What is the lower mandible of a bird?

The lower mandible is the lower half of a bird’s beak. It plays an important role in a bird’s ability to eat, drink, preen feathers, feed young, and defend itself. The lower mandible works together with the upper mandible to grasp, manipulate, and process food.

Anatomy of the Lower Mandible

The lower mandible consists of several key parts:

Bony Core

The main shaft of the lower mandible is made up of lightweight bone called ossified rostral bones. This gives the mandible strength and rigidity while minimizing weight. The size and shape of the bony core can vary significantly between bird species depending on diet. For example, seed-eating finches have short, thick mandibles for cracking hard seeds, while hummingbirds have long, slender mandibles for sipping nectar from flowers.

Rhamphotheca

Covering the bony core is a layer of keratin called the rhamphotheca. This is the outer covering that gives the mandible its color and texture. The rhamphotheca continues to grow throughout a bird’s life, helping to keep the beak sharp by replacing layers gradually worn away by use.

Tomia

The tomia are the inner sharp edges of the rhamphotheca that come together like a pair of scissors. When the upper and lower mandible close, the tomia align precisely, allowing birds like falcons and eagles to neatly slice meat. In species that crush food, the tomia are often rounded and blunt.

Nares

The nares are paired openings on each side of the mandible that connect to the nasal cavity within the skull. This allows birds to breathe while their beak is closed. A section of soft skin called the cere surrounds the nares.

Mandibular Symphyses

On the ventral side of the mandible is the mandibular symphysis, a fusion point where the left and right sides join together. This symphysis allows flexibility in the lower jaw.

Articular Process

At the posterior end of each mandibular ramus is an articular process. These bony knobs articulate with the quadrate bone of the skull to form the jaw joint. Powerful muscles attached here allow the mandible to open and close.

Upper vs. Lower Mandible

While the upper and lower mandible work closely together, they have some distinct differences:

• The upper mandible is firmly fused to the skull, while the lower mandible is separate and mobile, only attaching at the jaw joints.
• The upper mandible tends to be shorter and deeper than the lower mandible.
• The tomia of the upper mandible overlap those of the lower mandible.
• The upper mandible usually has a hooked tip to facilitate grasping and tearing food.

Mandible Strength

A bird’s mandibles contain a surprising amount of strength and bite force thanks to their specialized anatomy:

• The lightweight yet rigid bones minimize weight while providing strength.
• Kinetic skulls allow bending and flexing while retaining integrity.
• Powerful jaw muscles like the adductor mandibulae generate tremendous bite force.
• Serrated tomia and hooked tips provide gripping power.
• Symphysis allows flexing to withstand forces.

This strength allows small birds like hummingbirds to pierce tough flower corollas, while large predatory birds like eagles can crush bone. Researchers have measured huge bite forces in some species, like over 300 pounds in the hyacinth macaw!

Mandible Size and Shape

There is immense variation in the size and shape of bird mandibles depending on diet, habitat, and lifestyle:

Specialized mandible forms have evolved in many bird groups to perfectly match their dietary niche.

Sexual Dimorphism

In some species, males and females have different mandible sizes and shapes. This sexual dimorphism relates to differences in roles during feeding and reproduction:

• Female hummingbirds have longer, more curved mandibles to access nectar from a wider range of flowers.
• Male seed-cracking finches have thicker mandibles to exert more bite force when cracking hard shells.
• Male falcons have shorter, deeper mandibles optimized for defensive biting.

These specialized mandibles increase each sex’s feeding efficiency.

Coloration

While mainly horn-colored, mandibles can display a striking array of colors produced by pigments, blood flow, and structural colors:

• Red comes from increased blood flow near the surface.
• Black, brown, and gray from melanin deposits.
• Green, blue, and purple from nanostructures scattering specific wavelengths.
• Bright orange and yellow from carotenoids obtained through diet.

Vibrant mandible colors may play roles in species recognition, mating displays, or camouflage. However, the functions of elaborate mandible colors are still being researched.

Molt

The rhamphotheca undergoes regular molting and replacement as the mandible is worn down by use:

• The molt timing is often synchronized with wing and body feather replacement.
• Older layers of the rhamphotheca flake off while deeper layers continue growing.
• The molt helps maintain sharp tomia by replacing damaged outer layers.
• More frequent use causes faster wear and quicker molting cycles.

For example, hummingbirds molt their mandibles every 2-3 months due to frequent flower piercing and nectar feeding.

Development

Mandible growth begins early in a chick’s development:

• Specialized beak shapes develop in ovo before hatching.
• Rapid growth continues after hatching to facilitate feeding.
• Deceleration and final shaping happens a few weeks after fledging.
• The rhamphotheca takes time to completely harden and color.

Proper mandible development is critical for young birds to graduate towards adult food sources.

Diseases and Disorders

Bird mandibles are vulnerable to the following disorders:

• Injuries – Fractures, dislocations, wounds.
• Malformations – Crossbills, scissor beak, wry bill.
• Infections – Bacterial, fungal, viral.
• Nutritional – Hypovitaminosis A, calcium deficiency.

Captive birds may require mandible trimming or supportive care to manage these disorders.

Evolution

Mandibles evolved from the toothed jaws of early reptiles and dinosaurs:

• Keratinous beaks replaced teeth around 116 million years ago.
• Beaks were lighter while maintaining strength.
• Specialization into diverse forms enhanced feeding.
• Rhamphotheca allowed continuous growth and sharpening.

These evolutionary innovations produced the incredibly varied mandibles of modern birds.

Conclusion

The lower mandible is a critical anatomical structure that enables birds to effectively obtain, manipulate, and process food. Its lightweight yet strong architecture allows powerful biting forces. The shape and size are specialized for different dietary strategies. Molting maintains sharp tomia, while coloration can play diverse roles. Proper mandible development is essential for young birds. These superbly adapted mandibles allowed birds to diversify into specialized niches worldwide. The lower mandible works seamlessly with the upper mandible to allow birds to carry out all essential functions of life. It is one of the key adaptations that enabled the evolutionary success and ecological dominance of birds on the planet.