Archivos de diario de marzo 2021

08 de marzo de 2021

Surprising differences in displays of the tail between the blackbuck and other gazelles

All ruminants with visible tails can swish or flick the tail to shoo insects attracted to the anus and vulva. However, gazelles and their relatives (tribe Antilopini) are surprisingly variable in the other uses of their tails, along lines which I have not seen mentioned in the literature. Let me start with the genera Gazella, Eudorcas and Antilope.

Gazelles and their relatives display their tails mainly in reaction to the appearance of potential predators, and in social interactions within the group. These categories naturally tend to be blurred in playful behaviours that serve to rehearse reactions to danger.

Most species of Gazella and Eudorcas tend to wag the tail conspicuously as soon as they go from standing to walking and trotting, and then relax the tail again when galloping (e.g. see Gazella gazella in https://www.youtube.com/watch?v=mDXpdgATnBI). One way to interpret this is that the animals are signalling to the potential predator (including photographers) that the individual is energetic and alert, and thus not worth singling out for pursuit. However, Gazella subgutturosa and marica tend not to move the tail until running, when it is held more decidedly erect than in other gazelles (see https://www.istockphoto.com/video/goitered-gazelle-gm483199995-26191233). And Antilope leaves the tail inert throughout the locomotory sequence of reaction to potential predators (see https://www.youtube.com/watch?v=1I5jRQCI3VU), even sometimes when stotting (see https://www.gettyimages.com.au/detail/video/blackbuck-antelope-pronks-on-grassland-velavadar-stock-video-footage/918301414 and https://www.gettyimages.com.au/detail/video/blackbuck-antelope-pronk-on-grassland-velavadar-stock-video-footage/918308808).

The contrast can be illustrated by comparing Eudorcas thomsoni with Antilope cervicapra. Thomson's gazelle wags its black, long-tasselled tail with particular zeal when milling hesitantly in view of a safari vehicle (e.g. see https://www.shutterstock.com/da/video/clip-1021228609-herd-thomson-gazelles-drinking-waterhole-serengeti-tanzania), whereas no amount of nervousness will get the blackbuck to wag its nondescript tail - which lacks a noticeable tassel - in similar circumstances. Instead, the blackbuck tends to express its tension at a whole-body scale by leaping high into the air (see https://www.gettyimages.com.au/detail/video/pronking-blackbuck-females-run-and-leap-on-indian-stock-video-footage/1B02605_0001), in a way never seen in Thomson' gazelle.

Where the blackbuck - which is the most sexually dimorphic of antilopins - does display its tail is in masculine behaviour (rivalry and courtship, extending to lekking). Here, the adult male 'hypererects' the tail so that its tip touches the rump (see https://www.inaturalist.org/observations/66647799 and https://www.gettyimages.ca/detail/photo/indian-blackbuck-antelope-cervicapra-male-display-royalty-free-image/90065036?adppopup=true). This looks more like an olfactory than a visual display, because the tail tends to be rather redundant in the whole-body showiness of the black-and-white masculine figure as he walks in an unusual gait, which can be called a 'perfect amble'.

However, nobody seems to have found a scent-gland under the tail in the blackbuck. The displays, and lack thereof, of the tail thus remain an odd aspect of the blackbuck.

Ingresado el 08 de marzo de 2021 por milewski milewski | 0 comentarios | Deja un comentario

31 de marzo de 2021

Conspicuous features of colouration in giraffes

Giraffes (Giraffa, see https://giraffeconservation.org/giraffe-species/) are the largest land mammals with camouflage-colouration. However, they also feature several conspicuously pale features, as follows.

All species and subspecies of giraffes have spotless, sheeny surfaces on the back of the ears (see https://www.hemis.fr/image-photo/557797/kenya-great-rift-valley-lake-nakuru-national-park-baringo-giraffe-giraffa-cameleopardalis.html and https://www.mindenpictures.com/stock-photo-rothschilds-giraffes-giraffa-camelopardalis-rothschildi--in-savanna-naturephotography-image90720680.html and https://www.news24.com/news24/southafrica/news/tourist-critically-injured-after-giraffe-falls-onto-safari-vehicle-at-kruger-national-park-20191110 and https://upload.wikimedia.org/wikipedia/commons/4/43/Giraffes_in_Kruger_National_Park.jpg). This pattern of highlights, which can be called an auricular flag, accentuates the movement of figures walking away from the observer. In the case of Giraffa reticulata (see https://i.redd.it/3ju4klr11lv31.jpg and https://www.masterfile.com/image/en/841-02824842) this is the only conspicuous feature.

Several forms have a pale tract between the crook of the throat and the base of the ear (see https://peapix.com/bing/9393 and https://www.zoochat.com/community/media/reticulated-giraffe.478769/ and https://www.everypixel.com/image-8596535059091917737 and https://www.freepik.com/premium-photo/giraffe-near-large-tree-masai-mara-kenya_12050017.htm#&position=8). This is contiguous with the back of the ear in certain perspectives.

Several species and subspecies of giraffes have pedal flags consisting of spotless, conspicuously pale lower legs. In Giraffa camelopardalis the whole of the lower legs is pale (see https://www.twenty20.com/photos/130370da-9873-4cbf-b7b6-5b521bfcd379 and https://www.picfair.com/pics/06014049-rothchild-s-giraffe-giraffa-camelopardalis-nakuru-district-of-the-rift and https://society6.com/product/giraffe-animal-on-the-serengeti-national-park-tanzania-country-east-africa_stretched-canvas). In Giraffa tippelskirchi the pale is restricted to the pasterns and fetlocks (see https://en.wikipedia.org/wiki/Masai_giraffe#/media/File:Young_Maasai_Giraffes.jpg).

