Dr Ben Aldiss explores the rich world of the vertebrate in Norfolk’s natural world
Of the many vertebrates found in our county, I’m going to write about six of them in this month’s look at the Nature of Norfolk, but first, let’s investigate the meaning of the word. In terms of biological classification, vertebrates form by far the largest sub-group of the three that make up the phylum Chordata. All chordates have a rod of protein extending the length of their bodies at some stage in their development.
In the case of vertebrates, this rod, or notochord, becomes the spine with its row of articulated vertebrae which may be either of bone or – in the case of dogfish and sharks – the softer, but more flexible substance, cartilage.
Ninety-nine per cent of the world’s 66,000 vertebrate species have jaws, the other one percent consisting of an ancient group of jawless, eel-like fish – the lampreys and hagfish. The jawed vertebrates are subdivided into five familiar categories: the fish, amphibians, reptiles, birds and mammals.
So, let’s start our review of Norfolk’s vertebrates by looking at the very strange and rarely noticed brook lamprey. During their restoration work on various north Norfolk streams, the Norfolk Rivers Trust have proved the presence of this small, jawless fish. Although other lampreys feed by attaching themselves to other fish and rasping away their skin with circular rows of sharp teeth, the brook lamprey doesn’t feed at all as an adult.
Having hatched from its egg, the young, blind, toothless lamprey submerges itself in sediment at the bottom of a freshwater stream, where it feeds on suspended organic debris and plankton filtered from the water. Four years later it begins to transform into an adult.
This metamorphosis takes a further year, during which the lamprey stops eating and develops eyes and reproductive organs. It also grows teeth for the first time – but amazingly never uses them!
Having finally emerged from its muddy juvenile home, it simply swims to find a mate, then lays eggs and dies.
Among the more modern vertebrates – the ones with jaws – there is a distinct hierarchy. First to evolve were the fish, followed by amphibians, then reptiles and finally birds and mammals.
We’ve already looked at the brook lamprey, so let’s turn to a more typical fish, found throughout the streams of Norfolk. The three-spined stickleback wouldn’t win any prizes in a fishing competition, being only a few centimetres long when fully grown, but it shows all the characteristics typical of most other bony fish: scales, fins, two-chambered heart and an inability to control its body temperature.
Normally greenish above and silvery below, the male stickleback becomes a handsome fish every spring when he develops blue eyes and bright red head and throat. At this time, he finds a suitable territory and aggressively defends it against other males.
He then scrapes a depression in the sediment on the stream bottom and builds a nest out of bits of vegetation he collects and then glues together with a substance called spiggin. Finally, he lures a female into the nest, then fertilises the eggs she lays.
From then onwards he behaves as any proud father would, looking after the eggs and young sticklebacks until they can fend for themselves.
From an evolutionary point of view, amphibians are one rung up the ladder from fish, having developed the ability to leave the water, at least for part of the time. Like fish, they can’t control their body temperature, but instead of fins they have legs and rather than scales, they have smooth skin.
They include the frogs, toads, newts and salamanders. Of the UK’s seven native species, the natterjack toad is relatively rare, with the dune systems of Norfolk being one of its strongholds.
As with all amphibians, this toad begins life as an egg surrounded by protective jelly, laid in water. The young toad that hatches a fortnight later is the familiar tadpole and bears no resemblance to the adult. Adapted for an aquatic life, it is streamlined, limbless and uses gills to extract oxygen from the water.
Initially herbivorous, it becomes a carnivore later on, before metamorphosing into an air-breathing toad with legs and lungs. In the course of evolution reptiles superseded amphibians as the major vertebrates on Earth.
Their trick was to break the link with water by developing waterproof, scaly skins and eggs with tough waterproof shells. In Norfolk we have four of the UK’s six native reptiles: the common lizard, the slow-worm, the adder and the grass snake. Only one of these – the grass snake – is a typical reptile, with its young hatching from eggs.
All three of the others are viviparous. In other words their eggs hatch inside the mother’s body, so the young are born as mammals are – complete and ready to go. In addition to this abnormality, the slow-worm is neither a worm nor slow! It’s actually not even a snake, but a legless lizard.
Birds appear to have evolved from small, fast-running dinosaurs and were the next stage in the evolution of the vertebrates. As well as laying waterproof, shelled eggs, birds developed two further adaptations that set them apart from the reptiles: flight and the ability to control their body temperature. This ‘endothermism’ allowed them to exploit habitats impossible for either reptiles or amphibians – the polar oceans for example.
There are many bird species in Norfolk, but a good example of a relatively unusual one is the stone curlew, or Norfolk Plover as it is sometimes called. This strange bird with relatively huge yellow eyes and prominent swollen knees (hence another of its alternative names – the thick-knee), can be seen in small numbers every spring as it returns from its winter quarters in Africa to the sandy soils of the Brecks. It belongs to the same group as the waders, but is adapted for life on open plains and frequently nests in arable fields in the Thetford area.
Finally, at the top of the vertebrate evolutionary tree, come the mammals. Like the birds, they are endotherms, but insulate their bodies with fur rather than feathers. As we all know, apart from the strange Australian platypus and echidna, mammals give birth to their young, instead of laying eggs. For my example of a Norfolk mammal, I’ve chosen the stoat.
My wife and I saw one a number of times last winter as it ran along the garden wall and up onto the barn roof. It was in its distinctive seasonal ermine phase, with white coat and black-tipped tail.
This colour-change, from chestnut and cream in summer, seems to be triggered by day-length as well as temperature. Like its smaller cousin the weasel, the stoat is a fierce predator, but averaging 300g in weight compared to the weasel’s 70g, the stoat can tackle much larger prey and can even bring down rabbits and small hares, which it kills with a precision bite to the neck.
The stoat’s Latin name – Mustela erminea – is rather descriptive. Mustela literally means ‘spear-shaped mouse’ and erminea refers to the white winter fur, famously used in the past to make ermine coats and stoles.
Next month, for the letter W, I could have talked about mammals at the other end of the size-scale – whales – but instead my article will be on some of my favourite of all Norfolk’s wildlife: wasps!
Dr Ben Aldiss is a wildlife journalist and broadcaster, deputy managing editor of World Agriculture and an adviser in biodiversity and education on farms. waspsandwildlife.co.uk
