food source.

Lactation Without Nipples: How Monotremes Nourish Their Young

One of the defining characteristics of all mammals is the ability to produce milk, and monotremes are no exception. However, their method of lactation is highly unusual compared to other mammals.

• Unlike marsupials and placental mammals, monotremes do not have nipples. Instead, milk is secreted through specialised mammary glands located on the mother’s abdomen. The milk seeps out through pores in the skin and pools in grooves or patches of skin where the young lap it up.
• Monotreme milk is rich in nutrients and antibodies, helping to nourish and protect the vulnerable young. This is particularly important as monotreme hatchlings, especially in the case of platypuses, are highly underdeveloped at birth and rely on their mother’s milk for several months.
• For platypuses, the young stay in the burrow for about four months, feeding on milk while the mother hunts for food. Echidna puggles remain in the mother’s pouch for around 45 to 55 days before being left in a burrow to continue feeding on milk until they are ready to emerge.

The platypus and echidna are two of the most unusual and captivating creatures on Earth, with their unique adaptations like electroreception, venomous spurs, and spiny defences. These interesting and lesser-known facts highlight just how special Australia’s monotremes are, offering insights into their ancient evolutionary history and their important roles in Australian ecosystems.