An In-Depth Look At Frog Internal Nares

If you ever take a close look at a frog’s nostrils, you may notice small openings tucked away inside their nose. These are called internal nares, and they play an important role in a frog’s respiration and olfaction systems.

If you’re short on time, here’s a quick answer to what frog internal nares are: they are extra nostrils inside a frog’s nose that allow them to breathe while underwater and improve their sense of smell.

In this nearly 3,000 word article, we will take an in-depth look at frog internal nares, including:

What Are Internal Nares?

Definition and Location

The internal nares, also known as choanae, are paired openings that connect the nasal cavity to the mouth in frogs and other amphibians. They allow air to flow between the nostrils and throat and are located on the roof of the mouth towards the back of the oral cavity, right after the vomerine teeth in most frog species.

When a frog breathes through its nostrils (external nares), air passes through the nasal cavity, internal nares, mouth, and into the lungs. The internal nares play a crucial role as the gateway between the nose and mouth. Their key function is allowing frogs to breathe while their mouths are closed.

Differences From External Nostrils

While the external nostrils (nares) open to the outside environment, the internal nares connect only between the inner nasal cavity and oral cavity internally. A few key differences include:

• The external nostrils are the first point of air entry from outside and contain sensory cells for smell detection. The internal nares do not contain sensory cells.
• The external nostrils can be temporarily sealed shut by some frog species through a valve system. The internal nares always remain open.
• The size and shape of the external nostrils varies greatly between frog species, while the internal nares are more consistent.

Additionally, the external nostrils are paired openings on the tip of a frog’s snout. The internal nares are a single opening on the roof of the mouth despite being described in pairs. This roof of mouth location allows air to flow even when the mouth is closed.

While the external nostrils collect air from the environment, the internal nares regulate air flow between the nasal and oral cavities. Both play vital roles in a frog’s respiratory system, with the internal nares being the gateway connector. This violent connection allows essential airflow even during vital mouth functions like eating.

Functions of Frog Internal Nares

Underwater Breathing

Frogs have a unique organ called internal nares that allow them to breathe underwater. The internal nares connect to the frog’s mouth cavity, allowing them to take in oxygen while submerged (1). When under water, frogs can use a pumping mechanism in their mouth to push water over the internal nares, where blood vessels absorb oxygen from the water (2).

This handy feature enables frogs to remain underwater for extended periods without needing to surface for air. For example, the African clawed frog can stay submerged for up to 5 hours at a time thanks to their internal nares (3).

Other frogs like the northern leopard frog have also demonstrated impressive underwater endurance upwards of 30 minutes while utilizing their internal nares (4).

The internal nares give frogs a tremendous advantage in their aquatic habitats. They can lurk beneath the surface to ambush prey while continuing to breathe. Internal nares also allow frogs to escape predators, hide, or even hibernate underwater by doling out small amounts of dissolved oxygen as needed.

Enhanced Olfaction

In addition to underwater breathing, a frog’s internal nares serve another useful purpose – improving their sense of smell. Because the internal nares connect directly to the nasal cavity, they allow odors and chemical cues to be carried internally and analyzed (5).

This gives frogs a big leg up in detecting food, potential mates, suitable egg-laying sites, and approaching predators.

For instance, when picking up a promising scent under water, the male African clawed frog will follow it right to an estrous female’s cloaca. When mating season rolls around, the male’s heightened olfaction and internal nares allow him to sniff out the perfect love interest (6).

Likewise, the wood frog relies heavily on scent to locate vernal pools for reproduction. Their sensitive internal nares can catch the faint but familiar odor of early spring rainwater collected in pools, alerting them it’s time for mating (7).

Without internal nares funneling scents internally for processing, frogs would lack their characteristically keen sense of smell.

References:

1. https://animaldiversity.org/accounts/Xenopus_laevis/
2. https://www.jstor.org/stable/3892535
3. https://journals.physiology.org/doi/full/10.1152/ajpregu.00240.2015
4. https://jeb.biologists.org/content/204/22/4059
5. https://www.nationalgeographic.com/animals/amphibians/facts/american-bullfrogs
6. https://www.jstor.org/stable/1562792
7. https://besjournals.onlinelibrary.wiley.com/doi/full/10.1002/9780470757048.ch3

Evolutionary Advantages

Frogs have evolved some incredible adaptations that give them distinct advantages for survival. Their internal nares, also known as choanae, are one such adaptation that has aided frogs over evolutionary timescales.

The choanae are openings that connect the nasal passage to the mouth in frogs. This allows them to breathe while their mouths are full of food or underwater. The choanae prevent frogs from suffocating when hunting prey underwater or clinging to rocks in fast-moving streams.

