Archivos de diario de septiembre 2020

08 de septiembre de 2020

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?

Ingresado el 08 de septiembre de 2020 por milewski milewski | 0 comentarios | Deja un comentario

14 de septiembre de 2020

Is Gazella marica just an Arabian form of Gazella subgutturosa?

Based on geographical distribution, the sand gazelle (Gazella marica, see https://www.inaturalist.org/observations/55395922 and https://www.flickr.com/photos/120036191@N02/15075998166) seems to be a somewhat pallid Arabian form of the widespread goitered gazelle (Gazella subgutturosa, see https://alchetron.com/Goitered-gazelle#goitered-gazelle-f4036415-25e8-405c-9b90-ac086cae726-resize-750.jpeg and https://www.flickr.com/photos/korkeasaarizoo/48527469291/). However, the relationship between the two forms (which were previously regarded as subspecies) is complicated.

A problem with explaining the pallor of the sand gazelle as arid-adapted is that subspecies yarkandensis of the goitered gazelle (see https://mongolia.gogo.mn/r/156358 and https://rove.me/to/mongolia/wildlife-watching?photo=7) occurs in the extreme aridity of the Gobi Desert without particular pallor.

The sand gazelle is surprisingly smaller than the goitered gazelle. This diminutive status is in line with all the other large mammals of the Arabian Peninsula, but is hard to explain ecologically.

The larynx may differ in the two forms. The goitered gazelle has what is perhaps the oddest laryngeal anatomy of any ungulate (https://onlinelibrary.wiley.com/doi/10.1111/j.1469-7580.2011.01361.x). Nobody seems to have studied the larynx of the sand gazelle, and I have seen only one photo of this well-photographed species showing a (rutting?) male with a particularly noticeable larynx (see https://eol.org/pages/129520/media?license_group=cc_by_sa).

The female horns are surprisingly different even considering the whole genus Gazella: extremely well-developed in the sand gazelle (see https://www.inaturalist.org/observations/30681543 and https://www.inaturalist.org/observations/84177386) but extremely poorly developed in the goitered gazelle (see https://en.wikipedia.org/wiki/Goitered_gazelle#/media/File:Female_goitered_gazelle,_Shirvan_National_Park,_Azerbaijan.jpg and https://animaldiversity.org/accounts/Gazella_subgutturosa/pictures/collections/contributors/david_blank/goiteredgazelles8/).

The pattern on the flank may differ slightly. In the sand gazelle, the banding conforms with that in gazelles generally, in that the darker ventral flank-band runs obliquely towards the shoulder, leaving enough space for the paler flank-band above it to be triangular. In the goitered gazelle both bands tend to run horizontally, the darker ventral band running towards the scapula and the paler band tending linear rather than triangular.

Perhaps the clearest clue to the close relationship between the two is that both display their dark tails differently from other members of their genus. Whereas most species of Gazella display the tail mainly by wagging it briskly while walking, the sand gazelle and goitered gazelle display the tail mainly by holding it up while running (see, respectively, https://www.flickr.com/photos/lesnafi/11877272684 and https://www.flickr.com/photos/tom_martin/30486954365/).

Ingresado el 14 de septiembre de 2020 por milewski milewski | 0 comentarios | Deja un comentario

20 de septiembre de 2020

How to distinguish two confusing species of sand gazelles

True gazelles, of the genus Gazella, are bewilderingly speciose in North Africa and Arabia. Each species is individually variable; several 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 psammophilous species of gazelles inhabiting the desert dunes, the confusion is aggravated by the indiscriminate use of similar common names derived from the Arabic. 'Rhim' refers to Gazella leptoceros of the Sahara (see https://naturerules1.fandom.com/wiki/Slender-horned_Gazelle?file=Gazelle%252C_Rhim.jpg) while 'rheem' refers to Gazella marica of Arabia (see https://liloneoftheashes.com/2015/10/13/anantara-al-sahel-villa-resort-a-jungle-retreat/#jp-carousel-5884), 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. And this species is mislabelled even in professional publications (see cover photo of https://www.iucn.org/sites/dev/files/content/documents/2017/en_gacelas_web_160_media_1.pdf, which is Gazella leptoceros but incorrectly called Gazella cuvieri).

The following are distinguishing features.

All species of true gazelles are more or less fawn with whitish ventral parts separated from the fawn by a relatively dark flank-band. However, in the slender-horned gazelle (Gazella leptoceros), the fawn on the body, neck and legs is the most uniform of any species of true gazelle (see https://zoo-tycoon-movie.fandom.com/wiki/Rhim_Gazelle?file=891295197.jpg and https://www.flickr.com/photos/blacktigersdream/37334132881 and https://www.flickr.com/photos/pastough/6411619137/). By contrast, in the sand gazelle (Gazella marica) it is clearly differentiated into a pale upper flank-band, a pale lower-haunch, and pale legs (see https://www.naturepl.com/stock-photo/arabian-sand-gazelles-(gazella-subgutturosa-marica)-on-sir-bani-yas-island/search/detail-0_01655551.html).

A difference too subtle to see in most photos is on the feet. Those of the slender-horned gazelle are - at least in some individuals - marked with small-scale dark/pale contrasts near the hooves (see http://www.mammalogy.org/gazella-leptoceros-1153). The feet of the sand gazelle are pale like its legs, although technically the furred rim of the pasterns, adjacent to the hooves, can likewise be dark in some individuals.

All true gazelles share a certain detailed pattern on the face. The slender-horned gazelle shows this pattern in inconspicuous form, the only species-specific feature being an unusually pale patch on the rostrum (between forehead and nasal fur) in about half of all individuals (see https://www.flickr.com/photos/nikonpaul/46428414125/ and https://www.flickr.com/photos/truus_en_zoo/7836110296). In the sand gazelle the facial pattern is generalised in infants, but fades patchily as the animal grows. In adults of both sexes, but particularly males, the whole face tends to be conspicuously bleached (see https://www.inaturalist.org/observations/21921777 and https://www.flickr.com/photos/archaeologist_d/45817925995/), forming a facial flag. This is accompanied, in adult males, by a darkening around the eye (see https://www.flickr.com/photos/mikebarth/23973307372) that is not seen in the slender-horned gazelle.

Ingresado el 20 de septiembre de 2020 por milewski milewski | 0 comentarios | Deja un comentario

18 de septiembre de 2020

Why is the mountain goat all-white?

The 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 (e.g. see https://www.inaturalist.org/observations/24793211) in the way epitomised by the polar bear (Ursus maritimus). However, the sheer number of photos of the 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 innumerable photos of the 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 mountain goat - even in winter - tends to be rocky and dark rather than snowy and pale, making the animals conspicuous (e.g. see https://www.inaturalist.org/observations/38942159 and https://www.inaturalist.org/observations/77400383).

The 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 mountain goat the male is so intimidated by the female that his courtship requires juvenile-like postures of submission (see https://academic.oup.com/jmammal/article-abstract/45/4/551/852342?redirectedFrom=PDF and https://truehunts.com/mountain-goat-behavior-and-hunting-strategy/), and any naturalist familiar with the species knows how hard it can be to distinguish the males in any group.

The 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, extending the silhouette by means of furry adornments to make the female figure large and noticeable.

I suggest that it is in 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 mountain goat can best be understood. This species is all-white not to hide but instead to show off.

Ingresado el 18 de septiembre de 2020 por milewski milewski | 1 comentario | Deja un comentario