What's the Filthiest Animal?

By Daniel Kolitz on at

Ever since the 19th century, when disease was first linked to sewage-contaminated water, humans have gone to great lengths to escape their own filth. Meanwhile, animals have gone on revelling in the stuff—eating it, strategically dropping it, flinging it around just to pass the time, etc. Same goes for mud, piss, vomit, blood and rotting carcasses of every make and vintage. Most creatures just don’t have our hang-ups.

Still, there are degrees. A blissfully flea-bitten, shit-reeking dog might itself gag a bit at some of the animal kingdom’s most extreme swellings, its most frenzied fluid-play. For this week’s Giz Asks, we set out to find the worst offenders—the creatures who, from our admittedly squeamish/disease-shy perspective, are absolutely the filthiest. The animal experts we spoke to provided a diverse set of contenders, any one of which is gross enough to potentially claim that title.

Dave Gammon

Associate Professor, Biology, Elon University

Everybody knows that vultures are scavengers, that they feast on carcasses. What’s amazing, though, are some of the adaptations they’ve developed for that.

Carcasses might lay around for a long time before the vultures finally get to them—and a rotting carcass is going to attract a lot of toxins. But these vultures have ways of combating all that: Their stomach acid is so corrosive that they can digest things that would make other scavengers die. For example, vultures can readily consume a carcass that has anthrax in it, or cholera.

When they get to the carcass, they stuff themselves so full that what’s called their crop—a bag of skin within their throat—just bulges with the grody stuff that they’re eating. And if they get spooked by a predator while they’re full, they actually have to barf up some of their food so that they can fly away.

If that’s not enough, they actually will wee on themselves. And that turns out also to be an adaptive thing—we humans are not going to be able to relate to this, but if you went treading through a bunch of carcasses, your legs would be covered in bacteria, the kind of microbes that break down the carcass. By weeing on their legs, they’re actually helping to kill the bacteria that got on them—it’s a defense mechanism for them.

We think of birds as feathered creatures, but vultures don’t have any feathers on their head and neck. And if you don’t have feathers on your head, it allows you to stick your head right into the carcass and be less likely to get all the little bits of blood and gunk stuck to you and then be carried around all day. So maybe they care a little bit about cleanliness.

Dr Jason Bruck

Teaching Assistant Professor, Integrative Biology, Oklahoma State University

As a scientist, I think it is wrong to assume any animal is filthy, as it is inappropriate to ascribe our values onto non-humans. That being said, giraffes are a filthy disgusting bunch. To assess where the female is on her reproductive cycle, a male giraffe will bend down, put his head between a female’s rear legs, and slurp up her wee as she urinates in his face. After collecting a fair amount of her salty signal, the male will curl back his upper lip, expose his front teeth and inhale deeply to allow these fumes of fertility to stimulate a specialised sense structure called the Jacobson’s organ (or the vomeronasal organ). This is known as the Flehmen response, and most hooved animals perform some variation of this along with hedgehogs, pandas and big cats like lions and tigers (although not all do this with wee). You might even see your cat do a weird lip lift as it appears to be smelling something intensely. Anne Innis Dagg and J. Bristol Foster complied this information in a ‘dossier’ entitled The Giraffe: It’s Biology, Behavior and Ecology in 1976. Since then multiple reports of this crude giraffe conduct have been verified by thousands of zoo visitors annually.

Lixing Sun

Professor, Biology, Central Washington University

It depends on what we mean by ‘filthiest.’ In terms of, for example, disease-transmission, mosquitos. Several species of mosquitoes transmit malaria, which kill over a million people a year. Tsetse flies, in Africa, cause sleeping sickness. Tapeworms can live in the small intestines of humans, and can be as long as five feet. Sometimes these tapeworms take root underneath people’s skin. An infected person can see their tapeworm wriggling under there.

In terms of habit, of course, there are dung beetles. Free-living dogs can be very dirty. Some species of fish are slimy, like hagfish, and lampreys. And because sloths are so slow, parasites—even moths—grow on their fur.

