Smithsonian National Zoological Park l Friends of the National Zoo



Cracking the Egg

The egg reigns supreme as the animal kingdom's preferred reproductive method.

By Devin Murphy

Life begins with an egg. Most mammals, reptiles, amphibians, invertebrates, and birds at the Smithsonian’s National Zoo started out as nothing more or less than a fertilized egg. Some of those eggs transformed within their mothers’ wombs into cuddly cubs or precious pups. But most did not.


A Cuban crocodile hatches. (Jessie Cohen/NZP)

In many species, young grow outside their mother’s body. The encapsulating egg acts as a surrogate mother, protecting the growing embryo while providing it with nutritious yolk, water, oxygen, and room to grow.

Selecting the Sex

Eggs are such a fantastic life support system that it almost seems foolish to hatch. The Labord’s chameleon would agree. It lives only about 12 months and spends eight or nine of them in its egg, protected from Madagascar’s oppressive dry season.

Such a short life span is atypical among reptiles, some of which live for a century. Cuban crocodiles can live for 75 years, spending only 86 days in their eggs.

Crocodilian eggs do not act as shields from the environment to the extent that Labord’s chameleon eggs do. A crocodile egg actually needs direction from its environment. The temperature of the nest activates the sex chromosomes of a developing crocodile in its egg. Temperature-dependent sex determination, as this process is called, is not left up to chance, however. Female crocodiles are keenly sensitive to soil temperature as they build nests for their 30 to 40 eggs.

Green tree python

A Cuban crocodile lays eggs. (Mehgan Murphy/NZP)

To hatch males, Cuban crocodile mothers need to keep their nests between 30° and 30.5° Celsius. Any higher or lower, and the nest will yield females. But hatching a generation of all male or female crocodiles is not a recipe for evolutionary success. To prevent a generational disaster, different parts of the nest will hatch different sexes.

“Eggs that are closer to the surface may not be at the same temperature as eggs in the center or bottom of the nest,” says Barbara Watkins, a keeper at the Reptile Discovery Center (RDC). “Another factor is that females have been observed urinating on the nest once it is complete. Some think this encourages bacterial decomposition. That may affect the temperature.”

Because the Zoo is a captive setting, it is a little easier to hatch a desired sex. After a female crocodile lays a clutch of eggs, keepers remove them from the nest and place them into prepared and meticulously monitored incubators, which are kept at the perfect temperature for males or females. RDC curator Jim Murphy explains that keepers “almost always” move eggs to incubators, “because it gives us more control.”

Snake receives eye injection.

Zoo staff collect Cuban crocodile eggs. (Mehgan Murphy/NZP)

The window for moving eggs into an incubator is short. “I would say we have 24 to 48 hours after the eggs are laid to have them at the correct temperature,” says Watkins. Once the eggs are safely placed in an incubator, keepers will not touch them again until they hatch. “The embryo will attach itself to the egg within 24 hours of being laid,” Watkins explains. “Movement of the egg may result in the death of the embryo.”

The Zoo is planning, at some point down the road, to hatch four Cuban crocodiles to bolster the captive population in North America as the wild populating dwindles.

The iPhones of Eggs

Reptiles are seasoned egg-layers (crocodiles are relics from the Jurassic era), but they have competition from the air. “Reptiles are kind of like the Apple II computer, and birds are like the iPhone,” says bird curator Dan Boritt. That’s not to say that birds are total newcomers to the egg-laying game. “People will argue both ways on evolution and where birds came from. My take is birds are direct descendents of reptiles and dinosaurs. Most dinosaurs were egg-laying, but there has been quite a bit of change in the way birds incubate.”

Birds exclusively lay eggs; not a single species gives birth to live young. That’s because most birds rely on flight as their main defense against predators. “Birds have to be mobile,” says Boritt, and they can’t afford the extra weight of a developing embryo. Most birds lay eggs within two days of fertilization.

Of the more than 100 species of birds at the Zoo, the brown kiwi breaks all the rules. “Everything about them is just different,” says keeper Kathy Brader. Native to New Zealand, this endangered, flightless bird lays an egg that weighs about 20 percent of her own weight. The egg is so large, it fills her entire abdominal cavity. Such a huge egg means that there is little room for any metabolic activity. So the female stops eating a few days before she lays her egg.

Snake receives eye injection.

Kiwi chick Koa, a male, hatched in 2008.. (Mehgan Murphy/NZP)

“A lot of people will tell you, myself included, that the most strenuous thing a bird will ever do is hatch from an egg,” says Boritt. “A lot of birds will die in the hatching process.” Most birds and reptiles have an egg tooth: a calcification on their beaks or snouts. It is specially designed to help them break out of their egg. Kiwis lack an egg tooth. They kick their way out. Hatchlings begin foraging within a few days. Even though they are capable of taking care of themselves, they may still stay in their parents’ territory for up to six months.

Big Birds, Big Eggs

Kiwis are not the only birds with big eggs. The Zoo is also home to kori bustards, the largest birds that can fly. In the wild, these threatened birds live in southern and eastern Africa. Females lay eggs only during the wet season, because chicks need an abundance of insects to flourish. When the rains finally arrive, several females mate with a single male. Then each female lays two 150-gram eggs right on the ground—no nest required.

“The eggs are green,” explains biologist Sara Hallager. “They kind of look like something out of the army because they’re camouflage.” Their color helps them blend in with the ground and rocks that surround them, so they are safe when their mother leaves them to get food.

