Sundry aspects of the adaptive colouration of the pronghorn (Antilocapra americana)

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

https://www.kimballstock.com/preview.asp?db=a&image=WLD+21+TL0012+01

MASCULINE DISPLAYS

Kitchen D W and Bromley P T (1974) Agonistic behavior of territorial pronghorn bucks. Paper no. 18, pp. 363ff, in The behaviour of ungulates and its relation to management, ed. by V Geist and F Walther. IUCN Publications new series no. 24, vol. 1.

The territorial male reacts to trespassing conspecifics by snort-wheezing, "In 83.5% of the observations the snort-wheeze was associated with erection of the median gland...mane and upper 1/3 of the rump patch (Figure 4)". The caption of Figure 4 states "Stance of a buck doing the snort-wheeze call: mane, median-gland, and upper 1/3 of rump patch erect."

Closest-quarters mutual displays between males: "Partial erection of the mane and upper 1/3 of the rump patch, and a slight raising of the ears was noted in some encounters (Fig. 6)" (this is when the individual male has lost confidence). "Fig. 6. This buck has lost his confidence in an encounter with the buck in Fig. 5. Note the tail is raised, upper part of the rump patch and mane are partially erect, and the ears have moved to a neutral position."

Tooth-grinding by males can be heard by humans up to 10 meters away.

"Fig. 9. This is a territorial buck during a running chase, Note: (1) median gland is erect, (2) mane is fully erect, and (3) ears are almost fully depressed." "While in pursuit the [territorial male] depressed his ears, erected his mane, median gland, and on 13 occasions the upper 1/3 of his rump patch (Fig. 9)." "Fig. 12. This is the typical alarm posture with the rump patch and mane fully erect and the ears in a neutral position. The median gland is normally erect in this posture."

Excerpt from my notes: The exclusively male dark patch at the masseter is clearly associated with a masculine gland, in a position unusual among ungulates. This is perhaps the prime example, in all the ungulates, of a skin-gland unambiguously associated with gross-scale display of dark/pale contrast in the pelage. Apart from relatively large horns, this dark patch is the most sexually dimorphic feature of A. antilocapra, first appearing in infants. Its anatomical position is surprising (different from other ungulates) and the visual emphasis is surprisingly strong, the dark patch in maturity being so large and so tonally contrasting that it is visible at distance and from various perspectives. In this feature, no other ruminant is similar to the pronghorn, a species in which all the dark features (including forehead and horns) are on or near the head.

Certain spp. of Oryx have dark markings on the tract passing from the crook-of-throat to the masseter (https://www.shutterstock.com/da/image-photo/oryx-closeup-627303404 and https://www.shutterstock.com/da/image-photo/east-african-oryx-beisa-known-species-1302873655 and https://www.dreamstime.com/beautiful-close-up-oryx-head-magnificent-horns-gemsbok-against-pale-blue-clear-sky-image257816498 and https://www.masterfile.com/image/en/841-03507691/gemsbok-south-african-oryx-oryx and https://www.masterfile.com/image/en/700-02887439/oryx-buffalo-springs-national-reserve-kenya and https://www.agefotostock.com/age/en/details-photo/side-profile-of-an-oryx-in-the-kalagadi-transfrontier-park-south-africa/ESY-054018317 and https://www.shutterstock.com/da/image-photo/gemsbok-oryx-30641182?utm_campaign=image&utm_medium=googleimages&utm_source=schema and https://www.gettyimages.co.uk/detail/photo/oryx-antilope-leaks-minerals-kgalagadi-royalty-free-image/1215316063?phrase=gemsbok+face&adppopup=true and https://www.gettyimages.co.uk/detail/photo/gemsbok-drinking-water-royalty-free-image/121329915?phrase=gemsbok+face&adppopup=true).

However, these lack glands.

SEASONAL CHANGES

A noteworthy feature of Antilocapra americana is that it retains the same colouration throughout the seasonal cycle, despite having structurally/texturally distinct winter pelage.

Requiring investigation is the possibility of seasonal change in the colouration of the nape. In some photos, there is striking dark/pale contrast, partly owing to posteriorward extension of pale from the crown down each side of the dark mane.