Hippos can be very dirty as well—hippos living on the rivers during the dry season in Africa. The rivers dry up, so instead they become mud. Hippos’ skin is porous—which means they need water to keep their skin moist—so they’re pretty much wallowing in mud all day long. They also have a habit of beating their faeces—when they defecate, they use their tails to spread the faeces around, to mark their territory.

Matt Jones

PhD candidate, Entomology, Washington State University

Hands down, the filthiest animals must be dung beetles. They spend their adult life flying around in search of fresh faeces. When they find it, they lap up the liquid effluent, which contains nutritious microorganisms that the beetles can digest. After securing a stash of dung, beetles will mate, and the females lay eggs in the faeces. The cycle continues with the next generation larvae developing within the poo.

The evolution and ecology of these organisms is phenomenal; some specialise in finding and consuming incredibly specific faeces (like gopher tortoises) while others are less picky and will feed on a variety of nasty, faeces-like foods such as rotting carrion and plant matter. Dung beetles live worldwide on every continent except Antarctica.

However gross they might seem to some people, it’s important to recognise that dung beetles are super important to many healthy ecosystems. It’s widely understood that dung beetles play important roles in the healthy functioning of ecosystems, by providing services such as recycling nutrients back into the soil, improving soil structure, reducing greenhouse gases, and limiting the populations of dung-breeding flies by outcompeting them for the faeces. Additionally, we’ve just submitted a paper showing that dung beetles are critical in the natural suppression of human foodborne pathogens. Our research suggests that farming organically helps facilitate populations of dung beetles likely to reduce levels of pathogenic E. coli on vegetable farms. We’re excited about having another tool to help farmers mitigate this persistent food-safety risk.

In short, dung beetles are the best at doing a ridiculously dirty job that we often ignore. By spending their lives processing feces, they contribute greatly to healthier ecosystems.

Karin S. Pfennig

Professor and Associate Chair, Biology, University of North Carolina

I guess it depends on what one means by filthy—lots of animals are filthy in the just plain “dirty” sense of the word. For me the animal that fits the “oh gross” sense of the word’s meaning: parasitoid wasps.

Parasitoid wasps lay their eggs on on other host insects. Each parasitoid species typically specialises on a host species, and they will often lay their eggs on the host’s larval caterpillar stage. The baby parasitoids develop inside the host and eat the host alive. If their host is a caterpillar, they might even cause it to stop it from developing into the adult form in order to keep it around as a food source. After devouring the host from the inside out, they ultimately kill the host and burst out Alien-style. Yuck.

Ruth M. Colwill

Professor, Cognitive, Linguistic & Psychological Sciences, Brown University

Humans! We pollute the night skies with light that disorients hatchling sea turtles and migratory birds, sends thousands of moths into a tailspin ending in death by exhaustion, and shifts the birthing cycles of tammar wallabies. We have polluted our cities and oceans with noise that alters bird song communication and disrupts navigation by cetaceans, and our countryside with recreational noise and activity that stresses out the resident wildlife. Better known pollutants, perhaps, are the endocrine disrupting chemicals we manufactured that are now ubiquitous in the environment and suspected of causing behavioral anomalies in amphibians, birds, fish and mammals in addition to eggshell thinning in birds of prey and emasculation of alligators and gulls.

And in just a few decades, we have polluted our oceans with plastic trash sourced largely through the beverage industry and supermarkets. This plastic does not biodegrade; rather, it fragments into tiny pieces that absorb toxins from the water. These contaminated microplastics find their way into the food chain through the mouths of tiny zooplankton. The problem is so bad that seafood from the ocean can no longer be labelled organic. From a third to a half of seabird species consume plastic or feed it to their growing young, filling their stomachs with indigestible food that eventually starves them to death. The searing image of the emaciated flesh-footed shearwater with its stomach bulging with plastic, 175 pieces to be exact, screams out that the animal kingdom’s number one filthiest animal is the biggest polluter on the planet.

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