Why do animals yawn?
Everybody yawns, including unborn babies, most mammals, and some birds and reptiles and even fish. While we do not know why we yawn, there are many theories.
- When we are tired, we do not breathe as deeply as normal, and even in normal breathing, we only use a small portion of our full lung capacity. One theory suggests that by yawning, we are taking in more oxygen and exhaling more carbon dioxide, so yawning would be an involuntary reflex to control oxygen and carbon dioxide levels in the blood. However, studies have shown the breathing more oxygen doesn't decrease yawning, and breathing more carbon dioxide doesn't increase yawning, so this may not be the case.
- If parts of the lung do not receive fresh air, due to shallow breathing, they stiffen and collapse a little. Yawning may redistribute the oil-like surfactant that lubricates the lungs, reducing surface tension and preventing them from collapsing. So yawning may be a way of keeping the breathing apparatus functioning correctly.
- Another theory is that yawning stretches muscles and joints and increases heart rate and blood pressure, which promotes a more awake state.
- Yawning may indicate a mental or emotional state, in a similar way to smiling, crying or laughing. We yawn when we are tired or bored, and the physical action communicates those feelings to those around you.
The first two theories do not account for why unborn babies yawn, because they are not yet using their lungs to take in oxygen.
Yawning also seems to be a symptom of some medical conditions, include multiple sclerosis and lower brainstem damage, while sufferers of some mental disorders, such as psychoses, rarely yawn.
Low oxygen levels in the paraventricular nucleus (PVN) of the hypothalamus of the brain induces yawning. Yawning seems to require the production of nitric oxide by neruons in the PVN. By injecting neurotransmitters (i.e. dopamine, excitatory amino acids, nitric oxide) and neuropeptides into the hypothalamus, an increase in yawning has been observed. Yawning also requires feedback. A stifled yawn is usually unsatisfying.
Is yawning contagious?
- Yawning may be Nature's way of signalling to a group that it is time to rest. If one member of a group yawns, other members also yawn.
- More generally, yawning may signify changing conditions in the body, and the contagious nature of the yawn allows animals to synchronize their behaviour.
In
humans, these process are largely irrelevant, and yawning is an
evolutionary by-product that is no longer needed.