Intriguing new sexual dimorphism in the wapiti (Cervus canadensis)

(Also see https://www.inaturalist.org/journal/milewski/55532-previously-overlooked-facial-expressions-in-the-wapiti#.)

Readers, can you spot what is odd in these photos?
Anterior base of ear:
http://l7.alamy.com/zooms/5ff7b4e6ec474289b2ffbf8c7e36cfeb/usa-yellowstone-np-wyoming-female-wapiti-cervus-elaphus-canadensis-h6ha5h.jpg
Posterior base of ear:
 http://www.photos-voyages.com/canada/en/images/wapiti-femaleg.jpg

I have  noticed a sexual anomaly of adaptive colouration in the wapiti (Cervus canadensis, https://www.inaturalist.org/taxa/204114-Cervus-canadensis).

I have yet to establish the extent to which this occurs in other species of the genus Cervus, and other deer more broadly.

I can describe this sexual anomaly, as follows.
 
Everyone knows that, in the wapiti, the sexes differ. Males have large and complex antlers, whereas females lack antlers altogether.

Those with a good general knowledge will also know that the general colouration in the wapiti is similar between the sexes.

In other words, deer such as the wapiti show a pattern in which the main aspect of sexual dimorphism is the growth of antlers, and the appearance of the sexes is otherwise similar.
 
However, there is one aspect in which the colouration of females and males in the wapiti is surprisingly different. This is dark/pale contrasts at the bases of the ears.
 
Swivelling movements of the ears can signify mood and attention. Thus, the dark/pale contrasts at the bases of the ears of females of the wapiti may represent a kind of ‘sign language’ in this species, analogous with the eye-white displays in the human species (https://en.wikipedia.org/wiki/Cooperative_eye_hypothesis#).

Auricular semets, beyond just the wapiti, are an interesting topic for future exploration (e.g. see https://www.inaturalist.org/journal/milewski/56392-discovering-an-auricular-semet-in-a-kangaroo#).
 
However, in this Post I focus just on this particular aspect of subtle sexual dimorphism: the odd fact that this dark/pale contrast is lacking in mature males of the species, despite

• the showiness of their antlers, and
• the extreme ‘body language’ they use in other aspects of masculine display.

For some reason, males of the wapiti show a muting of the auricular displays of the species, despite

• accentuation of the antlers by the paleness of the points, and
• the conspicuous aspects of the overall colouration, in which a pale body is offset by a darkness on head, neck, legs, and belly.

Why do males lack the conspicuous pattern at the bases of the ears, despite being such visually striking animals in other ways?
 
For some reason, males of the wapiti do not just take the overall pattern of whole-body conspicuousness of the species, and add impressive antlers to the display. They also tone down the appearance of the ears.

This is an intriguing subtlety, because it indicates that the ‘socio-sexual language’ of deer can be surprisingly complex and nuanced.

It is as if males ‘trade’ the auricular display for an antler display.

One would not expect males to treat these displays as mutually exclusive. However, in this case: males seemingly grow antlers at the expense of their auricular language.
 
To put this in more familiar terms, in a semi-absurd analogy:
Imagine a human species in which the big, bearded, brawny men have brown eye-whites along the lines seen in chimpanzees and gorillas (no ape has the vivid eye-white displays in Homo sapiens), while the women retain the eye-white display.

Such a thing is unthinkable in humans, but seen in the wapiti.
   
The following (https://image.shutterstock.com/z/stock-photo-female-elk-or-wapiti-cervus-canadensis-near-cascade-pond-in-banff-national-park-alberta-canada-144023455.jpg) shows the dark/pale contrast at the base of the ear in adult females of the wapiti. 
 
The following (http://l450v.alamy.com/450v/gwgm3m/elk-or-wapiti-cervus-canadensis-gwgm3m.jpg) shows that this contrast is produced by both

• the extension of white from the ear-feather on to the anterior base of the ear, and
• the demarcation of this base with dark pelage.

The following (http://il5.picdn.net/shutterstock/videos/7043269/thumb/1.jpg) shows that a similar dark/pale contrast occurs on the posterior base of the ear.

What this means is that females of the wapiti show off their ear movements, regardless of the angle from which the head is viewed. Another individual can keep track of the ear movements not only by the motion of the ear pinna itself, but also by the shifting pattern of dark vs pale at the base of the swivelling ear.

Comparable auricular semets occur in various ruminants. However, their expression is extreme in females of the wapiti.
 
The following show the difference between female and male in the wapiti:
http://c8.alamy.com/comp/A73AN8/wapiti-elk-cervus-elaphus-canadensis-male-with-female-canada-alberta-A73AN8.jpg and http://l7.alamy.com/zooms/fa561e63b7b84b48acb680605aa319a0/wapiti-pair-in-rut-yellowstone-national-park-wyoming-usa-cervus-canadensis-dtrre5.jpg.

In adult males, the ear is not nearly as conspicuous as in females. 

The following is another view of the anterior base of the ear in females of the wapiti:
http://l450v.alamy.com/450v/dt0k6d/close-up-of-elk-wapiti-cervus-canadensis-female-in-the-rain-rocky-dt0k6d.jpg.

This shows the dark/pale contrast that

• accentuates any movement of the ear pinna, as the animal monitors its surroundings, and
• signifies mood and attention, which are of course of interest to other members of the group, in this gregarious species.

And the following (http://c8.alamy.com/comp/G1X6MC/wapiti-elk-cervus-canadensis-doe-banff-national-park-canadian-rockies-G1X6MC.jpg) helps to show how consistent this pattern is in females of the wapiti.
 
The following (http://us.123rf.com/450wm/jkraft5/jkraft51608/jkraft5160800008/61739330-portrait-of-an-elk-in-yellowstone-national-park.jpg?ver=6) again shows adult males of the species do retain a faint pattern at the anterior base of the ear. However, this is toned down/muted.

It is almost as if mature males of the wapiti suppress the auricular display so that

• the ears do not ‘compete’ with the antlers for the attention of conspecifics, and
• a small-scale grammar of 'whispering' is replaced by a large-scale grammar of 'shouting'.

The following (https://farm6.static.flickr.com/5543/30329552694_482b4caff1_b.jpg and https://farm9.static.flickr.com/8420/30315701445_e6da24256d_b.jpg) help to show how consistent this muting of the pattern at the anterior base of the ear is in mature males of the wapiti.

The following (https://thumbs.dreamstime.com/z/elk-wapiti-bull-antlers-2791169.jpg) shows the appearance of adult males of the wapiti in masculine display. The ears are hardly noticeable, greatly upstaged by the extreme antlers. If one looks closely, one will see that there is a pale patch at the anterior base of the ear. However, the colouration is not accentuated as it is in females.
 
The following confirm that adult males lack any conspicuous pale spot/patch at the posterior base of the ear:
https://thumb7.shutterstock.com/display_pic_with_logo/3277715/341336639/stock-photo-wapiti-elk-large-bull-grassing-rocky-mountains-canada-341336639.jpg and http://media.gettyimages.com/photos/canadian-elk-from-behind-picture-id542549827.

The following (http://il5.picdn.net/shutterstock/videos/7090150/thumb/1.jpg?i10c=img.resize(height:160)), although a photographic failure in every other way, does fortuitously show that the pale spot/patch at the posterior of the ear in adult females of the wapiti can be the most conspicuous feature of their colouration, when viewed from a certain angle.

Males have lost this feature. Can readers explain why this is so?

Posted on August 22, 2022 11:47 PM by milewski