In its seasonal uniformity in colouration, A. americana differs from sympatric cervids and possibly Bison bison.

The seasonal relative darkness of the cervids of snowy climates in the Northern Hemisphere tends to make them conspicuous in winter, even if they are inconspicuous in summer.

In its lack of seasonal change, A. americana resembles North American bovids (Ovis, Oreamnos) rather than cervids. However, A. americana tends to be particularly gregarious in winter (when females and males run together), so that conspicuousness may be enhanced by numbers and movement.

LEGS

The legs of Antilocapra americana are pale, and fairly uniformly so. There is no pedal flag. The ground-colour of the legs is paler than that of the palest parts of the torso, neck, and face.

On close scrutiny, the following features can be discerned:

  • the hooves, like the horns, are blackish, producing a nominal dark/pale contrast with the pasterns and fetlocks. Note that false hooves/dewclaws are absent;
  • whitish extends from the ventral surface of the torso to parts of the upper legs, on the posterior surface of the forelegs but the anterior surface of the hindlegs;
  • the elbow is white (https://www.nps.gov/articles/pronghorn.htm and https://www.sportsmansguide.com/article/twenty-pronghorn-pointers?id=2470), which can be thought of as an extension of the pale flank-band; the knee is also white, but less noticeably so.

The white tracts on the inner surfaces of the upper legs end with oblique borders, in opposite orientations on fore vs hind. However, the inner surfaces of the legs are such a pale tone of fawn that this pattern is easily overlooked.

Task: compare the patterns with Aepyceros and Antilope.

BACK-OF-HEAD PATTERN

There is a complex pattern on the posterior surface of the head in Antilocapra americana, extending to the nape and ear pinnae.

This consists of

  • pale pelage on the posterior of the crown, extended on to the nape at each side (left and right) of the mid-line, and
  • the dark pelage of the erectile mane of the nape.

The mane does not extend on to the crown, or even the occipital area of the skull.

The dark/pale contrast in this pattern is moderate, because the pale aspects are not white, while the dark aspects are brown rather than black. Furthermore, the display depends partly on the erection of the mane, which is narrow and inconspicuous when folded.

Then fairly pale posterior surfaces of the ear pinnae add to the display. However, this is rather ambivalent because

  • the paleness on the back-of-ear is not nearly white, and lacks any dark tip, and
  • one of the two ears is often turned inconspicuously backwards while the animal faces forward.

There is some analogy in display between the front and the back of the head. Both are complex and subtle, and subject to variation according to ontogeny/sex/subspecies. The mane differs in prominence between juveniles and adult males, and between e.g. subspecies mexicana and subspecies americana.

The dark mane, which seems most prominent in juveniles, may enhance the conspicuousness of the acetabulo-ischiopygal bleeze when viewed directly from behind. The dark mane adds tonal contrast despite being relatively far from the observer.

I have the following note in my files:

"Video on the Web, viewed June 2020, shows two adult male individuals of Antilocapra americana in winter pelage, with the horns reduced to dark spikes, shorter than the ear pinnae. This footage shows that A. americana does indeed have a kind of back-of-ear display, with enough dark/pale contrast to be noteworthy. The posterior surfaces of the ear pinnae are pale, with a tone similar to that of the lower legs. There is no dark tip on the ears, on either front or back. The main pale feature in the display is the whitish at the base of the ears. The main dark feature is the mane, which is narrow (not erect) in this view. The pale descending from the back-of-crown partway down each side of the mane is disjunct from the pale at the ear-bases. This back-of-ear/back-of-head/'nape' display is seen against the mainly dark-looking back and neck of the animal."

MANE

The mane is associated with masculinity in various mammals, which makes sense because it can enhance the apparent size of mature males in masculine rivalry.

As in giraffids, Antilocapra americana has a 'hyperprecocial' pattern, in which the mane is largest in and most conspicuous, relative to body size, in juveniles and females.

Then displaying if the mane is complex and subtle in A. americana. However, it is noteworthy that the mane is - unlike the dark subauricular patch of males and the piloerectile acetabulo-ischiopygal bleeze - not known to be glandular.

