Interpreting the buccal semet of the pronghorn (Antilocapra americana)

The pronghorn (Antilocapra americana) has an odd-looking mouth.

It also has an oddly graphic pattern of colouration at the mouth (see https://www.inaturalist.org/observations/74523202), which I suggest we call a buccal semet in view of its potential function of accentuating movements of the mouth as part of social communication within the species.

The mouth of adult males is shown in https://www.featheredphotography.com/blog/2013/08/28/odd-growths-on-the-face-of-a-pronghorn/.

That of adult females is shown in https://www.endangered.org/saving-the-sonoran-pronghorn-will-help-save-us/.

And https://www.zooborns.com/.a/6a010535647bf3970b01538fdcdd79970b-popup shows that this is the first feature of adult colouration to develop in infants.

The fact that there is a crisp dark/pale pattern at the mouth can be explained in terms of the mutual monitoring of cud-chewing among group-members.

Many deer and bovids have comparable patterns although the precise design is unique to the pronghorn. Keeping an eye out for any interruption of rumination in companions could signify alarm, because any individual noticing potential danger immediately interrupts the 'white noise' of the chewing to listen up in the appropriate direction.

Age, sex, season, and individuality affect various features, both large- and small-scale, of the colouration of the pronghorn. The pattern at the mouth is odd in being

• consistent in all individuals, and
• graphically black-and-white.

Most other ruminants have subtle, not stark, versions of the buccal semet (e.g. see https://www.gettyimages.com.au/detail/photo/chewing-buck-royalty-free-image/1252898190?adppopup=true, for Aepyceros melampus).

A possible explanation lies in a behavioural oddity of the pronghorn, which moves its lips in ways still poorly understood.

Lip-smacking is certainly a social signal, given e.g. when males are about to copulate.

However, compulsive lip-trembling is the really odd behaviour. Mueller-Schwarze and Mueller-Schwarze in 1972 wrote: "After sniffing materials such as crushed grass, rotten apples and the cud of deer, pronghorn of both sexes move their lips rhythmically at a rate of 150/minute for a long period, sometimes for longer than 20 minutes. During that period they may show interest in food...but are unable to eat. When put in the mouth the food falls out again".

Why would the pronghorn signify to group-members the discovery of bacterial odours in such an extreme way?

Posted on May 23, 2021 11:46 PM by milewski