Guidelines for discussing the adaptive colouration of ungulates

The topic of adaptive colouration in ungulates is like an unopened gift. Have we naturalists had a mental block in interpreting the appearance of the animals we are so keen to photograph? For example, we post thousands of photos of giraffes without discussing the obvious question 'why are they spotted?'

The initial question in every case is 'does the pattern hide the figure or body part, or show it off?' Is the animal hiding from predators or communicating with others of its kind? (see https://www.gettyimages.com.au/detail/photo/view-of-animals-on-land-royalty-free-image/1271835126?adppopup=true) Do prey species sometimes show off to their predators? The answers depend partly on scale, motion, illumination, background and the visual system of the onlooker. And each species of ungulate may have a different design depending on body size, habitat cover, gregariousness, nocturnal vs diurnal activity, social system, etc.

In a species living solitarily in forest, the adaptive colouration is predicted to be inconspicuous with the exception of certain small-scale patterns accentuated for social communication, such as markings on the tail or ears. By contrast, in extremely gregarious species of open ground, hiding may be unrealistic even at night; the colouration may be thoroughly conspicuous (https://www.gettyimages.com.au/detail/news-photo/arabian-oryx-whose-legendary-beauty-captivated-the-minds-of-news-photo/1229996518?adppopup=true and https://www.gettyimages.com.au/detail/photo/scenic-view-of-desert-al-ahsa-eastern-province-royalty-free-image/1280587970?adppopup=true), so that intraspecific communication can work continually at a whole-body scale to aid social means of evading predation.

Either way, hiding only works when the animal stays still, because the eyes of Carnivora are more sensitive to motion than is the case in humans. So an important principle is that the same body part can be coloured to look inconspicuous when motionless but conspicuous when moved.

It is the tonal, not the chromatic, aspects of colouration that matter for ungulates. Tonal refers to black/shades of grey/white, whereas chromatic refers to hues such as reddish. The eyes of ungulates and their predators are poor at seeing hues, but excellent at seeing motion in black and white. So a 'rich rufous' antelope may look vivid against a green background (see https://www.shutterstock.com/da/image-photo/antelope-bongos-zoo-1133237333) to the human eye, but it would look grey-on-grey to a conspecific or predator. And we may find that previously overlooked sheen effects are as important as pigmentation in the colouration of many ungulates (e.g. see how plain the back of the head is in Aepyceros melampus, https://www.gettyimages.com.au/detail/photo/side-view-of-deer-walking-on-grassy-field-royalty-free-image/1145793280?adppopup=true, and then watch this video of the same species: https://www.youtube.com/watch?v=9CGoEJtE9X8).

Although adaptive conspicuousness in ungulates always involves dark/pale contrasts, distance and scale are crucial. So, for example, the whole figure of the bontebok (Damaliscus pygargus pygargus, see https://blog.nature.org/science/files/2015/07/bontebok.jpg) is likely to be strikingly piebald to all viewers even from far away. But the black-tipped white ear of the impala (see https://www.catersnews.com/stories/animals/whats-going-on-ear-intrigued-impala-line-their-ears-up/) is noticeable only at close quarters. When viewed by a scanning predator it may just be 'camouflage-spotting' for the impala (see https://www.dreamstime.com/impala-aepyceros-melampus-male-standing-savanna-kruger-national-park-south-africa-impala-male-looking-alert-distance-image123979454).

It may be ho-hum that the colouration of animals is some complex combination amounting to an adaptive compromise between concealment and self-advertisement. But the topic is fascinating in ungulates because the patterns are so improbably diverse, from plain fawn to the bizarre striping of zebras, and from sexual uniformity to the male looking like a different species and behaving like a living flag.

Using these guidelines, can we naturalists begin to make sense of the wonderful blend of science and art that is on offer in the burgeoning collection of images in iNaturalist?

פורסם על-ידי milewski milewski, ספטמבר 08, 2020 12:33 לפנה"צ

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