Another peculiarity of the mane is that it ends abruptly, well-short of the base of the neck. When the head is held up, the mane seems to reach the base of the neck; however, when the bead is lowered to forage, the real shortfall becomes apparent. Either way, the mane certainly fails to reach the withers.

LACK OF GLOSS

Antilocapra americana does not have any gloss on its pelage, at any season. This is a point of difference from e.g. Nanger and Eudorcas, as well as alcelaphin bovids.

Even the horns are hardly glossy, being blackish but more-or-less matt.

The lack of gloss in A. americana is associated with the structure/texture of the pelage, which resembles that of e.g. Oreotragus and Hydropotes. This resemblance includes easy detachability, which is

  • presumably an anti-predator adaptation,
  • borne out by discussion among taxidermists on the Web, and
  • possibly greatest in winter pelage.

POSSIBLE CAUDAL FLAG

The tail of Antilocapra americana is grojnd-colour (fawn) on its upper (dorsal) surface, and white on its lower (ventral) surface. There is a narrow fawn-coloured midline separating the left from the right half of the acetabulo-ischiopygal bleeze; this mid-dorsal 'stripe' ends in the tail.

The tail is often lifted to a horizontal position during fleeing, making it a slightly noticeable feature by virtue of its (slight) projection.

The tai is displayed more than in Ovis canadensis, Ovis dalli, Oreamnos americanus, Alces alces, or Cervus canadensis, but less than in Odocoileus hemionus (check) and Rangifer tarandus. The latter erects the tail habitually, altering the silhouette more than in A. americana, despite the similarities of the tails.

It is noteworthy that, although males of A. americana have a (unpaired) median gland, exposed by erectile pelage, this is located not on the tail or at its base, but instead on the sacrum, anterior to the acetabulo-ischiopygal bleeze.

It is possible that Antilocapra americana possesses a caudal flag. This is despite the fact that the tail is small, and seemingly insignificant relative to the bleeze on the hindquarters.

The tail is sometimes erected and moved, gleaming white, without piloerection of the acetabulo-ischiopygal bleeze - which in its quiescent state is pale but not white. In other words, the tail - viewed in profile - sometimes flashes white as it is wagged. Each bout of wagging consists of a brief (split-second) series of a few rapid wags.

However, it is unlikely that any caudal flag in A. americana is deployed vs predators; its activation seems to be purely social/intraspecific.

Antilocapra americana does not erect the tail when piloerecting the acetabulo-ischiopygal bleeze. In this way it differs from Rangifer tarandus, which habitually raises the tail but does not seem able to piloerect the (small) patch of white pelage on the buttocks (https://churchillwild.com/caribou-antlers-and-their-growth/ and https://www.theguardian.com/world/2024/apr/23/wolf-culls-canada-caribou-saving-endangered-mountain-caribou).

The tail of A. americana is not inert, because it is

  • often raised to horizontal during running, and
  • sometimes flicked (flashing white) during standing.

However, the main function of the tail (and its base) in terms of display is to provide a fawn dividing line between the two 'hemispheres' of the acetabulo-ischiopygal bleeze, somewhat punctuating the whitish expanse of this bleeze.

Overall, the following summary pertains:

  • The tail of A. americana is sometimes hard to distinguish from an adjacent ruff of pelage at the edge of each half of the acetabulo-ischiopygal bleeze.
  • More than in any other ungulate, A. americana has specialised on emphasis by piloerection of an extensive, discrete, disc-like, white bleeze on the hindquarters.
  • This 'piloerectile emphasis' makes the tail relatively insignificant, even though the tail itself is actually similar to that of Rangifer, in which it is a major feature of the display on the hindquarters.
  • The tail of A. americana is far shorter than that if any antilopin bovid (check), fawn on the proximal dorsal surface and white on the ventral surface and the tip (which cannot be called a tassel).

Most of the ungulates of North America have short tails.

POSSIBLE ANTERIOLATERAL FLAG

https://www.jasonsavagephotography.com/tag/pronghorn/

SUBSPECIES PENINSULARIS (based on photos from the breeding programme in Los Angeles Zoo).

