A-Z of nature:
The roles of parasite and host are fascinating, as Dr Ben Aldiss explains
The grisly, gruesome and fascinating world of the parasite
It was the cuckoo calling in the garden back in the summer that clinched my choice of topic for the letter P and I’ll be honest with you – it’s a theme that’s not for the fainthearted!
Parasites affect us all at some stage or another and every plant or animal species in Norfolk is likely to be prey to at least one. Let’s start by deciding exactly what a parasite is.
The official definition is an organism that lives in or on another plant or animal and gains its food from it. According to this classification, fleas, ticks and worms are obviously parasites, but how does the cuckoo fit in?
Well, in the next few minutes I’ll explain and then go on to look at some other fascinating instances of this rather gruesome way of life. The cuckoo is a remarkable bird in many ways, but its most unusual characteristic is its habit of laying eggs in other birds’ nests.
This so-called brood parasitism is effectively a way of getting someone else to raise the kids and depends upon the fact that the foster parents’ urge to feed the clamouring cuckoo chick completely overrides any suspicion they may have that it doesn’t belong to them. For the deception to work, the female cuckoo has to sneak onto the nest of the chosen foster parents whilst neither of them is there.
Reed warblers and meadow pipits are often the victims. She then removes and eats one of their eggs and lays one of her own in its place.
Incredibly, the egg she lays matches almost exactly in colour, pattern and size, the eggs of the foster mother, despite the cuckoo being many times bigger.
With nothing else to do than mate and lay, the female cuckoo can produce a lot of offspring,
providing she can find enough nests to parasitise. In his remarkable book The Truth about the Cuckoo, Edgar P Chance describes how he spent five years between 1918 and 1922 filming the habits of a particular cuckoo. In one season, she laid 25 eggs, each one of which was hatched and reared by separate pairs of foster birds.
Once the young cuckoo has hatched, it immediately and instinctively heaves any other eggs or chicks out of the nest, after which it grows quickly, its ravenous appetite ensuring no rest for the tireless foster parents as they dart back and forth with food. At the time this article goes to print, all Norfolk cuckoos will be happily installed in sub-Saharan Africa where, interestingly, they don’t sing their characteristic song.
Unsurprisingly, therefore, a local bird expert I once met in Kenya couldn’t understand why these birds were called cuckoos!
You may think that parasites cause harm and broadly you’d be right, but as with all things in nature, life isn’t quite that simple. In the case of the cuckoo, the foster parents and their offspring clearly suffer, but are there any parasites that appear to cause few, if any ill effects?
Well, last month I mentioned that orchid seedlings need a fungus to help them grow. This fungus actually invades the orchid tissue and – at times – feeds on it. So, we could call the relationship at such times a parasitic one.
However, the orchid then fights back and the roles are reversed. It’s easy to see how this parasitic relationship has actually become beneficial to both partners over the course of evolution.
Many insects are parasites in their juvenile stages, but free-living as adults. As such, biologists call them parasitoids. A classic example is that large group of insects called ichneumons, all of which are parasitic as larvae.
Ichneumon comes from the Greek to track or to hunt and describes the way the adult, wasp-like female seeks out her prey – usually a caterpillar. Norfolk’s largest ichneumon is a fearsome-looking creature called Rhyssa persuasoria.
At over 7cm long, this handsome black ichneumon possesses a very long spike on her tail – just right for boring deeply into pine trees. Incredibly, she can detect the movements of the larva of one of Britain’s largest insects – the handsome yellow and black horntail – as it tunnels in the wood a centimetre or two below the surface.
With fine precision, she bores a thin tube to the unsuspecting larva, then squeezes an egg down beside it. When it hatches, the young ichneumon grub latches itself to the outside of the horntail’s body and gradually consumes it over the course of a month or so, only killing the larva at the last minute. The horntail killer then emerges as an adult ichneumon and flies off to find more unlucky victims.
Parasitic worms are found throughout the animal kingdom. Every bird and mammal in Norfolk is likely to harbour at least one.
As humans, we like to think we’re too civilised to suffer such parasites, so when we or our children get worms, we find it particularly disturbing. One of the most effective parasites is the tapeworm.
‘Once the young cuckoo has hatched, it immediately and instinctively heaves any other eggs or chicks out of the nest’
By effective I mean capable of sustaining itself with minimum ill effects on the host. It’s entirely possible to harbour a tapeworm for upwards of 20 years, the only obvious effects being a healthy appetite and an inability to put on weight, as the insidious parasite shares your digested food. It sounds horrific and rather unlikely, but a full-grown beef tapeworm inhabiting the human gut can grow to a length of 50 feet or 15 metres and produce a million eggs per day.
The beef tapeworm used to be relatively common in Norfolk before the Second World War, especially in people who liked their beef rare, but nowadays food hygiene and livestock husbandry have ensured that it’s a thing of the past.
Do be careful with rare or undercooked meat and fish in countries with poor hygiene records though – you could be returning to Norfolk after your holiday with more than you bargained for! 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
ABOVE: Ichneumon wasp with long ovipositor in flight
A cuckoo, one of nature’s most accomplished parasites