Kori bustard eggs hatch 23 days after being laid. “For a bird that size, it’s very short,” Hallager points out. But after they hatch, there is no rush to grow up and fly away from home. “They stay with mom for almost the first full year,” says Hallager.

At the Zoo, keepers pull kori bustard eggs for hand-rearing. The female birds receive dummy eggs, which they continue to faithfully incubate. Because kori bustards are so skittish and wary of humans, it is easier to hand-rear them in captivity so they are less shy as mature birds. The Zoo has raised 55 chicks since 1997 and was one of the first zoos to raise koris.

Kori bustard eggs are placed in incubators and need to be turned every two hours. Two days before the egg hatches, it will begin to turn itself. After a chick hatches, it no longer has a use for the shell, which lies cracked into pieces, but keepers still do. “When a chick hatches, there are a lot of residual blood veins. You can actually take the egg shell and send it into a lab, and they can sex the chick from the DNA,” says Hallager.

Different Parenting Styles

Like reptiles and birds, amphibians lay eggs. But they don’t stop there. Amphibians coat their eggs with a transparent jelly. The substance protects the eggs and also traps heat, regulating the temperature of the young amphibians until they are capable of doing so. Being semi-permeable, the jelly also helps hydrate the developing animals.

Solomon Island leaf frogs skip the tadpole stage, hatching as froglets. (Mehgan Murphy/NZP)

Frogs can lay hundreds of eggs in a single clutch. The American bullfrog does so, counting on sheer volume to ensure the survival of at least some of its offspring. “That’s a perfectly good way of making new frogs and standard in the temperate part of the world,” says biologist Edwin Smith. But “when you get down to the tropics, wow! How everything changes.”

The strawberry poison dart frog, native to Central and South America, invests in hard work rather than large clutch sizes. Females lay only six or seven eggs, bunched together in a clear jelly, on a sheltered leaf on the forest floor. The male waters the clutch by emptying his bladder on it. Meanwhile, the female patiently waits about eight days for the embryos to grow into tadpoles. Each is the size of a grain of rice.

When a tadpole hatches, the mother frog places it on her back. She then climbs into the upper level of the rainforest to find a small pool of water nestled in the leaves of a bromeliad. She leaves the tadpole there. The mother repeats the trek for each tadpole.

For the next 80 days, the mother returns regularly to feed the tadpoles. On each visit, she lays an unfertilized egg—nutritious food that helps the tadpole survive metamorphosis.

Last year, the Zoo bred strawberry dart frogs for the first time in its history. Keepers customarily raise eggs and tadpoles away from their parents, but they cannot do that with strawberry dart frogs. “It’s not really that difficult; you just have to have the right environment for it,” says Amazonia keeper Justin Graves. “The female strawberry dart frogs have to feed the juvenile tadpoles unfertilized eggs.”

The frogs’ terrarium includes bromeliads that can serve as tadpole nurseries. They are about three feet up from the bottom—a long way for a frog that is smaller than a quarter.

Hordes and Hitchikers

Add all those egg-laying reptiles, birds, and amphibians together, and you’d still come nowhere near the number of the largest group of egg layers—invertebrates. These spineless creatures account for 99 percent of the animal kingdom.

One of the Zoo’s most popular invertebrates, the giant Pacific octopus, puts all its eggs in one basket, proverbially speaking.
For females, breeding is literally a once-in-a-lifetime experience. They find a cave or crevice that can serve as a den. There, they lay more than 80,000 eggs in strands that resemble miniature grape stems.

Tending the clutch is a labor of love. The female fastidiously bathes the eggs with oxygenated water, which she squirts over them with her siphon. This intense work keeps the octopus too busy for food. “Octopuses may live only two months to six months after that because they’re not eating,” says invertebrates curator Alan Peters.

When the octopuses hatch two months later, they are no bigger than a pinkie fingernail. Translucent, they have two red eyes. Their tentacles are nothing more than little stubs, and their mantles make up the bulk of their miniscule bodies. “They’re really small, and they often get eaten,” says Peters. Only a lucky few get the chance to live up to their “giant” name.

These octopuses share that adjective with another resident of the Zoo’s Invertebrate Exhibit. Giant water bugs can grow an inch and a half long, not too shabby for a bug. Females have a surefire way of ensuring paternal care for their eggs: They lay them on the father’s back! “It’s his responsibility to hatch the eggs,” says keeper Donna Stockton.

Snake receives eye injection.

A male giant water bug carries eggs on its back.(Mehgan Murphy/NZP)

The female attaches the eggs to the male’s back by secreting a glue-like substance that holds them in place. She then carpets the male’s back with eggs. “They fill his whole back,” says Stockton. “That can be anywhere from 20 to 100 eggs.”

When the larvae are ready to hatch, the father swims to the surface of the water so that his offspring can breathe. (Though aquatic, giant water bugs breathe air.) The newly hatched larvae are no bigger than M&M candies. They are white for an hour or two before their exoskeletons harden. At the Zoo, Stockton separates the newly hatched larvae quickly to prevent them from eating each other.

Practically Perfect

Obviously, some species, notably ours, reproduce successfully without laying eggs. And eggs are far from foolproof. Predators eat them, some fail to hatch, and many young don’t make it to adulthood. Still there’s no denying that, for animal offspring of countless species, shapes, and sizes, eggs make nearly perfect packages.


—DEVIN MURPHY is the editorial intern for Smithsonian Zoogoer.


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Smithsonian Zoogoer 40(2) 2011. Copyright 2011 Friends of the National Zoo. All rights reserved.