It was a cold overcast day during the autumn school break at Odense Zoo in central Denmark. But there was an unmistakable buzz of excitement in the air among the children and adults who were waiting to see the zoo’s lion cub. Just after noon, a keeper brought the male lion out. But instead of running and jumping about playfully, the young cub lay perfectly still on its side. No one seemed to think anything was amiss — they knew the lion had died eight months earlier.
In February of last year, the zoo had eight lions, too many for their enclosure. Keepers couldn’t find a suitable institution elsewhere to take in the three cubs, which were never named to avoid the staff getting too attached to them. So to avoid overcrowding the den, the zoo decided to euthanize the cubs. On Oct. 15, keepers removed the male lion from the freezer and publicly dissected it in front of more than 300 people. The event revived the debate about killing healthy animals; it was reminiscent of the heated protests that erupted in early 2014. That’s when Copenhagen Zoo, also in Denmark, euthanized a healthy 2-year-old giraffe named Marius whose genes were deemed too common for breeding.
Zoos have limited space for animals to live and breed. So they have to carefully control population numbers, either by restricting breeding or by culling surplus animals. Zookeepers sometimes disagree over which method to use and whether one is better than the other. Views can differ from zoo to zoo and there is an apparent cross-Atlantic divide. While contraception advocates say that their approach saves healthy animals from being killed, proponents of culling insist that contraception creates unnatural conditions for animals. Still others resist having to choose between the two.
“It’s not really one or the other,” says Kirsten Pullen, director of the British and Irish Association of Zoos and Aquariums. “They are different tools we use to help us manage healthy populations.”
Culling and contraception are two ways of achieving the same goal — conserving wildlife. Zoos and aquariums play a vital role in preserving biodiversity and preventing the extinction of endangered species. Their conservation breeding efforts helped save 16 vertebrate species between 1996 and 2008, according to the International Union for Conservation of Nature.
To conserve species, experts agree zoos must take an active role in managing their animal populations. Under natural conditions, disease and predation keep animal numbers in check, says Gerald Dick, director of the World Association of Zoos and Aquariums based in Switzerland. “But we’re in unnatural conditions. Animals in zoos don’t have the same factors acting on them as they do in nature.”
Zoo officials control animal populations to prevent inbreeding and to maintain genetic diversity. Inbreeding occurs when closely related animals mate, such as a parent and its offspring or two siblings. Progeny from these matches tend to have deformities or be less healthy. More broadly, genetic diversity across a species fosters healthy populations. The greater the differences in their genes, the less likely populations of animals will succumb to diseases or to changes in their environment.
“The aim is to have viable and healthy zoo populations,” says Dick.
Many zoos in Europe work towards this goal by euthanizing surplus animals. “Animal populations that are managed through culling can be more genetically and demographically diverse,” says David Williams-Mitchell, spokesman for the European Association of Zoos and Aquaria in Amsterdam, the Netherlands.
Culling is a widely practiced method for keeping animal populations to an optimum number — and not just in zoos. “We maintain healthy population sizes like this in all parts of the world,” says Bengt Holst, director of research and conservation at the Copenhagen Zoo. Thus wild rabbits in England, white-tailed deer in the United States and wallabies in Australia are routinely hunted or culled in order to keep their numbers in check.
In zoos, staff members ensure culling is carried out in a humane manner. “Culling has to be done according to national laws,” Holst says. “And as quickly and painlessly as possible.”
Culling always starts with leading the animal out of its enclosure so as not to distress the other animals, explains Pullen from the British and Irish Association of Zoos and Aquariums. The animal is then sedated and given a lethal injection, similar to what a vet would do with a sick pet. Larger animals are euthanized with a bolt gun, a device that delivers an instantaneous fatal concussion.
Some people shudder at the thought of killing a healthy animal, as evident from the uproar generated over the giraffe and lions culled by the Danish zoos. This could be a matter of perspective, though. “As civilized societies, we have become more disconnected from nature,” says Dick from the World Association of Zoos and Aquariums. Many people don’t give a second thought as to where meat in the supermarket comes from, yet balk at the idea of killing healthy animals.
So some zoos favor a different method of controlling animal populations: contraception. “Contraception is just as important as culling,” says the European Association of Zoos and Aquaria’s Williams-Mitchell. Although the general perception is that European zoos prefer culling to contraception, this simply isn’t true, he says. Religious and cultural differences exist across the continent and culling is frowned upon in some countries like Germany, Italy, Poland and the United Kingdom. According to Williams-Mitchell, the latest figures from EAZA demonstrate that both methods are used across its 349 member zoos in 44 European countries — with 150 large mammals euthanized and more than 4,800 animals (mostly mammals) receiving contraception in 2013.
Across the Atlantic, many zoos tend to favor contraception. “American zoos have been using contraception much longer and more extensively than European zoos,” says Cheryl Asa, head of reproductive management at the Association of Zoos and Aquariums, based at the Saint Louis Zoo in Missouri.
Zoos use a range of birth control methods. Animals can be implanted with a contraceptive device under their skin or given pills that are crushed up and hidden in their food. “For primates like orangutans, we can use birth control pills used in humans,” Asa says. “We crush up the pills up in yogurt or honey — food they like — so they can’t find them and keep them in their cheek pouch to spit it out later.”
Females are normally the ones who receive contraception because there are more products available for females and their effects are more obvious, Asa explains. You can easily observe if a female displays estrus behavior or attracts male attention to see if the contraception worked. In contrast, a male animal has to be put under anesthesia — a more invasive procedure — to check its sperm levels.
However, contraception can have serious, long-lasting effects on a female’s reproductive tract and has been known to cause cysts and tumors. Nina Collatz Christensen, director of animal keepers at Odense Zoo, remembers when they tried using contraception on a lioness several years ago, before the three lion cubs were euthanized at her zoo. The lioness walked around the enclosure listless and was constantly sick, Collatz Christensen says. “Her uterus was basically spoiled and she couldn’t give birth after that.”
“It’s quite a big decision to make,” says Pullen about contraception. “You might be risking having an animal that can’t breed in the future.”
Contraception use may also affect an animal’s social status within a group. Pullen cites the example of Hamadryas baboons, native to the horn of Africa and Saudi Arabia, which live in groups comprising one male and two or three females. Contraception prevents a female from going into heat and having her bottom become bright red and swollen. This causes her to lose the attention of the male who is in charge of the group, thereby altering her place within the social hierarchy, says Pullen.
Contraception use in male animals can also cause problems. Pullen notes the ways in which hormone-based contraception can affect how secondary sexual characteristics develop. “Male lions may lose their mane or muscle mass because their testosterone levels fall,” she says.
Perhaps one of the biggest arguments against contraceptive use is that it prevents the female from breeding and experiencing parenthood, which is a huge part of natural behavior, says Holst.
But not everyone agrees with this argument. “Most individuals do get to reproduce at some time in their life,” says Asa. “Just not at every single opportunity with every male they come into contact with.”
Despite the apparent continental divide, zoos on either side of the Atlantic utilize both culling and contraception to manage their animal populations. “It’s not really one or the other,” says Pullen. “There’s a scale in between.”
“Both are scientifically valid methods,” says Williams-Mitchell. “We’re just trying to ensure that animal populations remain sustainable in the long-run.”