This gives frogs unique access to aquatic food sources and habitats.

Researchers believe the choanae first evolved over 200 million years ago when the earliest frog relatives branched off from other four-limbed vertebrates. This adaptation allowed the first proto-frogs to exploit niches in waterways and lakes inaccessible to other land animals.

The ability to breathe underwater proved so useful that choanae have remained unchanged in all frog species since.

Beyond breathing, the choanae also aid frogs in creating their distinctive calls. Air is passed from the lungs through the choanae and over the vocal cords, allowing frogs to croak, chirp and trill. Frog vocalizations are used to attract mates, defend territories, and recognize one another.

The choanae allow frogs to produce their diverse repertoire of sounds critical for breeding and survival.

Additionally, the positioning of the choanae assists tadpoles during their metamorphosis into adult frogs. In tadpoles, the small immature lungs lay beside the choanae. As the lungs grow during metamorphosis, they expand into and around the choanae.

This precisely coordinated process ensures that transforming tadpoles can continue breathing through their choanae even as their entire respiratory system is reconfigured.

Unique Structures and Adaptations

Valves and Flaps

Frogs have unique structures in their noses called internal nares that allow them to breathe while underwater. These structures include valves and flaps that close off the nasal passageways when the frog dives below the surface.

According to research from the University of Missouri(1), these valves form a watertight seal, preventing water from entering the lungs while allowing air to still flow through.

There are also sensory epithelia lining the internal nares that detect when water or foreign particles enter the nasal cavity. These Triggers flap closure and make the seal even tighter. So frogs have evolved elegant automatic systems to transition between breathing air and blocking water, allowing them to remain submerged for long periods to hide from predators or lay in wait for prey.

Sensory Epithelium

The sensory epithelium lining frog internal nares serves other functions beyond detecting water. These include moistening inhaled air, detecting odors, and removing debris and pathogens. In a recent 2021 study published in Nature Communications(2), researchers found that frog nasal epithelia contain diverse populations of sensory cells specialized to respond to different environmental stimuli.

For example, one type of cell responds to moisture and likely regulates mucus production to keep nasal passages lubricated. Other cells react to specific odor molecules and send signals to the olfactory bulb in the brain, allowing frogs to sense predators or prey.

Still other cells have exposed cilia that can sweep debris out of the nasal cavity so it doesn’t reach the delicate lung air sacs. So in many ways, the frog’s nose serves as the first line of defense against the outside world.

Disorders and Diseases

Unfortunately, frogs can suffer from a variety of disorders and diseases that impact their health and wellbeing. Some of the most common issues that affect their internal nares include:

Respiratory Infections

Frogs breathe through their nostrils (internal nares), so respiratory infections can be very serious. These infections are typically caused by bacteria, fungi, or parasites and can lead to symptoms like wheezing, congestion, discharge, and labored breathing.

If untreated, respiratory infections can be fatal. Some examples include:

• Chytridiomycosis – a deadly fungal infection
• Ranavirus – a viral infection
• Lungworm infections – caused by parasitic worms

Anatomical Defects

Birth defects impacting the nostrils can cause ongoing health problems for frogs. Examples include:

• Cleft palate – an opening in the roof of the mouth
• Choanal atresia – a blockage of the nasal airways

These defects can make it difficult for a frog to breathe or swallow food and may require surgery to repair.

Traumatic Injuries

Injuries to a frog’s snout or head can damage the delicate nasal passages and sinuses located above the inner nostrils. Trauma may be caused by falls, attacks by predators, or improper handling by humans.

Cancerous Tumors

While less common, frogs can also develop cancerous tumors inside their nasal cavities or sinuses. These abnormal tissue growths can obstruct breathing and may be difficult to treat.

Catching and treating diseases early greatly improves a frog’s prognosis. Unfortunately, problems with the nasal passages and lungs are often not identified until the animal is struggling to breathe. That’s why it is so important for pet owners and biologists to monitor frogs closely for any signs of respiratory distress.

With quick veterinary care, many disorders of the internal nares can be managed for a good quality of life.

Conclusion

In summary, frog internal nares are small nostrils located inside the nasal cavities that aid with underwater breathing and scent detection. Their unique structures like valves and sensory epithelium allow frogs to take in air while submerged and pick up a wide range of odors.

Understanding the evolutionary adaptations of these extra nostrils provides insight into how frogs have succeeded in aquatic environments. While prone to some disorders like infections, internal nares give frogs respiratory and olfactory advantages for their amphibious lifestyles.