Beginning a study of the malar stripe in bovids

Many species of bovids, in the antilopin, aepycerotin, caprin and hippotragin tribes, have a feature of colouration called the malar stripe. This is a relatively dark stripe running from the orbit above the eye, diagonally across the side of the face, to near the corner of the mouth. The malar stripe is particularly typical of gazelles, but it also occurs in goats and oryxes, among others.

The adaptive function is deeply ambivalent. On one hand, running through the eye suggests a disruptive effect reducing the conspicuousness of the animal to predators. The eye tends to be one of the most noticeable parts of an animal, and disguising it can be disproportionately effective in helping to hide the whole animal. On the other hand, where the stripe is bold enough, and offset by strikingly pale fur, it can contribute to a flag or even a bleeze, advertising the animal for social purposes. These contrary functions could occur within a given species, according to age or sex.

Here I illustrate a few examples of how subtle the effects remain, even once the dichotomy is understood between reducing conspicuousness and boosting conspicuousness.

In gazelles, the malar stripe tends in most species to be effacing. However, in Soemmerring's gazelle (Nanger soemmerringi, see https://www.inaturalist.org/photos/45006655) it has been drafted to form one border of a bold black-and-white pattern on the front of the face. This species accordingly possesses a facial flag or even a frontal bleeze. In the case of the springbok (Antidorcas marsupialis), which has a showily whitish face, the malar stripe seems perverse. It is not bold enough to offset the pale when viewed at distance, and by crossing out the eye seems to detract from the facial advertisement (see https://www.inaturalist.org/observations/56238889).

The malar stripe of the gemsbok (Oryx gazella) is part of a conspicuous pattern of dark and pale on the face of the adult (see https://www.inaturalist.org/observations/74839633). (In the same species, it plausibly disguises the face shortly after birth when the infant relies on lying low; see the fourth photo in https://fossilrim.org/animals/gemsbok). However, the scimitar-horned oryx (Oryx dammah, see https://www.inaturalist.org/photos/122472718) is analogous to the springbok in being, overall, a species with conspicuously pale colouration. And, as in the case of the springbok, the malar stripe seems to detract from the showiness of the face, which leaves us puzzled as to its adaptive value.

Publicado por milewski milewski, 10 de junio de 2021

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