Tetrapods: Out on a limb

Mammals are one of the most diverse and fascinating groups in the animal kingdom. Whether running, flying, swimming, or burrowing, they have adapted to nearly every environment on Earth. Beyond their diversity, mammals play vital roles in maintaining ecological balance—from pollinating plants to acting as top predators. Understanding mammals also helps us appreciate our own place in the natural world, as humans are mammals too.

What Defines a Mammal?

While mammals vary wildly in appearance, they all share a specific set of characteristics that distinguish them from reptiles, birds, and amphibians.

• Mammary Glands: Females possess glands that produce milk to nourish their young, providing essential nutrients and antibodies for a strong start in life.

• Hair or Fur: Used for insulation and protection, this trait helps mammals regulate their body temperature across diverse climates.

• Middle Ear Bones: A specialised jaw structure containing three middle ear bones significantly enhances their hearing.

• Warm-Bloodedness (Endothermy): Mammals generate their own body heat internally, allowing them to remain active in cold environments.

• Four-Chambered Heart: This efficient system pumps oxygenated blood throughout the body to support a high metabolic rate.

• Specialised Teeth: Mammals have various teeth shapes adapted for specific diets, such as tearing meat or grinding plants.

• Live Birth: Most mammals are viviparous (give birth to live young). Notable exceptions include the platypus and echidna, which lay eggs.

The Three Main Groups of Mammals

Mammals first evolved from reptile-like ancestors called synapsids approximately 220 million years ago. Today, the roughly 6,500 species are divided into three groups based on how they reproduce:

Group Reproductive Strategy Examples

Placentals Young are nourished in the uterus via a placenta. Humans, dogs, whales, elephants

Marsupials Give birth to underdeveloped young that finish growing in a pouch. Kangaroos, koalas, opossums

Monotremes Rare, primitive mammals that still lay eggs. Platypus, echidna

Local Spotlight: Mammals of the Sid Valley

In areas like the Sid Valley, mammals occupy a variety of habitats, including woodlands, hedgerows, and even urban gardens.

Notable Local Species:

• Bats: All 17 UK species can be found in or near woodlands. They are nocturnal insectivores that use linear features like hedges to navigate.

• Otters: These top carnivores are regularly seen on the River Sid. Their numbers are recovering after a dramatic decline in the 1950s due to pesticides.

• Hazel Dormice: Elusive and arboreal, these mammals spend most of their time in trees and are a "stronghold" species for Devon.

• Badgers: Known for their vast underground "setts," badgers are active foragers that search for worms and beetles.

Threats and Conservation

Mammal populations face significant challenges today, primarily from habitat loss due to intensive agriculture and urban development. The loss of hedgerows, which act as vital "wildlife corridors," has particularly impacted smaller mammals.

Invasive species also play a role: the American Mink has contributed to the local disappearance of the water vole, while Grey Squirrels carry a virus that has devastated native Red Squirrel populations.

What You Can Do: You can support local mammals by managing gardens for wildlife—avoiding chemicals, providing food/shelter, and ensuring fences have gaps for hedgehogs to pass through.