Molting is a widespread biological process in which animals periodically cast off a part of their outer structure, such as skin, feathers, or a shell, and replace it with new growth. This shedding is necessary for survival and health across many different groups in the animal kingdom. The process allows an organism to maintain its protective outer layer, which can become damaged, worn down, or too small for the growing body. This renewal is governed by biological signals and environmental cues, ensuring the animal’s physical structure remains functional.
What Molting Is and Why It Happens
Molting, sometimes referred to as shedding, is a hormonally controlled event that serves several physiological purposes. The most straightforward reason is to accommodate growth, particularly in species whose body covering is rigid and unable to expand. As an animal increases in size, its external layer must be cast off so a larger structure can be formed underneath.
Beyond growth, molting is a regular maintenance activity that replaces old or compromised tissues with fresh ones. Feathers, skin, and exoskeletons all experience wear and tear from friction, parasites, or environmental exposure, reducing their protective or functional capabilities. Replacing these layers is a form of biological repair that restores the integrity of the animal’s outer barrier.
Many species also molt in response to seasonal or environmental changes, often regulated by changes in daylight hours (photoperiod). This seasonal molt allows animals to change their appearance for insulation or camouflage, such as growing a denser coat for winter or changing plumage color for breeding displays. The timing of the molt is regulated to ensure the animal is protected and ready for the demands of the coming season.
How Arthropods Shed Their Outer Shell
For arthropods (including insects, crustaceans, and arachnids), the process of shedding their rigid, non-living exoskeleton is known specifically as ecdysis. Since the hard outer shell cannot stretch, ecdysis is the only way for the animal to increase its body size. The process begins with the separation of the old cuticle from the underlying epidermal cells, a phase called apolysis.
During apolysis, the epidermal layer secretes a new, soft cuticle beneath the old exoskeleton, and a molting fluid is released into the space between the layers. Enzymes within this fluid partially digest the inner layer of the old shell, allowing the animal to reabsorb some of the raw materials before the final shed. This recycling of resources minimizes the metabolic cost of forming a new structure.
The actual shedding (ecdysis) occurs when the arthropod uses muscular contractions and increased internal fluid pressure to split the old exoskeleton along weak lines. The animal then extracts itself from the old shell, which is left behind as the exuviae. Immediately after emerging, the newly molted animal is in a soft, vulnerable state (often called teneral). It must quickly inflate its body by taking in air or water to stretch the new cuticle before it hardens. This final hardening process, known as sclerotization or tanning, stiffens and darkens the new shell, a procedure that can take hours or days.
Molting in Birds and Reptiles
Vertebrates like birds and reptiles molt, but their process differs significantly from the wholesale replacement seen in arthropods, focusing on gradual maintenance. Birds periodically shed and replace their feathers, which is necessary because feathers are dead structures that become frayed and damaged over time. Most bird species undergo a partial molt, replacing feathers symmetrically and gradually to maintain their ability to fly and regulate body temperature.
The timing of a bird’s molt is linked to the end of the breeding season, after the energetic demands of reproduction have passed and before migration begins. Some waterfowl undergo a complete molt of their flight feathers all at once, making them temporarily flightless until the new feathers grow in. The process is governed by hormonal responses to seasonal changes in the length of daylight.
Reptiles, such as snakes and lizards, shed their outer layer of skin in a process called sloughing. This shedding is triggered by growth, injury, or the need to remove parasites. Snakes typically shed their skin as a single, inverted piece, often starting at the head and working it off by rubbing against rough surfaces. Before the event, a snake’s skin color dulls, and its eyes may take on a milky appearance as a layer of fluid forms between the old and new skin. Unlike the arthropod molt, the reptile and bird molt is a mechanism for maintaining a healthy and functional outer covering.