A-Z of na­ture:

The roles of par­a­site and host are fas­ci­nat­ing, as Dr Ben Ald­iss ex­plains

EDP Norfolk - - INSIDE - Dr Ben Ald­iss wildlife jour­nal­ist and broad­caster, deputy man­ag­ing ed­i­tor of World Agri­cul­ture

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

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