The main difference from the nominate subspecies seems to be a reduced pattern of dark/pale contrast on the cheeks. The pale patch of the cheek, typical in the nominate ssp., is longitudinally divided by fawn in ssp. peninsularis (https://animalworld.tumblr.com/post/188640547595/peninsular-pronghorn-antilocapra-americana). This is individually variable.

Other points noted for ssp. peninsularis:

  • The orbits of the skull also seem to be less prominent than in the nominate ssp.
  • The forehead of adult males is dark.
  • The pelage on the rump is slightly sexually dimorphic in that the vicinity of the 'caudal' gland in males.
  • The horns of males are short and forward-directed, with a 'ruff' of pale pelage at the bases, with a narrow strip of fawn separating the horn-bases from the whitish pelage of the ear-bases and posterior crown. The latter whitish is, in turn, more-or-less separated from the white of the facial streak posterior to the subauricular gland.
  • There is a whitish band, in both sexes, transversely across the crown, just anterior to the ear-bases.

ONTOGENY

None of the conspicuous features of colouration is fully-developed in infants of Antilocapra americana. However, all are incipient (partly visible) shortly after birth.

Infants possess the dark/pale contrast at the nose/mouth, with the rest of the malar flag developing only later in life. The pelage of the subauricular gland starts to darken in juveniles (i.e. its colouration is precocial). However, the dark spot is initially small. It gradually spreads as adulthood and maternity are reached.

The conspicuous pale feature on the flanks is particularly dim in infants, becoming fully-developed in juveniles.

The acetabulo-ischiopygal bleeze is functional already in infants, in the sense that it can be piloerected to show conspicuous white. However, without such piloerection the feature is inconspicuous in infants, because

  • the superficial hairs of the bleeze are pale fawn rather than white, and
  • the hindquarters are so poorly-developed in infants that the expanse of the feature is limited.

Overall, the colouration of infants is certainly adapted for concealment. Infants possess a functionally (dependent on piloerection) conspicuous feature only on the hindquarters, and not on the flanks or the cheeks.

Infants of A. americana are less precocial in colouration than are gazelles. The most similar gazelle in this respect is Antidorcas marsupialis. This is partly because its infants lack the facial pattern of juveniles and adults.

COMPARISON WITH GAZELLES

The adult body mass of Antilocapra americana is about double that in Gazella-Eudorcas, and intermediate between those of Antidorcas marsupialis and Damaliscus pygargus.

Antilocapra americana resembles gazelles in that its adaptive colouration is a subtle combination of conspicuous and inconspicuous features, at various scales. Because it is larger-bodied than most gazelles, it tends to be less able to hide in open environments.

If it were the case that A. americana were as unambivalently conspicuous as Antidorcas marsupialis, then

  • the whitish pelage would be white, and
  • there would be dark features on both the hindquarters and the flanks.

The erectile bleeze is not frequently activated in earnest in A. marsupialis. By contrast, the erectile bleeze of A. americana is frequently activated, flashing pure white, almost as a 'contingent substitute' for the whole-figure conspicuousness of A. marsupialis.

I.e. while there is analogy in the erectile bleezes of the two spp., another way of looking at it is that A. americana

  • is less committed to conspicuousness than is A. marsupialis, and
  • boosts its conspicuousness more readily - by means of 'flareing' - than does A. marsupialis.

In any collection of about 100 photos of A. americana, one will find many inadvertent depictions of its erectile bleeze. By contrast, in any 100 photos if A. marsupialis, I would not expect to see any depictions of its erectile bleeze, especially if one specifies displays in earnest rather than in play. The ratio could be about 10:1.

Antidorcas marsupialis possesses a lateral bleeze, whereas A. americana does not. In the latter, the whitish on the flank has high clearance, thus catching the light. However, this feature is not necessarily white, and it cannot be whitened by 'flareing' in the manner of the bleeze on the hindquarters. Furthermore, the bold pattern in the flanks is precocial in A. marsupialis, but not in A. americana.

Just as the colouration on the faces of most spp. of gazelles (Gazella and Eudorcas) is disruptive, so the patterns on the face and anterior surface of the neck of A. americana are disruptive.

Posted on May 01, 2024 02:08 AM by milewski milewski

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