Yes, birds do have an epiglottis. The epiglottis is a flap of elastic cartilage that covers the entrance to the larynx or voice box in birds and other animals. When birds swallow food or water, the epiglottis closes over the glottis (opening of the larynx), preventing food and liquid from entering the trachea and lungs. After swallowing, the epiglottis returns to its open position, allowing air to flow into the lungs for respiration. The epiglottis plays an essential role in birds, enabling them to breathe while eating and drinking.
Anatomy of the Avian Epiglottis
The avian epiglottis is located at the base of the tongue, where the oral cavity connects to the larynx and trachea. It is attached to the hyoid bones near the glottis. Structurally, the epiglottis consists of elastic cartilage covered in a thin mucous membrane. The cartilage provides rigidity to maintain the flap-like shape of the epiglottis while allowing flexibility for opening and closing movements.
In most bird species, the epiglottis is a spoon-shaped structure with the concave surface facing up towards the tongue. However, the exact size and shape can vary between species. For instance, hummingbirds have a long, thin epiglottis that can project beyond the tip of the beak. Meanwhile, geese have a large, wide epiglottis proportionate to their broad beaks. Overall, the epiglottis tends to be larger and more prominent in birds compared to mammals.
Function of the Epiglottis
The primary function of the epiglottis in birds is to protect the airway during swallowing. As birds consume food or water, the epiglottis closes over the glottis, temporarily blocking off the opening to the trachea. This prevents food and liquid from entering the respiratory system. Instead, the substances move down the esophagus towards the stomach.
Once swallowing is complete, the epiglottis returns to its resting open position, allowing air to again flow freely through the glottis and trachea during breathing. The opening and closing movement of the epiglottis occurs rapidly and repetitively with each swallow.
In addition to swallowing, the epiglottis may close briefly when birds vocalize to protect the trachea. However, the primary protective function relates to eating and drinking. Without an epiglottis, birds would constantly risk choking or aspiration of food into their airways.
Development of the Epiglottis
The epiglottis begins developing fairly early during avian embryogenesis. In chickens, the primordial tissues and cartilage that will form the epiglottis are present by embryonic day 6. Initially, the epiglottis appears as two small ridges or swellings on either side of the glottis.
| Embryonic Age | Developmental Milestones |
|---|---|
| Day 6 | Epiglottis primordium present as lateral swellings |
| Day 8 | Formation of epiglottis cartilage |
| Day 10 | Epiglottis fuses over glottis |
| Day 12 | Epiglottis assumes spoon-shape |
Between days 8 and 10, the cartilage condenses to form the central support structure of the epiglottis. The lateral swellings then expand medially to fuse together over the laryngeal opening by day 10. Subsequently, the characteristic spoon-shape becomes recognizable by day 12 as the epiglottis angles downward towards the glottis.
Although the epiglottis forms early, it is not functionally mature until later in development. Coordinated swallowing reflexes involving epiglottis movement do not appear until later embryonic or early post-hatching stages.
Evolutionary Origins
Birds inherited the epiglottis from their reptilian ancestors. All living reptiles have an epiglottis projecting over their glottis. This includes the crocodilians, which are the closest living relatives of birds.
During the evolution from early reptiles to modern birds, the epiglottis became larger and more flexible. This enhanced its protective abilities to accompany the high rates of breathing and eating in birds. For instance, hummingbirds have an especially long, agile epiglottis to match their rapid food and water intake while hovering.
In mammals, the epiglottis also evolved from the same primordial structures present in reptiles. However, in some mammals like horses and cats, the epiglottis became highly reduced or lost entirely. This demonstrates how the protective role of the epiglottis was not essential in all species.
Absence of Epiglottis in Ratites
One group of birds that lack an epiglottis are the large flightless ratites. Ratites include ostriches, emus, rheas, cassowaries, and kiwis. These birds have completely lost the epiglottis over the course of evolution.
It is not entirely clear why ratites lost this seemingly important anatomical structure. One hypothesis is that the absence of epiglottis is related to their elongated trachea. Ratites have very long S-shaped tracheas up to 1.5 meters long in ostriches. This long trachea may function as an alternative mechanism to protect the lungs from aspirated food and liquid.
Interestingly, during embryonic development ratite chicks initially form a small epiglottis that later regresses and degenerates before hatching. This indicates the evolutionary loss of epiglottis occurred fairly recently in this lineage.
Diseases Involving the Avian Epiglottis
Though rare, there are some diseases that can affect the epiglottis in birds:
Epiglottitis – This is inflammation of the epiglottis, typically caused by bacterial or viral infections. It can lead to swelling and obstruction of airflow through the larynx.
Epiglottic entrapment – The epiglottis can sometimes become trapped over the glottis, obstructing breathing. This often occurs in birds that are improperly restrained or relaxed under anesthesia.
Neoplasia – Tumors arising from the tissue of the epiglottis have been reported in pet birds on occasion. They may impede normal epiglottis movements.
Trauma – Direct damage to the delicate epiglottis can occur, for example an injury from swallowing a sharp foreign object. This could affect its protective function.
However, overall diseases of the epiglottis are uncommon in wild and healthy birds. Proper care should be taken when handling birds under anesthesia or sedation to avoid trauma or entrapment of the epiglottis.
Investigating the Avian Epiglottis
Researchers use several techniques to study the anatomy and function of the epiglottis in birds:
Dissection – Direct examination of the epiglottis in dissected specimens provides details on size, shape, and attachments. Stains can highlight cartilage and soft tissues.
Endoscopy – Passing a small camera into the trachea allows observation of epiglottis movements in live birds during swallowing and breathing.
Histology – Microscopic examination of epiglottis tissue architecture reveals cellular composition and morphology.
Embryonic development – Tracking developmental changes in the epiglottis elucidates the embryological formation in birds.
High-speed video – Recording swallowing at high frame rates can analyze the rapid kinematics of epiglottis function.
Computer modeling – Computational simulations help study the biomechanics of epiglottis movements.
Continued research on this small but vital structure provides insights into avian anatomy, physiology, and evolution. The unique characteristics of the bird epiglottis equip it for the demands of flight and the elevated metabolic rates of birds.
Conclusion
In summary, nearly all living birds possess an epiglottis – an elastic cartilage flap covering the larynx. During swallowing, the epiglottis closes to prevent aspiration, while opening during breathing. This anatomical structure evolved early in reptiles and persists in birds, enabling their unique breathing and feeding physiology. Though rarely implicated in disease, the epiglottis performs a critical role in protecting the airway and facilitating the active lifestyles of birds. Understanding its form and function continues to be an active area of ornithological research.
