Dungeness crab shells washed up on a beach in Washington State.

Lee H. Rentz

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The Magic of Molting

Some animals shed their exoskeletons, skins, and more as they grow

Back in April, the Washington State Department of Fish and Wildlife began receiving numerous phone calls and emails from concerned residents. Many had spotted an alarming sight along Cherry Point beach a few miles south of the Canadian border: thousands of crabs washed up on the sand. The department sent officials out to investigate. Upon closer inspection, they discovered that what covered the beach wasn’t dead crabs, but their empty shells—the result of a natural process called molting.

Many animals undergo molting as a means of shedding their outer layer—feathers, hair, skin, or exoskeleton—so they can grow bigger or prepare for their next life stage. “It’s a critical event in the life cycle of an organism,” says Donald Mykles, a biologist at Colorado State University. 

Back in April, the Washington State Department of Fish and Wildlife received a lot of phone calls and emails from worried people. They had spotted an alarming sight on a beach a few miles south of the Canadian border. It looked like thousands of crabs had washed up on the sand. The department sent officials out to investigate. After getting a closer look, they discovered that dead crabs weren't covering the beach. It was just their empty shells. This was the result of a natural process called molting.

Many animals undergo molting in order to shed their outer layer. This lets them grow bigger or prepare for their next life stage. Animals might shed feathers, hair, skin, or an exoskeleton. An exoskeleton is a hard outer skeleton that protects an animal's body. “It’s a critical event in the life cycle of an organism,” says Donald Mykles, a biologist at Colorado State University. 

Humans have skin and bones that stretch and grow with us over time, but animals like the Dungeness crab in Washington have rigid outer bodies that can’t expand to accommodate their growth. Instead, these animals undergo multiple molts as they mature into adults.

All arthropods—including crustaceans, spiders, and insects—must regularly go through the molting process. But animals like snakes, birds, and dogs molt too.

“There are lots of different organisms that molt,” says Mykles, “but the process is very, very different for each one.” Read on to learn more about how and why different  species molt.

Humans have skin and bones that stretch and grow with us over time. But animals like the Dungeness crab in Washington have rigid outer bodies. Their bodies can’t expand with them as they grow. Instead, these animals undergo multiple molts as they mature into adults.

All arthropods, including crustaceans, spiders, and insects, must regularly go through the molting process. But animals like snakes, birds, and dogs molt too.

“There are lots of different organisms that molt,” says Mykles. “The process is very, very different for each one.” Read on to learn more about how and why different species molt.

Adhi Prayoga/Solent News

GRASSHOPPER

During its year-long life, a grasshopper develops from an egg to a nymph to an adult.  Molting occurs five to six times, but only during the nymph stage. That’s when it resembles a small adult without wings. Unlike other arthropods, which molt throughout their entire lives, most insects—including grasshoppers—stop molting once they become adults.

When it’s time to molt, a grasshopper’s body produces hormones that signal it can no longer support its increasing mass. The insect then begins to grow a new exoskeleton inside its old one. “It’s like growing a sleeping bag inside a sleeping bag,” says John Ewer, an entomologist at the University of Valparaíso in Chile.

Once the new shell is ready, the insect gulps in as much air as possible and breaks out of its old shell. Then it quickly attaches its muscles and nerves to the new shell. In this state they are easy prey for predators, “so they molt as quickly as possible, often in a few minutes,” says Ewer.

Grasshoppers live only for a year. In that time, it develops from an egg to a nymph to an adult.  It molts five to six times, but only during the nymph stage. That’s when it resembles a small adult without wings. Most arthropods molt throughout their entire lives. But most insects, including grasshoppers, stop molting once they become adults.

A grasshopper knows it’s time to molt because its body produces a special hormone. This chemical signal tells the grasshopper's body that's it's time to molt. The insect then begins to grow a new exoskeleton inside its old one. “It’s like growing a sleeping bag inside a sleeping bag,” says John Ewer. He studies insects at the University of Valparaíso in Chile.

Once the new shell is ready, the insect gulps in as much air as possible and breaks out of its old shell. Then it quickly attaches its muscles and nerves to the new shell. In this state the insects are easy prey for predators. “They molt as quickly as possible, often in a few minutes,” says Ewer.

Leonard Lee Rue III/Science Source/Getty Images

SNAKE

When snakes molt, they leave behind their old skin in one piece. Because they don’t have exoskeletons like arthropods, snakes remove their skin in a process that’s also called shedding. “The shedding of skin all in one piece is unusual for vertebrates,” says biologist Harvey B. Lillywhite from the University of Florida.

Environmental factors such as temperature and the amount of food available can affect the frequency of molting. Most terrestrial snakes shed their skin two to three times a year, says Lillywhite.

During the two-week molting period, terrestrial snakes can get aggressive. That’s because they shed their eye caps, which are special scales that cover their eyes. This impairs their vision, making them feel particularly vulnerable. 

When snakes molt, they leave behind their old skin in one piece. Snakes don’t have exoskeletons like arthropods. When they molt, they remove their skin. This process is also called shedding. “The shedding of skin all in one piece is unusual for vertebrates,” says biologist Harvey B. Lillywhite from the University of Florida.

