Birds, like mammals, are endothermic organisms meaning they
have to change physically or behaviourally in order to maintain an optimal
internal temperature. This is important in maintaining homeostasis for
circulatory and osmoregulatory function in the body. In the previous post, the mammalian
thermoregulatory adaptation of sweating was discussed; which is an evolved physical
adaptation. Birds however have evolved without sweat glands, and depend on techniques
such as gular fluttering and behavioural techniques such as shading and
wing-drooping.
Gular fluttering is both a behavioural and physical
adaptation for reducing internal heat in birds. The bird will stretch its mouth
up and open, expanding the gular skin sac whilst pulsing the hyoid apparatus
(Lasiewski R.C et al. 1966). Stretching the throat increases the surface area
of the gular skin, which contains four pairs of blood vessels (McSweeney T.
1988). The interior of the gular sac and buccal cavity is cooled by increasing the
speed of air flow in and out of the mouth. Heat moves from the blood vessels
close to the skin surface, and through moist membranes into the fast moving air
where it is pushed out of the birds’ mouth (Fowler M.E et al. 2003).
Fig. 1 - Diagram of Gular Flutter Cooling Mechanism
(Harrington 2012)
(Harrington 2012)
Lasiewski R. C. and G. A. Bartholomew were among the first
to study gular fluttering and found that in poor-wills (nightjar) the birds
opened their mouths and commenced gular fluttering upon reaching a temperature
of 39°C and higher. Cooling
of the gular skin occurred instantly after commencement of gular fluttering. The
temperature of the gular skin was found to be 1.5 to 3°C cooler than body temperature, and up to 9°C cooler than external air
temperatures. This suggests that gular fluttering is a very successful thermoregulatory
adaptation for birds (Lasiewski R.C et al. 1966).
Gular fluttering is crucial to the prevention of heat
exhaustion, hyperthermia and dehydration in many bird species. The temperature
of gular skin as a result of fluttering is reduced greatly and in conjunction
with increased blood flow, reduces overall body temperature (Lasiewski R.C et
al. 1966). Gular fluttering in birds is an effective way to reduce internal
body heat with minimal water loss.
References:
Lasiewski, Robert C and
Bartholomew, George A. 1966, 'Evaporative Cooling in the Poor-Will and the
Tawny Frogmouth’, The Condor, Volume
68, no. 3
McSweeney, Terese and
Stoskopf, Michael K. 1988, Selected Anatomical Features of the Neck and Gular
Sac of the Brown Pelican', The Journal of Zoo Animal Medicine, Volume 19,
no. 3, pp. 116-121
Fowler, Murray E. and Miller, Eric R. 2003, ‘Zoo and Wild Animal
Medicine: Current Therapy’, Volume 6.
Figures:
Harrington, Emily. 2012, ‘Gular Flutter Cooling Mechanism’,
Diagram, asknature.org, viewed
12/3/16, URL: http://www.asknature.org/media/image/17403
That’s a very interesting mechanism. Is this mechanism similar to the cooling we see when elephants flap their ears (in terms of cooling blood in the peripheries) or when bats fan their wings?
ReplyDeleteYes basically the same process. All involves elevating heart rate to send faster pumping blood to an area of the body with thinner skin and multiple blood vessels where it is cooled quickly and circulated throughout the body. The cooled blood reduces heat stress on cells in different parts of the body.
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