Mammals

Mammals are animals characterised by a suite of unique features, some of the most obvious are that they secrete milk from glands on their skin, have hair, a diaphragm and a three-boned middle ear.

Overview

What is a mammal? Am I a mammal?

Kangaroos, rats, bats and humans (i.e us!) are all examples of animals that can be identified as mammals because they share the following features:

Milk-making glands (mammary glands) from which new-born young take nourishment. Mammals (Mammalia) are actually named after these glands.
Hair that partially or wholly covers the body (although in some groups like whales, dolphins and dugong the hairs are reduced to bristles or absent).
A lower jaw comprised of two single bones (‘joined’ at the front). In other vertebrate groups like reptiles or fish each side of the jaw is comprised of two or more unfused bones.
A three-boned middle ear containing the stapes (stirrup), incus (anvil) and malleus (hammer).
A left-curving aortic arch. In birds, the main artery leaving the heart curves to the right. In other vertebrates more than one main artery leaves the heart.
A diaphragm. No other animal group has this sheet of muscle and tendon that separates the heart and lungs from the liver, kidneys, stomach and intestines.

Australia has unique mammals

Australia (and nearby New Guinea) are the only places in the world where you can find representatives of the world’s three major groups of mammals living together – Monotremes, Placentals and Marsupials. While all these groups have mammal characteristics, they show major differences in how they reproduce.

Placental Mammals are characterised by complex placenta that provides nourishment to young developing in the mother’s womb. Placental mammals dominate mammal communities on every continent in the world except Australia. In Australia the only native non-marine placental mammals are bats and rodents (mice and rats).

Marsupials give birth to young at very early stages of development (often described as pink jellybeans!) that are then nursed in a pouch on the mother’s abdomen. Kangaroos, Koalas, Wombats, Bandicoots and Quolls are all examples of marsupials. A very unusual and distinctive aspect of Australia is that it is dominated by marsupials, with over 70% of the world’s species occurring here.

Monotremes are remarkable mammals that lay eggs. While they show regular mammalian features that include body hair and mammary glands, they also have no nipples and the milk they produce must be lapped up by their young as it is secreted from ducts on the mother’s belly. They also have electrosensitive bills (platypus) or beaks (echidnas) and spurs associated with a venom system. Monotremes are only found in Australian and New Guinea – so we are lucky to have them!

Common questions

Everywhere in Australia has some interesting mammals – there is nowhere else in the world where you can see a giant hopping marsupial like a kangaroo or monotreme like an echidna.

However, one area that is particularly special is the wet tropics rainforest of north Queensland. This area is home to a particularly diverse set of mammals that are not found in the rest of the world. For example, lemuroid ringtail possum and the musky rat-kangaroo, a tiny kangaroo relative.

For the last 35 million years Australia has not had any connections with any other continent (in contrast Asia, Europe and Africa have all been connected to each other). Mammals are also generally not very good at dispersing over water. For these reasons placental mammals like monkeys and deer which evolved in the Northern Hemisphere were never able to naturally colonise Australia. In contrast marsupials and monotremes were present before Australia became isolated and still dominate Australian mammal diversity.

Australia has the worst record in the world for mammal extinctions. There are many reasons for this, but in many cases the key overarching driver appears to involve interactions with introduced placental mammal species such as cats, foxes, rabbits, sheep and cows. It seems that the long evolutionary isolation of Australia’s mammals may have also rendered them uniquely vulnerable to introduced species. Habitat loss and modification, and climate change are also major challenges.

The koala has a wide distribution range and has coped better with human settlement than many of Australia’s mammal species. However, it lives in the most densely settled and rapidly growing area of Australia. In south-east Queensland an estimated three quarters of key koala habitat has been lost since 1960, clearing continues and many remaining patches of habitat are small and close to busy roads. Increasingly extreme weather and heatwaves are also a problem for koalas. For these reasons koalas are rapidly declining in many areas. Investment in preserving and expanding key habitat, and effective efforts to slow down anthropogenic climate change would certainly improve the conservation outlook of this species.