Journal archives for September 2020

September 18, 2020

Why is the rocky mountain goat all-white?

The rocky mountain goat (Oreamnos americanus) is the only ungulate species on Earth which possesses all-white pelage, in both sexes and at all ages. What is the adaptive reason for this extreme specialisation?

The obvious answer is 'nivicolous crypsis', i.e. hiding in the snow and ice in the way epitomised by the polar bear (Ursus maritimus). However, the sheer number of photos of the rocky mountain goat in iNaturalist suggests that this interpretation is incorrect. How could it possibly be that a prey-animal coloured to blend into its typical environment is so easy to spot, and so extremely photogenic?

Instead, it should be clear from the thousands of photos of the rocky mountain goat on the internet that its white pelage advertises it in its normal surrounds, that it is particularly gregarious in winter, that it does not attempt to crouch or 'freeze' when alarmed, and that even the newborns do not hide in the way of most ungulates. The terrain chosen by the rocky mountain goat - even in winter - tends to be rocky and dark rather than snowy and pale, making the animals conspicuous.

The rocky mountain goat is one of the most extreme examples, among ungulates, of the female emulating the male in muscularity, weaponry (deadly horns), adornment (e.g. beard, mane, pantaloons), aggressiveness, and self-advertisement. A similar syndrome occurs in adaptively conspicuous bovids such as wildebeests (Connochaetes) and oryxes (Oryx). However, in the case of the rocky mountain goat the male is so intimidated by the female that his courtship requires juvenile-like postures of submission, and any naturalist familiar with the species knows how hard it can be to distinguish the males in any group.

The rocky mountain goat is so specialised for taking deliberate, sure-footed refuge from predators on cliffs that it has abandoned any attempt to hide, even at night or in infancy, and it has nearly abandoned running when alarmed. It seems to be compatible with this strategy that it has also feminised the bravado usually associated with masculinity in mammals, extending the silhouette by means of furry adornments to make the female figure big and noticeable.

I suggest that it is view of this combination of specialisations - anti-predator reliance on cliffs and maternal defence, going together with extreme 'feminist' self-assertiveness - that the conspicuous monotonal whiteness of the rocky mountain goat can best be understood. This species is all-white not to hide but instead to show off.

Posted on September 18, 2020 23:02 by milewski milewski | 0 comments | Leave a comment

September 20, 2020

How to distinguish two confusing species of sand gazelles

True gazelles, of the genus Gazella, are bewilderingly speciose in Arabia and North Africa. Each species is individually variable. Many of the species have local subspecies. Photographers tend to focus on males. As a result, identification from photos - particularly in the case of females - can be difficult even for naturalists with plenty of experience with ungulates.

In the case of two species of sand gazelles inhabiting the desert dunes, the confusion is aggravated by the indiscriminate use of similar common names derived from the Arabic. 'Rhim' refers to the Saharan Gazella leptoceros while 'rheem' refers to the Arabian Gazella marica, and the two seem to be mislabelled interchangeably on the internet even when the specimens are in zoos.

The name 'slender-horned gazelle' for G. leptoceros hardly helps because both species have long, slender, somewhat asymmetrical horns in most females.

In reality, the two can easily be told apart if you know what to look for, as follows.

All species of true gazelles have are more or less fawn with whitish ventral parts separated from the fawn by a relatively dark flank-band. All also have intricate markings on the face.

First, look at the fawn of the body, neck and legs. In G. leptoceros, this is the most uniform of any species of true gazelle, whereas in G. marica it is clearly carved into a pale upper flank-band, a pale lower-haunch, and pale legs. Another difference which I have never seen mentioned in field guides is that the feet of G. leptoceros are marked with subtle dark/pale contrasts near the hooves, whereas those of G. marica are the same plain pale as the legs.

Now look at the face. In G. leptoceros there is just the standard, inconspicuous pattern typical of gazelles, except for a small whitish patch above the nose in about half of all individuals. In G. marica, the whole face tends to be conspicuously whitened.

Posted on September 20, 2020 00:21 by milewski milewski | 0 comments | Leave a comment

September 14, 2020

Is Gazella marica just an Arabian form of Gazella subgutturosa?

Based on geographical distribution, Gazella marica seems to be a pallid Arabian form of the widespread Gazella subgutturosa. However, the relationship between the two forms (which were previously regarded as subspecies) is complicated.

It is hard to explain the pallor of Gazella marica as arid-adapted because subspecies yarkandensis of Gazella subgutturosa occurs in the extreme aridity of the Gobi Desert without similar pallor.

Gazella marica is surprisingly smaller than Gazella subgutturosa, the adult females weighing respectively about 19 kg and 25 kg. This diminutive status, which is in line with all the other large mammals of the Arabian Peninsula, is hard to explain ecologically.

The larynx may differ in the two forms. Gazella subgutturosa has what is perhaps the oddest laryngeal anatomy of any ungulate. Nobody seems to have studied the larynx of Gazella marica, but I have yet to see a photo of this well-photographed species showing a rutting male with a particularly noticeable larynx.

The female horns are surprisingly different even considering the whole genus Gazella: extremely well-developed in marica but extremely poorly developed in subgutturosa.

Furthermore, I have noticed that the colouration on the flank differs, as follows. In marica, the flank-banding conforms with that in gazelles generally, in that the darkest, most ventral flank-band runs obliquely towards the shoulder, leaving enough space for the paler flank-band above it to be triangular. By contrast, in subgutturosa the flank-bands run horizontally, the darkest, most ventral band running towards the scapula and the paler band being converted from a long triangle to something approaching a line.

Perhaps the clearest clue to the close relationship between marica and subgutturosa is that both display their tails differently from other members of their genus. Whereas most species of Gazella display the dark tail mainly by wagging it briskly while walking, marica and subgutturosa display the tail mainly by holding it erect while running.

Posted on September 14, 2020 23:59 by milewski milewski | 0 comments | Leave a comment

September 08, 2020

Guidelines for the adaptive colouration in ungulates

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

The most basic question in every case is 'is the pattern designed to hide the figure or body part, or to show it off?' Is the animal hiding from predators or communicating with others of its kind? 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 be differently configured depending on body size, habitat cover, gregariousness, nocturnal vs diurnal activity, etc.

For example, in a species living solitarily in forest, the colouration may be adapted 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 and the colouration may be thoroughly conspicuous, 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 twitched.

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 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.

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) is likely to be strikingly piebald to all viewers even from far away. But the black-tipped white ear of the impala (Aepyceros melampus) is noticeable only at close quarters. When viewed by a scanning predator it may just be 'camouflage-spotting' for the impala.

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?

Posted on September 08, 2020 00:33 by milewski milewski | 0 comments | Leave a comment