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,

• the most conspicuous dark feature (subauricular) corresponds to the location of a gland of olfactory significance (see https://www.inaturalist.org/observations/45924357 and https://www.researchgate.net/profile/William-Wood-5/publication/266375985_2-Ethyl-3-methylpyrazine_in_the_subauricular_and_median_glands_of_pronghorn_Antilocapra_americana/links/59ad766c0f7e9bdd115c4860/2-Ethyl-3-methylpyrazine-in-the-subauricular-and-median-glands-of-pronghorn-Antilocapra-americana.pdf), and
• the most conspicuous pale feature (an acetabulo-ischiopygal bleeze) corresponds to the location of a gland of olfactory significance, viz. the ischiadic gland (https://anatomypubs.onlinelibrary.wiley.com/doi/abs/10.1002/aja.1001290106 and https://journals.co.za/doi/abs/10.10520/AJA00445096_278 and https://ui.adsabs.harvard.edu/abs/2002BioSE..30..361W/abstract).

Furthermore, in the pronghorn,

• the subauricular gland occurs only in males (https://www.science.org/doi/abs/10.1126/science.183.4127.860), and
• the dark patch of pelage covering this gland is also restricted to males (https://www.shutterstock.com/da/image-photo/tight-portrait-pronghorn-1979951672 and https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1439-0310.1997.tb00182.x).

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 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#).

Publicado el 21 de mayo de 2021 a las 05:13 PM por milewski