Mammals of the British Isles - The Squirrel

Grey Squirrel

Strangely enough, the cute and lovable grey squirrels (Sciurus carolinensis) we see playing in the grounds of Stover School are in fact rodents – the same group of mammals as rats. Since their introduction from North America, they have had a catastrophic effect on the native red squirrel (Sciurus vulgaris) throughout the British Isles. Grey squirrels also cause significant damage to woodlands through ‘bark stripping’ and reduce density and diversity in populations of woodland birds.

Wild grey squirrels first appeared in Britain in the late 19th Century, with some possibly escaping from London Zoo. In Italy two pairs escaped from an ambassador's garden in Turin in 1948, although some sources suggest that pet grey squirrels had already escaped in Piedmont by the late 1800s. The spread of grey squirrels, at the expense of native red squirrels, appears to have had a more marked effect in the British Isles than on mainland Europe – so far at least.

Red Squirrel

There are now thought to be just 140,000 red squirrels in Britain, whilst there are over 2.5 million greys, and the future of the red squirrel is becoming increasingly uncertain as they are now extinct in southern England (except for a few on the Isle of Wight). Red Squirrels are still widespread in the North of England and Scotland, but even here their range is contracting. The grey squirrel is having such a profound impact on British wildlife that the International Union for the Conservation of Nature has now listed it on their global list of the 100 worst invasive species. The UK Forestry Commission is working with partners in projects across Britain to develop a long-term conservation strategy that deters greys and encourages reds.

The Grey Squirrel is larger and more aggressive than its European cousin, and greys out-compete reds for food, feeding more on the ground and being able to digest acorns, which reds can’t. Contrary to popular opinion, red squirrels do not hibernate – but as greys can build up and store more fat they are better at surviving cold winters. Grey squirrels are also more generalist feeders eating: nuts, flowers, fruits, seeds, tree bark, fungi, bird eggs, nestlings and frogs - whilst reds prefer the seeds of coniferous forests, although they will diversify under pressure. Grey squirrels are also thought to have introduced the squirrelpox virus from North America which is deadly to red squirrels, although greys have a natural immunity.

Distribution of Grey Squirrel

Distribution of Red Squirrel

Grey squirrels, which can live at high population densities in broadleaved woodland, cause significant damage to trees such as sycamore, beech, oak, sweet chestnut, pine, Norway spruce and larch, by bark-stripping. This dramatically reduces the economic value of woodland – costing more than £10 million per year. They may also be partly responsible for recent declines in many woodland bird species - through predation of eggs and young chicks, competing for nest sites or because they consume food which would otherwise be available for birds.

Squirrels build large nests, called dreys, often in the forks of tree trunks. They can breed twice in a season, in spring and in late summer, but usually only breed once. There are between one and six young in a litter. The young are born naked and blind. Young squirrels may stay with their mother until she has her next litter. Mortality is quite high, with only one in five surviving to their first winter. Adult squirrels have few natural predators, as they are far too quick and agile for most.

this article is based on work by Jane Chen, Year 10

Year 11 Pupils Investigate the Kidney

As part of their GCSE Biology course, Year 11 pupils today investigated the structure of the kidney.

The Dissection Begins

The kidneys are two bean-shaped organs attached to the rear wall of the abdominal cavity. They have a rich blood supply, and as the blood passes through them it is filtered by millions of little structures called nephrons, consisting of: a glomerulus (a coiled ball of capillary blood vessels), and a kidney tubule (of which there are over 1 million, or 60 km worth per kidney).

A Nephron

Finishing Touches

Blood flows into each glomerulus from the renal artery. Small molecules ‘leak out’ under pressure filtration, into the Bowman’s capsule of the tubule. The small molecules are: water, urea, sugar and salt. The urea and most of the water flow down the tubule to a collecting duct and ultimately into the ureter and bladder to be excreted as urine. Useful materials however, like glucose and salt, are actively transported back into the capillaries surrounding each tubule – a process known as selective reabsorption.  

In cross-section, a kidney has two main regions: the outer cortex is lighter coloured, and contains the glomeruli and the beginning and end of each nephron. The inner medulla contains the middle u-shaped part of each nephron (e.g. the loop of Henlé). The medulla extends into the central ‘pelvis’ of the kidney as projections called pyramids.

Kidney in Cross-Section

Kidney Structure

Further Research

Great Thinkers: Democritus

Democritus

Democritus was the first person to develop a view of the world based on atoms, although he was more of a ‘natural philosopher’ than a true scientist.

He was born in Abdera in Thrace, northern Greece, in 460 BC, and died at the remarkable age of 90 (or perhaps even older). Democritus was a follower of Leucippus and together they put forward the concept of ‘atoms’. Contrary to many of their more famous rivals they interpreted Nature in a mechanistic way – believing that natural phenomena were free from the interventions of gods and supernatural causes. Their ideas on the nature of matter and the workings of the body etc., were thus remarkably close to our contemporary science-based world view. For over 1,000 years however the writings of their arch opponents Plato and Aristotle held sway, and matter was generally considered to be composed of four primordial components: water, air, fire and earth – in a world largely ruled by the whims of gods.

Democritus believed that things consisted of an infinite number of very small particles which he called atoms (from the Greek atomos indivisible). These atoms were seen as eternal, and it was believed that there were many different kinds of atom, which could move about randomly – occasionally colliding and joining to form a new substance. Whilst each atom was indestructible, the things they created by combining with others were not. Democritus summed up his views by saying “nothing exists except atoms and empty space; everything else is opinion”. 

His ‘atomic viewpoint’ gave Democritus interesting insights into areas such as reproduction and evolution. He said that all parts of the body contribute to the seed from which a new animal grows, and both parents contribute seed. Parental characteristics are inherited when one parent’s seed predominates over the other for a particular character. He thought that species do not exist for ever, unlike the atoms from which individuals are made. Democritus also wrote extensively on: ethics, the senses, theology (which he disliked), geometry and the nature of the soul or psychê. Although none of his original works have survived we know of them through the writings of Epicurus and others.

Democritus came from a well-to-do family, and travelled extensively around the classical world. He was known as ‘the laughing philosopher’, apparently due to the high value he placed on cheerfulness, and he believed that “the best way for a man to lead his life is to have been as cheerful as possible and to have suffered as little as possible...The right-minded man is he who is not grieved by what he has not, but enjoys what he has.” There is a lesson for all of us there somewhere!