Beneficially bloodshot, from birds to buffaloes

As everyone knows, reddish hues in feathers or fur of birds and mammals are usually owing to pigments such as carotenoids (e.g. see,skin%20produce%20most%20feather%20colours.&text=Bright%20red%2C%20yellow%20and%20orange,generally%20get%20from%20eating%20plants.).

However, when bare skin changes from flesh-coloured to reddish, this is usually the colour of blood, not just pigments.

Skin can look blood-red, because the capillaries just beneath the skin dilate enough for oxygenated blood to show through. The red is the hue of hemoglobin (, which can be called a pigment but is the substance that transports oxygen in the blood.

And, in photogenic animals ranging from birds to large mammals, bare skin can be designed to become bloodshot. In some cases this is a social/sexual display, and in others the red skin acts as a radiator of excessive bodily heat in hot weather. This aid to thermoregulation can be particularly valuable in dry climates where water tends to be unaffordable for sweating.

The most familiar example is the comb of the domestic fowl (Gallus gallus). The skin and the capillary walls are translucent enough that the comb can look blood-red ( and and Although this may intensify in hot weather, its functions are mainly social/sexual in birds.

A similar but less obvious effect occurs in the ostrich (Struthio camelus): the neck of adult males becomes bloodshot in the breeding season ( and and Although this is for sexual display rather than thermoregulation, the important point is that the hue in the skin is that of blood, not that of the carotenoid pigments for which the feathers of flamingoes are so well-known (

In mammals, subtle effects of a similar kind have been overlooked. This is partly because they tend to be restricted to the ear pinnae, which are widely assumed to be large for hearing rather than the radiation of excessive heat. For example, who has previously noticed the following?

So, let us look at some large-eared mammals adapted to hot climates.

Some photos of hares (Lepus) suggest bloodshot ears in hot weather. In cool weather, the ear pinnae are translucent but flesh-coloured: However, in the following the hue has intensified to reddish: The following are not necessarily bloodshot but clearly show the blood vessels: and

Among ungulates, I have found the following examples. In each case I illustrate the ear first flesh-coloured, then bloodshot in what is presumably hot weather.

Odocoileus hemionus

Strepsiceros strepsiceros

Ammelaphus imberbis

Syncerus caffer

(for comparison, in the following there is a bleeding injury on the ear pinna:

Bos indicus

Publicado por milewski milewski, 14 de septiembre de 2021


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