The peculiar showiness of the subauricular gland of the pronghorn (Antilocapra americana)

@ptexis @variani18

See https://www.inaturalist.org/journal/milewski/93100-an-index-to-my-posts-about-the-pronghorn-antilocapra-americana#

The pronghorn (Antilocapra americana) is the only ungulate possessing a subauricular gland.

This gland 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 and https://tradbow.com/watch-pronghorn-patch/).

The pronghorn is also the only ungulate in which, in the colouration of the whole body,

Furthermore, in the pronghorn,

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.

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.

I suggest the term 'malar flag' for the pattern on the side of the face in adult males, comprising

  • crisply-defined white band on the cheek, uppermost throat, and base of the ear pinna,
  • dark on the rostrum, and
  • the dark subauricular patch.

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 face (including the ear pinnae in females and juveniles), throat, and neck feature a complex series of chevron-like markings (see https://www.inaturalist.org/observations/63215153 and https://www.inaturalist.org/observations/15710472).

This pattern distracts rather than attracting the eye of the onlooker at the distances relevant to scanning predators (https://www.inaturalist.org/journal/milewski/52221-why-the-pronghorn-antilocapra-americana-has-a-striped-neck#).

In females, the face of the pronghorn 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 males, 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, in the pronghorn, the subauricular visual accentuation is developed to the point that it is noticeable even at a distance. This reveals the animal to predators in what seems like an unnecessary way.

However, this anomaly can be considered in the context that the pronghorn is hypercursorial (https://www.inaturalist.org/journal/milewski/93032-a-new-explanation-for-the-hypercursoriality-of-the-pronghorn-antilocapra-americana-selenium-as-a-crucial-micronutrient#).

Posted on 21 May, 2021 17:13 by milewski milewski

Comments

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.

Posted by taogirl almost 3 years ago

The dark marking on the subauricular gland of males starts to form in infancy, 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.

Posted by milewski almost 3 years ago

The following describe how adult males display the subauricular gland and its associated dark patch:

The behaviour of ungulates and its relation to management, ed. by V Geist and F Walther. IUCN Publications new series no. 24, vol. 1. file:///C:/Users/Antoni%20Milewski/Downloads/NS-024-1%20(1).pdf

Kitchen D W and Bromley P T (1974) Agonistic behavior of territorial pronghorn bucks. Paper no. 18, pp. 363ff.

"An interesting variation of the [territorial male's] slow approach is the cheek patch display, which develops from the head-low, ears-back approach. As the slow stiff walk begins the [territorial male] raises his head above the level of his withers, tilts his muzzle up, and slowly rotates his head to the side, ears depressed...There is a general erection of the neck hair, but not of the mane. The mane may be partially erect, but in 36 of 40 encounters it was not."

Bromley P T and Kitchen D W (1974) Courtship in the pronghorn. Paper no. 17, pp. 354ff.

The sexually aroused male whines to the female, while piloerecting the mane and the hair of the median gland. The male waves its head to display the dark patch on the cheek. I infer that females do not erect the mane or the pelage of the neck during courtship.

Posted by milewski about 17 hours ago

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