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Why Do Small Dogs Live Longer Than Large Dogs?

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Why do small dogs live longer than large dogs?

Adriana Heguy:

The issue of body size and lifespan is a fascinating topic in biology. It’s strange that across species, at least in mammals, large-bodied animals live longer than small-sized animals. For example, elephants live a lot longer than mice. The theory is that
bigger animals have slower metabolisms than small animals, and that faster metabolisms result in more accumulation of free radicals that damage tissue and DNA. But this doesn't always hold for all animals and the “rate of living” theory is not widely accepted. What we cannot clearly understand remains fascinating.

But now if we look at within a given species, lifespan and body size are inversely correlated. This is definitively the case for dogs and mice, and it has been proposed that this is the case for humans, too. Why would this be? A possible explanation is that larger dogs (or mice, or people) grow faster than their smaller counterparts because they reach a larger size in more or less the same time, and that faster growth could be correlated with higher cancer rates.

We do not have a clear understanding of why growing faster leads to accelerated aging. But it seems that it is an accelerated rate of aging, or senescence, that causes larger dogs to have shorter lifespans than little dogs.

The figure above is from Ageing: It’s a Dog’s Life. The data is from 32 breeds. Note that the inverse correlation is pretty good, however some large dog breeds, at around 40 to 50 kg (or about 88 to 110 pounds), live 12 or 13 years in average while some other dog breeds of equal body size live only eight or nine years on average. This is due to dogs being a special case, as they were artificially bred by humans to select for looks or behavior and not necessarily health, and that considerable inbreeding was necessary to produce “purebred” dogs. For example, boxers are big dogs, but their higher cancer rates may result in a shorter lifespan. However, the really giant breeds all consistently live eight to nine years on average. So there is something going on besides simple breeding quirks that led to bad genetics and ill health. Something more general.

A few years ago, a large study [PDF] was published using mortality data from thousands of dogs across 74 breeds, testing three hypotheses: Large dogs may die younger than small dogs because of (1) an earlier onset of senescence, (2) a higher minimum mortality hazard, or (3) an increased rate of aging. The conclusion from their study is that aging starts more or less at the same age in small and large breeds, but large breeds age faster. We do not have a clear understanding of the underlying mechanism for faster aging in dogs. It seems that when we selected for large body size, we selected for faster aging as well. But we do not know all the genetic components of this. We know that there are at least three genes that determine large body size in dogs: IRS4 and IGSF1, involved in thyroid hormone pathways which affect growth, and ACSL4, involved in muscle growth, and back fat thickness.

But how this accelerates aging is still speculation. More studies are needed, but dogs seem to be a great model to study the evolution of body size and its relationship to aging.

This post originally appeared on Quora. Click here to view.

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Big Questions
What Are Curlers Yelling About?
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WANG ZHAO/AFP/Getty Images

Curling is a sport that prides itself on civility—in fact, one of its key tenets is known as the “Spirit of Curling,” a term that illustrates the respect that the athletes have for both their own teammates and their opponents. But if you’re one of the millions of people who get absorbed by the sport once every four years, you probably noticed one quirk that is decidedly uncivilized: the yelling.

Watch any curling match and you’ll hear skips—or captains—on both sides barking and shouting as the 42-pound stone rumbles down the ice. This isn’t trash talk; it’s strategy. And, of course, curlers have their own jargon, so while their screams won’t make a whole lot of sense to the uninitiated, they could decide whether or not a team will have a spot on the podium once these Olympics are over.

For instance, when you hear a skip shouting “Whoa!” it means he or she needs their teammates to stop sweeping. Shouting “Hard!” means the others need to start sweeping faster. If that’s still not getting the job done, yelling “Hurry hard!” will likely drive the point home: pick up the intensity and sweep with downward pressure. A "Clean!" yell means put a brush on the ice but apply no pressure. This will clear the ice so the stone can glide more easily.

There's no regulation for the shouts, though—curler Erika Brown says she shouts “Right off!” and “Whoa!” to get her teammates to stop sweeping. And when it's time for the team to start sweeping, you might hear "Yes!" or "Sweep!" or "Get on it!" The actual terminology isn't as important as how the phrase is shouted. Curling is a sport predicated on feel, and it’s often the volume and urgency in the skip’s voice (and what shade of red they’re turning) that’s the most important aspect of the shouting.

If you need any more reason to make curling your favorite winter sport, once all that yelling is over and a winner is declared, it's not uncommon for both teams to go out for a round of drinks afterwards (with the winners picking up the tab, obviously). Find out how you can pick up a brush and learn the ins and outs of curling with our beginner's guide.

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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Why You Should Never Take Your Shoes Off On an Airplane
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What should be worn during takeoff?

Tony Luna:

If you are a frequent flyer, you may often notice that some passengers like to kick off their shoes the moment they've settled down into their seats.

As an ex-flight attendant, I'm here to tell you that it is a dangerous thing to do. Why?

Besides stinking up the whole cabin, footwear is essential during an airplane emergency, even though it is not part of the flight safety information.

During an emergency, all sorts of debris and unpleasant ground surfaces will block your way toward the exit, as well as outside the aircraft. If your feet aren't properly covered, you'll have a hard time making your way to safety.

Imagine destroying your bare feet as you run down the aisle covered with broken glass, fires, and metal shards. Kind of like John McClane in Die Hard, but worse. Ouch!

Bruce Willis stars in 'Die Hard' (1988)
20th Century Fox Home Entertainment

A mere couple of seconds delay during an emergency evacuation can be a matter of life and death, especially in an enclosed environment. Not to mention the entire aircraft will likely be engulfed in panic and chaos.

So, the next time you go on a plane trip, please keep your shoes on during takeoff, even if it is uncomfortable.

You can slip on a pair of bathroom slippers if you really need to let your toes breathe. They're pretty useless in a real emergency evacuation, but at least they're better than going barefoot.

This post originally appeared on Quora. Click here to view.

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