Why Do Animals Have Soft White Underbellies?

Slate’s animal blog.
Dec. 25 2013 11:45 AM

Why Do Animals Have Soft White Underbellies?

A rescued Magellanic Penguin from South America swims in the water at the Aquarium of the Pacific in Long Beach, California, May 2012.
My, what a soft belly you have.

Photo by Frederic J. Brown/AFP/GettyImages

The phrase “soft white underbelly” describes the anatomy of a broad spectrum of animals ranging from sharks to amphibians to birds and even some mammals. Why is the most notorious biological vulnerability on Earth so ubiquitous?

Pale underbellies are most often found among animals that need to worry about danger from below. Creatures that swim, fly, or climb may blend into the background water or sky above if their bellies are light. Animals can also benefit from a camouflage concept called countershading. By having darker pigment on the part of the body that receives the most light and lighter pigmentation on the part that receives less light, the animal’s appearance tends to flatten out and is less conspicuous.

But what explains the softness?

One might think that the basic skeletal design of vertebrates has always neglected to protect abdomens—the rib cage covers the chest and opens up around the belly in most familiar species. But this was not always so. Many prehistoric animals had a group of protective bones called gastralia that covered their bellies like a second set of ribs. Gastralia provided not only some measure of physical protection but also attachment points for abdominal muscles and other tissue.

Advertisement

Theropod dinosaurs such as Tyrannosaurus rex had gastralia, as did pterosaurs and champsosaurs. Archaeopteryx had gastralia, though modern birds do not. It isn’t hard to imagine why birds lost those abdominal bones. Getting off the ground quickly requires being as light as possible. Birds have hollow bones because of this evolutionary pressure. If the gastralia were adding weight and the birds didn’t tend to be attacked in the stomach very often, then birds born without those bones may have been able to get into the air and stay there more easily.

Perhaps the birds illustrate why other species lost or never bothered with abdominal bones. Extra bones or armor take a lot of calories and calcium to grow and maintain. It’s like the MRAP armored vehicles that the U.S. military used in Iraq. Soldiers and Marines needed the bellies of their vehicles heavily armored to protect against roadside bombs. Once they left Iraq and the threat of sudden death from below was lower, the military discarded the MRAPs in favor of lighter, faster vehicles that also happen to use less fuel. As in nature, military vehicles evolved from having soft bellies, to armored ones, and then back to being soft-bellied.

Today there are still a few animals with gastralia. The crocodilians, an ancient group of animals that dates back to the time of the early dinosaurs, all have gastralia within their white underbellies. But for the most part, animals really do expose soft white underbellies to the modern wild.

Jackson Landers is the author of Eating Aliens. He recently spent a year and a half hunting and eating invasive species throughout North America. Email him here.

TODAY IN SLATE

War Stories

The Right Target

Why Obama’s airstrikes against ISIS may be more effective than people expect.

The NFL Has No Business Punishing Players for Off-Field Conduct. Leave That to the Teams.

Meet the Allies the U.S. Won’t Admit It Needs in Its Fight Against ISIS

I Stand With Emma Watson on Women’s Rights

Even though I know I’m going to get flak for it.

Should You Recline Your Seat? Two Economists Weigh In.

Medical Examiner

How to Stop Ebola

Survivors might be immune. Let’s recruit them to care for the infected.

History

America in Africa

The tragic, misunderstood history of Liberia—and why the United States has a special obligation to help it fight the Ebola epidemic.

New GOP Claim: Hillary Clinton’s Wealth and Celebrity Are Tricks to Disguise Her Socialism

Why the Byzantine Hiring Process at Universities Drives Academics Batty

Moneybox
Sept. 23 2014 3:29 PM The Fascinating Origins of Savannah, Georgia’s Distinctive Typeface
  News & Politics
History
Sept. 23 2014 11:45 PM America in Africa The tragic, misunderstood history of Liberia—and why the United States has a special obligation to help it fight the Ebola epidemic.
  Business
Moneybox
Sept. 23 2014 2:08 PM Home Depot’s Former Lead Security Engineer Had a Legacy of Sabotage
  Life
Education
Sept. 23 2014 11:45 PM Why Your Cousin With a Ph.D. Is a Basket Case  Understanding the Byzantine hiring process that drives academics up the wall.
  Double X
The XX Factor
Sept. 23 2014 2:32 PM Politico Asks: Why Is Gabby Giffords So “Ruthless” on Gun Control?
  Slate Plus
Political Gabfest
Sept. 23 2014 3:04 PM Chicago Gabfest How to get your tickets before anyone else.
  Arts
Brow Beat
Sept. 23 2014 8:38 PM “No One in This World” Is One of Kutiman’s Best, Most Impressive Songs
  Technology
Future Tense
Sept. 23 2014 5:36 PM This Climate Change Poem Moved World Leaders to Tears Today
  Health & Science
Medical Examiner
Sept. 23 2014 11:37 PM How to Stop Ebola Could survivors safely care for the infected?
  Sports
Sports Nut
Sept. 23 2014 7:27 PM You’re Fired, Roger Goodell If the commissioner gets the ax, the NFL would still need a better justice system. What would that look like?