Giraffa giraffa giraffa is the only form of giraffe featuring a conspicuously pale, sheeny area on the buttocks and haunches (see https://www.flickr.com/photos/27727912@N03/49219139176/ and https://www.agefotostock.com/age/en/details-photo/south-african-giraffes-giraffa-giraffa-giraffa-feeding-on-leaves-kgalagadi-transfrontier-park-northern-cape-south-africa-africa/ZI6-3020214 and https://www.flickr.com/photos/manuelromaris/50203975892 and https://www.123rf.com/photo_111847852_giraffe-in-kruger-national-park-south-africa-specie-giraffa-camelopardalis-family-of-giraffidae.html).

Ingresado el 31 de marzo de 2021 por milewski milewski | 0 comentarios | Deja un comentario

06 de marzo de 2021

Extreme sexual incongruity in the adaptive colouration of the blackbuck

In its colouration, the blackbuck (Antilope cervicapra) of India seems to function as two incongruous species in one (see https://www.shutterstock.com/nb/image-photo/blackbuck-known-indian-antelope-found-indianepal-751018990).

The blackbuck is a gazelle, phylogenetically and ecologically. However, females and juvenile males lack the conspicuous dark/pale contrast on the hindquarters that is seen in gazelles of the genera Gazella and Eudorcas. At the same time, mature males are more extensively dark than any other gazelle (see https://sustain.round.glass/columns/blackbucks-dung-attract-mates/ and https://www.overdrive.in/news-cars-auto/features/never-stop-discovering-velavadar-blackbuck-national-park/).

Among gazelles, conspicuous colouration is correlated with gregariousness in open environments, where hiding from predators seems to be less successful than the self-advertisement of alertness and locomotory fitness. The hindquarters tend to have bold patterns of dark (tail-tassel and pygal bands) and pale (buttocks and escutcheon), which stand out in posteriolateral view even when the animals stand still. The display is accentuated by movement of the tail and flaring of white on the buttocks, particularly when the animals stot in demonstration, to any scanning predator, of a current capacity to flee so rapidly and enduringly that pursuit of the individual in question would likely be futile.

The ecology and behaviour of the blackbuck would predict more-or-less normal colouration for a gazelle. After all, this species is in line with other gazelles in being gregarious and living in treeless grassland. It stots with vigour and versatility: it leaps high (see https://www.saevus.in/velavadar-land-of-blackbucks/ and https://www.india.com/travel/articles/if-you-want-to-spot-blackbucks-in-their-natural-habitat-head-to-rehekuri-wildlife-sanctuary-3564384/ and https://hive.natureinfocus.in/photo_sharing/the-leaping-buck and https://www.shutterstock.com/nb/image-photo/jumping-blackbuck-captured-national-park-velavadar-1800223201), bounces stiff-legged (see https://www.shutterstock.com/nb/image-photo/young-blackbuck-jumping-grassland-talchapar-rajasthsan-1890026977 and https://www.inaturalist.org/observations/30385400 and https://www.shutterstock.com/nb/image-photo/jumping-blackbuck-clicked-velavadae-1630599001), or style-trots, and is capable of flaring the white of the buttocks and erecting the tail (see http://www.thejunglelook.com/gallery2/main.php?g2_itemId=5388 and https://walkthewilderness.net/most-beautiful-indian-antelope-blackbuck/).

However, the patterns typical of gazelles have been lost in an extreme sexual dimorphism. Males become more conspicuous than any gazelle as much of their body turns blackish in maturity (see https://www.shutterstock.com/nb/image-photo/blackbucks-antelope-foraging-vast-grasslands-blackbuck-1696197772 and https://www.flickr.com/photos/cirdan-travels/44441395342 and https://www.francisjtaylor.co.uk/product/blackbuck-group/). This pattern has little relationship with predation and instead functions in masculine rivalry and courtship (see https://www.freeart.com/artwork/art-print/a-young-male-blackbuck-antilope-ce_fa14647235.html and https://www.francisjtaylor.co.uk/product/blackbuck-courtship-display/). For their part, females and juveniles (see https://www.shutterstock.com/nb/image-photo/blackbucks-velavadar-sanctuary-gujarat-india-1678657045 and https://www.shutterstock.com/nb/image-photo/blackbuck-antilope-cervicapra-known-indian-antelope-1771445606) have lost all the noticeably dark features of gazelles (flank-band, pygal bands and tail tassel). And the tail, having lost its tassel, is usually left inert in gaits and situations in which it would be flicked or erected demonstratively in other gazelles.

The only conspicuous aspect of colouration in females and juveniles is the ventral whitish, which extends higher than in any other gazelle on to the elbow region and the ventral surface of the neck (see https://www.inaturalist.org/observations/36571904 and https://www.shutterstock.com/nb/image-photo/blackbuck-antilope-cervicapra-indian-antelope-group-1840517536). This in itself is a puzzling pattern, partly because it has been lost in the southern- and easternmost populations.

In mature males, the effectively black-and-white colouration seems superfluously conspicuous to predators. In females and juveniles, the colouration is incongruous in a converse way, because no other gregarious species gazelle living in the open, and frequently stotting and bounding, has colouration lacking all dark features.

And since the sexes live together most of the time, we are left with an overall puzzle. Regardless of whether the females and juveniles can blend into such exposed environments, their presence is likely to be divulged anyway by the outlandish appearance of the mature males among them. So how does the overall colouration of the blackbuck function in a single adaptive strategy with respect to predation?

Ingresado el 06 de marzo de 2021 por milewski milewski | 0 comentarios | Deja un comentario