Wednesday, March 16, 2016

Thermal Adaptation - Gular Fluttering 


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)

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

2 comments:

  1. 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?

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    1. Yes 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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