Mammals belong to the vertebrate group of animals. They are characterised by having bodies covered with hair or fur, by giving birth to live young—except for monotremes, which lay eggs—and by feeding their offspring with milk produced by mammary glands. Mammals are warm-blooded animals, meaning they are able to regulate their body temperature and maintain it at a constant level. Their body shape and size vary considerably depending on their habitat and taxonomic group. Almost all mammals possess four five-digit limbs, which have evolved into a wide variety of forms according to their mode of locomotion.
According to their diet, mammals can be classified as herbivores, which feed on plants, such as rabbits; carnivores, which eat meat, such as canids; and omnivores, which consume both animal and plant matter, as some mustelids do.
Around 5,500 mammal species have been described worldwide, representing a wide range of lifestyles and grouped into 22 orders. Rodents are the most diverse order, with around 2,000 species distributed across every continent except Antarctica, followed by bats with approximately 1,100 species. There is growing concern worldwide about the status and distribution of mammal populations. Changes in land-use practices, habitat loss and fragmentation, climate change, human persecution, disease, pollution, pesticide use and increasing competition between species have all contributed to the decline of numerous mammal populations.
The red squirrel
The red squirrel (Sciurus vulgaris) s a rodent that is easily recognised by its long, bushy tail, which helps it maintain balance. It has large dark eyes and brown fur that is more reddish in summer and greyer in winter. Herbivorous and granivorous, it feeds mainly on pine nuts, fruits, seeds and fungi. It is a diurnal and highly agile animal that lives in trees and moves easily from branch to branch. It inhabits all types of woodland, from sea level to elevations of around 2,000 metres. In Catalonia, it is a protected species, relatively uncommon but with a stable population.