Environmental factors such as temperature and the amount of food available can affect how often a snake molts. Most terrestrial snakes shed their skin two to three times a year, says Lillywhite.

Snakes can get aggressive during the two-week molting period. That’s because they shed their eye caps. These are special scales that cover their eyes. This makes it hard for the snake to see, making them feel particularly vulnerable. 

Louise Heusinkveld/Alamy Stock Photo

BIGHORN SHEEP

Molting isn’t just for insects and snakes. Some mammals molt too, shedding their pelage—the fur, wool, or hair that covers their body—on a seasonal or year-round basis. Bighorn sheep, the largest wild sheep in North America, shed their heavy winter coats every June and July, when the weather warms up.

Losing their wool is a critical process for the survival of bighorn sheep—molting helps them avoid heat stress in the summer. It also helps them to more easily navigate the rugged regions at higher elevations.

Wild sheep, like the bighorn, shed their wool naturally. But domestic breeds, such as Merino sheep, don’t molt. Their wool grows continuously, producing anywhere from 10 to 40 pounds annually. Farmers have to shear the animals once or twice a year.

Molting isn’t just for insects and snakes. Some mammals molt too. They shed their fur, wool, or body hair on a seasonal or year-round basis. The largest wild sheep in North America are the bighorn sheep. They shed their heavy winter coats every June and July, when the weather warms up.

Losing their wool is a very important process for the bighorn sheep’s survival. Molting helps them avoid heat stress in the summer. It also helps them to more easily navigate the rugged regions high up in the mountains.

Wild sheep, like the bighorn, shed their wool naturally. But types of sheep raised by farmers don’t molt. Their wool grows continuously, producing anywhere from 10 to 40 pounds annually. Farmers have to shear the animals once or twice a year.

Use the information to answer questions about how different animals molt. Round your answers to the nearest whole percent. Record your work and answers on our answer sheet.

Use the information to answer questions about how different animals molt. Round your answers to the nearest whole percent. Record your work and answers on our answer sheet.

In its first year, a Dungeness crab molts 6 times. By the time it’s fully mature at age 3, it’s molted a total of 15 times. What percent of a mature crab’s molts happen in its first year of life?

In its first year, a Dungeness crab molts 6 times. By the time it’s fully mature at age 3, it’s molted a total of 15 times. What percent of a mature crab’s molts happen in its first year of life?

If a Dungeness crab can live up to 13 years, what percent of its total expected life span has a 3-year-old crab reached?

If a Dungeness crab can live up to 13 years, what percent of its total expected life span has a 3-year-old crab reached?

The long-horned grasshopper spends up to 108 days in the nymph stage, when it undergoes molting. What percent of its 365-day-long lifetime does it spend as a nymph?

The long-horned grasshopper spends up to 108 days in the nymph stage, when it undergoes molting. What percent of its 365-day-long lifetime does it spend as a nymph?

An American grasshopper weighs 10 milligrams as a new nymph. Six weeks later, it’s an adult at 2,500 milligrams. What percent of an adult’s weight is a nymph’s weight, rounded to the nearest tenth?

An American grasshopper weighs 10 milligrams as a new nymph. Six weeks later, it’s an adult at 2,500 milligrams. What percent of an adult’s weight is a nymph’s weight, rounded to the nearest tenth?

The number of rattles on a rattlesnake’s tail corresponds to the number of times it’s molted. If a rattlesnake has 18 rattles, and shed its skin 7 times in its first year of life, what percent of shedding occurred that first year?  

The number of rattles on a rattlesnake’s tail corresponds to the number of times it’s molted. If a rattlesnake has 18 rattles, and shed its skin 7 times in its first year of life, what percent of shedding occurred that first year?  

In its first year, a king cobra sheds its skin once every month. If a shedding cycle takes about 1.8 weeks to complete, what percentage of time does a cobra spend shedding in its first year of life? (Hint: There are 52 weeks in a year.)

In its first year, a king cobra sheds its skin once every month. If a shedding cycle takes about 1.8 weeks to complete, what percentage of time does a cobra spend shedding in its first year of life? (Hint: There are 52 weeks in a year.)

Two species of wild sheep live in North America: Dall’s and bighorn sheep. A female Dall’s sheep weighs 110 pounds, and a female bighorn is 200 pounds. A Dall’s sheep’s weight is what percent of a bighorn’s weight?

Two species of wild sheep live in North America: Dall’s and bighorn sheep. A female Dall’s sheep weighs 110 pounds, and a female bighorn is 200 pounds. A Dall’s sheep’s weight is what percent of a bighorn’s weight?

A male Merino sheep weighs about 180 pounds and produces 25 pounds of wool. A female weighs about 120 pounds and produces 18 pounds of wool. Which produces a greater percent of its body weight in wool?

A male Merino sheep weighs about 180 pounds and produces 25 pounds of wool. A female weighs about 120 pounds and produces 18 pounds of wool. Which produces a greater percent of its body weight in wool?

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