Image copyright AFP

When Copenhagen Zoo put down a healthy male giraffe earlier this month, much of the world was horrified. But those in the know say it's quite normal - a fate that befalls thousands of zoo animals across Europe every year.

"This is not a thing that should go anywhere outside Denmark," says Copenhagen Zoo's Scientific Director Bengt Holst, responding to the barrage of critical news coverage. "We all know it's done every day."

Every day? It's not actually quite that often, but Holst says it's difficult to give a precise number.

"We do it when it's necessary," he says. "If I should take an average over 10 years - it could be probably something like 20, 30 [per year]."

That figure includes some smaller animals, not just the big "charismatic megafauna" that have the potential to make headline news. At the larger end of the scale, Copenhagen Zoo has put down leopards, tigers, lions, bears, antelopes and hippos in recent years, as well as the young giraffe, Marius.

One gorilla would generate more interest and more column inches than 10,000 rats - so the numbers game for me is kind of irrelevant Simon Tonge, South West Environmental Parks

Other Danish zoos are also quite open about culling. Just a week before Marius was put to sleep, another Danish zoo killed two lions.

"We have around about 2,000 animals here in Odense zoo," says curator Michael Sorenson. "The number is less than 10 a year which we have in surplus. So, if you look at the numbers it's a small fraction."

What about the rest of Europe?

It's often hard to get any information, but the 340 zoos that belong to the European Association of Zoos and Aquaria (EAZA) must sign up to the organisation's various breeding programmes, and for each species in the programme there is a studbook - a kind of inventory which records every animal's birth, genetic make-up, and death.

EAZA does not publish these records or advertise the number of healthy animals that have been culled, but executive director Dr Lesley Dickie estimates that somewhere between 3,000 and 5,000 animals are "management-euthanised" in European zoos in any given year.

EAZA euthanasia policy Euthanasia should be considered, the EAZA says, when: The animal poses a threat to human safety

The animal is suffering from a disease

The animal would otherwise be transferred to substandard accommodation

Where the continued presence of an individual animal is disruptive to the natural dynamic of a group within an individual collection and/or the demographic or genetic health and development of an EAZA approved ex-situ conservation programme The statement continues: "We are ethically obliged to strike an informed balance between the life of an individual and maintaining the long term viability of a managed population." Read the full euthanasia policy

"That's our estimate for all animals management-euthanised in the zoo, be it tadpoles up until a giraffe," she says.

What about the numbers of larger animals - lions, tigers, bears, giraffes and so on?

"I would expect that's less than a few hundred," says Dickie.

A careful examination of the studbooks would no doubt yield a more precise figure. But Dickie warns that sometimes an animal is just listed as having died, without any indication whether it was for medical or management reasons.

Zoos are instructed to be as clear as possible when recording data on deaths - but they are "fallible", she says, and sometimes information will be missing.

It is possible to find some studbooks online and just as Dickie says, it's not always recorded how an animal died - just that they have. Others give much more useful information. If a healthy animal has been put down, the key phrase is "Death by: Euthanasia (cull)"

Image copyright Getty Images Image caption Hippos are among the animals listed as culled in zoo studbooks

From these and other conversations the BBC has had with studbook keepers, we have learned that:

Five giraffes have been put down in Europe since October 2012, all of them in Denmark

Four hippos were killed across Europe in 2012 - in Portugal, Spain, Germany and Denmark

Twenty-two healthy zebras were put down between 2000 and 2012, including one at Marwell in the UK.

Eleven Arabian Oryx were killed in Edinburgh, London, Rotterdam and Zurich between 2000 and 2009, plus dozens more at zoos in Qatar and the United Arab Emirates (which are part of the European breeding programme)

Laws compared Germany's Animal Welfare act bans the killing of a vertebrate "without reasonable cause"

Animal Welfare act bans the killing of a vertebrate "without reasonable cause" Such a law only exists in a few other European countries, including Austria

Swiss law prohibits only "malicious killings" Source: Ex Situ Conservation, Surplus Animals and Euthanasia - by Goetz Hildebrandt et al

Four German zookeepers were also prosecuted in 2010 for culling three tiger cubs at Magdeburg Zoo "without reasonable cause" (though the EAZA judged the step "entirely reasonable and scientifically valid").

Some zoo professionals caution against attaching numbers to the culls at all.

"The numbers game can be made to sound awful," says Simon Tonge Executive Director of South West Environmental Parks, which runs Paignton and Newquay zoos in the UK.

The headline "Zoos euthanise thousands of animals per year" would be misleading, he says.

"Well OK, but you know most of those animals were rats or mice or something like that.

Find out more Image copyright Thinkstock Hannah Barnes's report on the debate over euthanasia in zoos can be heard on BBC Radio 4 at 20:00 on Thursday, or afterwards on the iPlayer The Report: Zoogenics?

"If we ever got to the point of having to consider euthanasia for a gorilla… I would argue that that one gorilla would generate more interest and more column inches than 10,000 rats. So the numbers game for me is kind of irrelevant."

There is a further problem, he argues. It is not always easy to say when an animal has been culled - some cases fall into a grey area.

"Suppose we have two animals of a species," he explains. "Both are ill, but we know that with a year or more's intensive veterinary effort, we can make them well again.

"One of them is never going to breed because it's genetically not important enough, but the other one is more important. Because we just don't have that time and the money to invest in both of those animals… we euthanise the least [genetically] related one. Is that a veterinary euthanasia or is that management euthanasia? I genuinely don't know. I wouldn't know which column to put it in in the inventory… we just don't count them really."

Image copyright Thinkstock Image caption Zoos have run successful breeding programmes for giraffes

So, how many healthy animals are being put down by zoos? We do not really know.

We cannot just expand the zoo - we have to realise, 'OK, what do we do with the surplus?' Bengt Holst

There is good reason to think it may be higher than the European Association of Zoos and Aquaria estimates, because some zoos are not always explicitly recording these deaths, and others are not members of EAZA in the first place.

Whatever the number is, there are good reasons to assume it is probably growing.

"Twenty years ago the giraffes didn't breed as well as they do today, so… automatically we run into these problems," says Bengt Holst, Scientific Director of Copenhagen Zoo.

"We cannot just expand the zoo. Of course we have to realise, 'OK, what do we then do with the surplus?' and we made our decision."

There is evidence that a number of European breeding programmes for other species are also going very well - perhaps too well.

The EAZA Yearbook 2007/2008 (the latest publicly available edition) states clearly that a "breed and cull" policy should be followed for some animals, like the pygmy hippopotamus.

Surpluses are a problem with a number of species, including monkeys and baboons, it notes.

So, unless European zoos adopt the American practice of using contraception to prevent the birth of surplus animals, culling is here to stay.

You can listen to Hannah Barnes' report on BBC Radio 4's The Report at 20:00 GMT on Thursday, or afterwards on the iPlayer.

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