The peculiar showiness of the subauricular gland of the pronghorn (Antilocapra americana)
The pronghorn (Antilocapra americana) is the only ungulate possessing a subauricular gland. This is located between the base of the ear and the crook of the throat (see https://www.animalspot.net/wp-content/uploads/2015/05/Pronghorn-Male.jpg).
It is also the only ungulate in which, in the colouration of the whole body,
- the most conspicuous dark feature (subauricular) corresponds to the location of a gland of olfactory significance (see https://www.inaturalist.org/observations/45924357), and
- the most conspicuous pale feature (an acetabulo-ischiopygal bleeze) corresponds to the location of a gland of olfactory significance.
Furthermore, in the pronghorn,
- the subauricular gland occurs only in males, and
- the dark patch of pelage covering this gland is also restricted to males.
The gland has a well-known function in courtship: the male presents the side of the face to the female in an obvious posture (see https://sonoranimages.wordpress.com/2017/09/27/pronghorn-unrequited-love/).
The presentation has both an olfactory and a visual component. This is because the location of the gland - which becomes surprisingly large when in full use - is accentuated by a large patch of blackish, in contrast with the white cheek.
Indeed, this bold pattern, on the side of the face in males, is the most clear-cut and consistent feature of dark/pale in the entire colouration of the pronghorn. Is the dark subauricular feature a flag or a semet?
The overall colouration of the front of the pronghorn, in both sexes and at all ages, tends to conceal the animal by disrupting the outline of the neck and head (e.g. see https://www.inaturalist.org/observations/32645239).
This is because the front of the neck and the front of the face have a complex series of chevron-like markings (see https://www.inaturalist.org/observations/63215153 and https://www.inaturalist.org/observations/15710472). These distract rather than attracting the eye of the onlooker at the distances relevant to scanning predators.
In the female the face becomes hardly more conspicuous when it is turned to profile (see https://www.inaturalist.org/observations/76386666 and https://www.inaturalist.org/observations/34721203).
However, in the male the facial profile reveals a pattern so bold as to form an advertisement rather than a feature of camouflage (see https://www.inaturalist.org/observations/31304964).
It is unsurprising for a subcutaneous gland
- to be sexually dimorphic, and
- to have some visual accentuation.
What is surprising is that the subauricular visual accentuation is developed to the point that it is noticeable even at a distance, revealing the animal to predators in what seems like an unnecessary way.
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interesting journals, now I have a reason to go back and look at all my observations of this beautiful animal that I see semi-regularly on the Adobe Badlands. I have wondered about the reason also for the apparent predator target on their rears. But guess that would also function for their young to see them.
The dark marking on the subauricular gland of males starts to form as soon as infancy is over, enlarging progressively as the juvenile becomes adolescent. This provides an easy way to tell the sexes apart in juveniles, despite the difficulty of seeing the prepuce. The following juvenile is male: https://www.inaturalist.org/observations/69561058.
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