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Take a three-tonne African elephant and a scrawny New York subway rat and you have yourself two creatures that bear little resemblance to each other. One thing that connects them is some small and furry mammalian ancestor that lived on planet earth, say, 65 million years ago. And now here’s another: birth control.

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Last year, researchers in South Africa confirmed that a contraceptive vaccine being trialled at several wildlife reserves across the country was working to control problematic elephant populations. Fast forward a few months, and it looks like rodent residents in the Big Apple might be in for a birth-control experiment of their own.

Family planning and the reproductively gifted brown rat do seem like a natural fit. If you take world domination as your yardstick, Rattus norvegicus is probably the earth’s most successful mammal after humans (having conquered every continent aside from Antarctica) and it thrives in the big-city context (according to that infamous statistic, there are more rats than people in NYC, perhaps four per person). But when it comes to elephants, threatened as they are in most parts of Africa by rampant poaching and habitat loss, why would fertility ever be a problem?

“Family planning and the reproductively gifted brown rat do seem like a natural fit. If you take world domination as your yardstick, Rattus norvegicus is probably the earth’s most successful mammal after humans”

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Surprising it may be, but elephant populations, particularly in some of South Africa's smaller game parks, do at times require downsizing. When numbers outstrip available space, hungry herds can wreak havoc on their surroundings, mowing down vegetation and raiding nearby areas in search of food. To tackle the problem, some parks that may once have relied on bullets and culling now choose to attack chemically: elephant females are darted from helicopters with a dose of the porcine zona pellucida (PZP) vaccine, which prevents sperm from fertilising their eggs (a strategy that, like culling, is not without its critics). 

Meanwhile, across continents, rats in New York wreak their own brand of havoc, chewing their way through everything from electrical circuits to lead, pilfering garbage, harbouring leptospirosis and just being plain scary (for some unsettling rats-on-the-subway videos, the Gothamist website is your best bet). After waging deadly war with traps and poison, and even launching a Rodent Control Academy, city PMPs (that's pest-management professionals, in case you're wondering) are also ready to go chemical. 

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Their new weapon will come in the form of a product called ContraPest that's being developed by biotechnology company SenesTech Inc. The target: the ovary of the female rat. Unlike the elephant vaccine, ContraPest is designed to induce early menopause, effectively sterilising its recipient. But since rats cannot be darted from helicopters (as compelling as that scenario might look in your head), PMPs will need to rely on bait stations stocked with tasty and contraceptive-laced titbits. Nourished as they have been on a wholesome American diet of stale doughnuts, pizza crusts and other fast-food delicacies (according to one renowned rodentologist, chicken nuggets are a particular favourite), the rats may prove tricky to entice.

Whether the test subject is the African elephant or the big-city rat, managing animal populations with birth control may seem like bio-tampering on a dangerous scale -– but over the last decade or so, it's been catching on.

Elk, monkeyssquirrels and kangaroos have all been targeted. And as the subways of New York City begin to fill with the scuttling sounds of chemically sterilised rats, it looks like, for better or worse, wildlife contraception may be here to stay.