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Why Do Worms Surface After Rain?

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Reader Bruce wrote in to ask, “When there is a heavy rain, worms climb out on the pavement, only to die when the rain stops. Why do worms commit suicide?”

Worms coming topside during and after rain used to be explained as them simply trying not to drown in water-logged soil. As biologists learned more about annelids (the group that includes earthworms, ragworms, and leeches), though, we figured out that some of them breathe a little differently than most land animals, and don’t drown as easily as you or I would in a watery hole in the ground.

Earthworms breathe by passing oxygen and carbon dioxide through their skin. For the oxygen to get through the skin and into the worms’ bloodstream properly, there needs to be some moisture on the skin, so the worms produce mucus to keep them moist and slimy. Moisture in the ground keeps the mucus from drying out, so soil that’s a little wet actually helps the worms breathe easy. Even if the soil is very saturated or an earthworm is submerged in water, it can survive as long as there’s sufficient oxygen to pull in through the skin.

Drowning in a rain shower, then, isn’t really a big concern—most of the time. Different species consume oxygen at different rates, sometimes at different times during the day, so some are more at risk for running out of oxygen in wet soil than others. In 2008, zoologists in Taiwan looked at two worm species, one that surfaces when it rains and one that doesn’t. They found that the surfacing worm consumed oxygen at a faster rate—especially at night—and didn’t tolerate total water submersion all that well. They could stay underground for a little while when it rained during the day, but had to come up sooner if it rained at night. The other worm consumed oxygen at a lower rate and could survive underground with lower oxygen concentrations.

So, drowning in wet soil is a concern for some earthworms, but not all of them. But is there something else that drives them aboveground?

Some biologists think that worms come up to reproduce and/or make long trips, both of which are easier to do aboveground than in the confines of the soil. Since they need to stay moist to breathe, they hold off on these activities until the surface is nice and wet with rain and there’s a lower risk of drying out.

The mating 'n movement hypothesis is supported by the type of worms you see during and after rain. As Phil Nixon from the University of Illinois explains:

 “If the other factors were the driving influences, large numbers of juvenile worms should also be present, but the vast majority appears to be adult red worms with occasional adult nightcrawlers. When worms are brought to the surface with electricity, worm-grunting, or chemicals, many more worms are present. It is obvious that only a small percentage of the adult red worms are emerging during heavy rains. Perhaps this represents the small percentage of the population with the colonial spirit.* It appears somewhat similar to the small percentage of the world's people with a colonial spirit that immigrated to this continent and became our ancestors.”

Now, coming to the surface can be risky. There’s hungry birds, careless humans, and the possibility that a worm will get marooned on a sidewalk and dry out. If they’re coming up to mate, though, the risk involved may be overcome by the drive to reproduce. Surfacing isn’t necessarily suicidal, either. The worms don’t die, like Bruce thought, as soon as the rain stops, and many do make it back underground before they’re in danger. If death was guaranteed, we’d have no surfacing worms left to wonder about.

*Or maybe the ones that are able and ready to reproduce.

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Big Questions
Do Cats Fart?
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Certain philosophical questions can invade even the most disciplined of minds. Do aliens exist? Can a soul ever be measured? Do cats fart?

While the latter may not have weighed heavily on some of history’s great brains, it’s certainly no less deserving of an answer. And in contrast to existential queries, there’s a pretty definitive response: Yes, they do. We just don’t really hear it.

According to veterinarians who have realized their job sometimes involves answering inane questions about animals passing gas, cats have all the biological hardware necessary for a fart: a gastrointestinal system and an anus. When excess air builds up as a result of gulping breaths or gut bacteria, a pungent cloud will be released from their rear ends. Smell a kitten’s butt sometime and you’ll walk away convinced that cats fart.

The discretion, or lack of audible farts, is probably due to the fact that cats don’t gulp their food like dogs do, leading to less air accumulating in their digestive tract.

So, yes, cats do fart. But they do it with the same grace and stealth they use to approach everything else. Think about that the next time you blame the dog.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Animals
Squirrels Are Probably More Organized Than You, Study Finds
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Despite having a brain that's slightly bigger than the size of a peanut M&M, squirrels have a fascinating, razor-sharp instinct when it comes to survival. They know that acorns that are high in fat and sprout late are perfect for long-term storage, so they salvage them for winter and eat the less nutritionally dense white-oak acorns right away. They also tend to remember where they put their acorn stash rather than relying solely on smell. Like nature's perfect stunt performer, they can even fall out of trees in a way that minimizes physical damage. Now, researchers at the University of California, Berkeley have unveiled a newly discovered part of a squirrel's hoarding strategy, Atlas Obscura reports.

The researchers tracked 45 wild fox squirrels on the UC-Berkeley campus for nearly two years. They made available to the squirrels four different types of nuts—walnuts, pecans, almonds, and hazelnuts. Sometimes the animals were given a single type of nut, and other times the nuts were mixed. Either way, the squirrels promptly sorted and stored their food according to type—walnuts went in one hiding place, almonds in another, and so on.

This type of behavior is known as "chunking" and makes it easier to retrieve data in memory. In doing this, a squirrel won't have to visit several different places looking for pecans: They know just where the main supply is. Squirrels can stockpile up to 10,000 nuts a year, so it's essential for them to know which type of nut is where.

The study, published in Royal Society Open Science, also indicated that squirrels seem to understand nuts have weight, choosing to carry heavier acquisitions to a different location than lighter nuts.

Squirrels being squirrels, they were happy to be gifted an assortment of nuts during the experiment, but there was one wrinkle: Rather than stash them away, sometimes they'd just eat them on the spot.

[h/t Atlas Obscura]

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