Humans have very strict tradition of hygiene, such as washing hands before eating, using utensils, and in general keeping as much distance between food and dirt as possible.

At the same time, most animals would eat off the floor or eat food with foreign particles stuck to it ( after being dropped on the floor). Many animals, like cats groom themselves, practically licking off (and swallowing?) dirt off their coats.

Do animals suffer adverse effects, such as indigestion or disease from consuming food that is dirty by human standards?

Or is it something about the human digestive system that cannot handle the same kinds of dirt that animals can?

  • 2
    $\begingroup$ Hygiene is very much cultural, compare fastidious Americans that use Clorox wipes on everything to indigenous cultures that have very few taboos against dirt. $\endgroup$
    – MattDMo
    Aug 10, 2013 at 23:58
  • $\begingroup$ There's a parallel to be drawn here with carnivorous animals eating raw meat, too. I naively expect that they just get more parasites (ie. that our admittedly cultural components of cooking do in fact also help protect against infection), but I'd be interested to hear if there's more to the answer. $\endgroup$ Aug 11, 2013 at 0:28

2 Answers 2


I'm sure it varies wildly based on the animal and what they're eating. In general, if in the course of an animal's natural feeding process it picks up a little dirt, it has evolved to cope with that. Animal's behaviors and guts have evolved to fit their food source and lifestyle. For a behavioral example, seals will eat rough rocks to help breakdown bones (and barf them up when they get too smooth). Guts have evolved symbiotic relationships with microbes that are specifically adapted to the food source, termites have microbes to help them break down wood, whereas we don't have the microbe population to pull that off. Interesting side note: it was found that one of the gut microbes in some Japanese people has gotten a horizontally transferred gene from a bacteria that lives on seaweed, probably helping them break down seaweed.

There could be a problem when the body is challenged with dirt that contains uncommon substances that the body isn't adapted to. I'm sure if a cat rolled around in soil contaminated with a lot of arsenic it probably wouldn't be doing very well after licking off all the dirt.

This does bring up an interesting current medical hypothesis called the Hygiene Hypothesis. Basically, these things you are talking about, utensils, antibacterial soap, etc. are all very very recent developments in human evolution. As a result, proponents of this hypothesis think that our bodies are adapted to challenges by microbes and parasites, and depriving our bodies of that exposure when we are young totally screws up the development of our immune systems. This would explain why we see a huge increase in allergies and autoimmune diseases in the developed world, our immune systems are totally miscalibrated, and freak out over harmless things like pollen (allergies), or worse, attack self-antigens (anti-insulin antibodies in type I diabetes). Human children are animals, and baby animals should play in the dirt!

  • $\begingroup$ Horizontally transferred seaweed-eating gene? Wow! $\endgroup$ Aug 11, 2013 at 11:12
  • $\begingroup$ One of my all-time favorites :) $\endgroup$
    – gchadwick
    Aug 11, 2013 at 15:02

Animals certainly occasionally get sick from eating contaminated food, just as humans used to before our standards of sanitation improved. There are various adaptations/considerations for this. Apologies for not being able to find scholarly references. Here are some anecdotes, some of which are speculation/urban legend:

  • Predators eat fresh meat, which is less likely to be spoiled
  • Vultures lack feathers on their heads and bathe frequently to minimize exposure
  • Dogs supposedly eat grass to induce vomiting when they've eating something bad (fairly disputed)
  • Rats and other scavengers have excellent senses of smell, and can likely detect common contamination or over-spoilage
  • Some scavengers may have more aggressive digestive systems to reduce infections

Only 100 years ago, humans weren't so well off ourselves. While food hygiene is only part of it, our increased sanitation has reduced mortality from infectious diseases substantially. From the CDC:

In 1900, the three leading causes of death were pneumonia, tuberculosis (TB), and diarrhea and enteritis, which (together with diphtheria) caused one third of all deaths (Figure 2). Of these deaths, 40% were among children aged less than 5 years (1). In 1997, heart disease and cancers accounted for 54.7% of all deaths, with 4.5% attributable to pneumonia, influenza, and human immunodeficiency virus (HIV) infection (2).

It would be interesting to find a break down of the improvement from agriculture/cooking, water/sewage, medicine, etc.


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