ארכיון יומן של ספטמבר 2020

ספטמבר 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.

פורסם ב ספטמבר 18, 2020 11:02 אחה"צ על־ידי milewski milewski | 0 comments | הוספת תגובה

ספטמבר 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.

פורסם ב ספטמבר 20, 2020 12:21 לפנה"צ על־ידי milewski milewski | 0 comments | הוספת תגובה

ספטמבר 08, 2020

Is the colouration of Dorcas gazelle conspicuous or inconspicuous?

The colouration of the Dorcas gazelle is ambivalent. On one hand, it could be that the adaptive function is to hide the animal in plain sight in its environment, which is open because this is a species of the desert. On the other hand, it could be that gazelles, far from trying to hide, advertise themselves for the sake of gregariousness and the display of fitness, which are arguably their best strategy versus predators given their inevitable exposure.

In the past, almost all authors have assumed that the Dorcas gazelle's colouration blends into the desert, following the rationale of camouflage. However, I see the animal differently: Gleaming out like a beacon in most illuminations. In my view, it is not that this gazelle is pale to match the pale sand; instead, it is so pale that it becomes a living highlight. The most important element in its pattern, ensuring conspicuousness, is the extension of the white underparts to a level so high on the flank and rump that, even at noon, the white is likely to draw attention to the figure.

So which of these opposing interpretations is true?

What is not readily apparent from photos is that the Dorcas gazelle continually wags its tail, which is particularly conspicuous in being the only black part of the body. Because the eyes of ungulates and predators alike are extremely sensitive to motion, this continual movement is a real giveaway as to the antipredator strategy of this gazelle. Instead of 'freezing' in alarm and crouching low as camouflaged antelopes would do in dense vegetation, the Dorcas gazelle advertises itself and its fitness, relying on its ability to outrun the predator.

For its part, the predator relies on spotting any individual which is sick, old or injured, as betrayed by any slight failure to stot vigorously, to alarm-sneeze loudly or to keep up the flagging of the tail.

In reality, the colouration of most ungulates is probably some complex combination representing an adaptive compromise between hiding and self-advertisement. For example, the colouration of the large ears of the Dorcas gazelle suggests camouflage of that part of the figure at least. And it is possible that the same overall pattern which gleams by day actually hides the whole body when the light is dim. One way to investigate this would be to see whether gazelles behave differently when alarmed by night, freezing, keeping quiet and facing the threat rather than presenting the bold hindquarters as they typically do by day.

פורסם ב ספטמבר 08, 2020 12:33 לפנה"צ על־ידי milewski milewski | 0 comments | הוספת תגובה

ספטמבר 14, 2020

Gazella marica is not just an Arabian form of Gazella subgutturosa

At first sight, it might seem that Gazella marica is the form of the widespread Gazella subgutturosa which occurs on the Arabian peninsula. So there was some surprise when a genetic study a decade ago found the two forms to be different.

However, even without genetic information marica can be seen to be more than just an extremely pale, desertic subspecies of gutturosa.

Firstly, marica is considerably smaller than subgutturosa, the adult females weighing respectively about 19 kg and 25 kg. Gazella marica is not just desertic but also rather diminutive.

Secondly, the larynx is different in the two forms. In marica the larynx seems normal for gazelles and ruminants generally, whereas subgutturosa possesses what is perhaps the oddest laryngeal anatomy of any ungulate.

Thirdly, the horns are different in the female: Extremely well-developed in marica but extremely poorly developed in subgutturosa.

And fourthly, the arrangement of the flank-banding is surprisingly different, 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 reduced to a line instead of a long triangle.

As far as I know, the difference in pattern of colouration has never been pointed out before.

Furthermore, it is hard to see the extreme pallor of marica as essentially arid-adapted because subspecies yarkandensis of species subgutturosa occurs in the extreme aridity of the Gobi desert without similar pallor.

פורסם ב ספטמבר 14, 2020 11:59 אחה"צ על־ידי milewski milewski | 0 comments | הוספת תגובה