A comparison of the goitred gazelle and the springbok

Two gazelles, the goitred gazelle (Gazella subgutturosa, see https://www.istockphoto.com/video/goitered-gazelle-gm473245265-26192520 and https://www.bbc.com/azeri/azerbaijan-44417549 and https://www.pedalverde.net/azerbaijan) of the Iranian region and the springbok (Antidorcas marsupialis) of South Africa, make for an obvious comparison because they live on similar semi-arid plains in opposite hemispheres. Adults can hardly hide in these exposed environments. And a puzzling difference in adaptive colouration is that the goitred gazelle tends to blend into its surroundings whereas the springbok tends to stand out even at a distance.

What makes the springbok so conspicuous is its boldly dark flank-band offset by white highlights on the face, rump, flanks and upper legs (see https://www.inaturalist.org/observations/85319775 and https://www.inaturalist.org/observations/70140445). In the goitred gazelle the basic pattern is similar but toned down to a semblance of camouflage (see https://commons.wikimedia.org/wiki/File:Ceyran.jpg and https://www.dreamstime.com/goitered-gazelle-jeyran-field-wildlife-nature-reserve-goitered-gazelle-jeyran-field-wildlife-nature-reserve-field-image216752540 and video in https://animalia.bio/goitered-gazelle and https://www.shutterstock.com/da/image-photo/gazella-subgutturosa-goitered-gazelle-1155749653).

Both species greet scanning predators with stotting displays, but these accentuate the differences. The goitred gazelle shows the white buttocks as it flees (see https://www.pond5.com/stock-footage/item/130905132-herd-goitered-gazelles-running-desert-001 and https://www.pond5.com/stock-footage/item/130905144-herd-goitered-gazelles-running-desert-004) and stots by bouncing up and down with its black tail erect (see https://www.istockphoto.com/video/running-goitered-gazelle-gm473245311-26196643 and video in https://discover.hubpages.com/education/Goitered-Gazelle-History-Facts-and-Efforts-to-Recover-It and https://www.diana-hunting.com/game/goitered-gazelle). However, the springbok has an extreme performance, bouncing while blazening an uniquely extended white double-crest which is normally folded invisible on the back, rump and buttocks (see https://www.youtube.com/watch?v=jMIiB9DnRXg and https://www.atlasobscura.com/articles/watch-this-gazelle-spring-across-the-african-savanna-like-a-pogo-stick).

Consider the different regimes of natural predation in the Iranian region and South Africa. Here, the counterintuitive principle is: the more intense the predation, the more conspicuous certain ungulates evolve to be.

In both regions, the natural predators included large cursorial canids (wolf Canis lupus vs painted hunting dog Lycaon pictus) which scanned the groups of gazelles by day to choose the least-fit member before starting a chase of endurance. This preliminary scrutiny means a selective pressure for self-advertisement ('showing off') instead of keeping a low profile.

A crucial difference was that the northern species was the only wild ruminant in most of its habitat, whereas the southern species coexisted with the red hartebeest (Alcelaphus caama), the blesbok (Damaliscus pygargus phillipsi) and/or the black wildebeest (Connochaetes gnou). The greater reliability of prey in South Africa is likely to have maintained a denser population of predators, thus boosting the risk of any particular group of the springbok being encountered by the painted hunting dog. Imagine an evolutionary tipping point, at which the adaptive colouration of adults of the springbok lost any concealing features and became thoroughly revealing.

The best chance for the individual springbok is not to self-efface (either in the landscape or in the group) but to announce itself as vitally as possible, disqualifying itself from being worth chasing. In the case of the goitred gazelle as for most other antelopes, encounters with predators remained infrequent enough that it made sense to bet-hedge rather than committing to conspicuousness. Red hartebeest (see https://www.picfair.com/pics/06662183-red-hartebeest-alcelaphus-buselaphus), blesbok (see https://www.123rf.com/photo_19680505_a-blesbok-antelope-damaliscus-pygargus-south-africa.html?vti=mcypwdomf20ai7hkf7-1-5) and black wildebeest (see https://rangerdiaries.com/diary/black-wildebeest-2/) also have dark/pale features obvious to predators in certain settings, but their overall colouration remains ambivalent. This may be because all are larger and more enduring in flight than the springbok, and thus less vulnerable to the painted hunting dog.

Publicado por milewski milewski, 14 de mayo de 2021

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Whereas the goitred gazelle flees and stots with its tail rigidly erect (see https://www.inaturalist.org/observations/504792 and https://www.inaturalist.org/observations/38272914), the springbok never even raises its tail to the horizontal while fleeing (see https://www.alamy.com/springbok-antidorcas-marsupialis-running-namibia-etosha-national-park-image151881858.html) or stotting (see https://www.shutterstock.com/da/image-photo/pronking-springbok-37655779 and https://elements.envato.com/springbok-pronking-in-the-grass-X6ZPTS2). This difference makes sense partly because in the springbok the dark tassel is too small to add much to the display, and the base of the tapering tail is less flexible than in the goitred gazelle (compare https://www.alamy.com/tail-end-of-goitered-gazelle-gazella-subgutturosa-image211285398.html with https://www.alamy.com/springbuck-springbok-antidorcas-marsupialis-herd-from-behind-namibia-image5449818.html).

Publicado por milewski hace alrededor de 1 año (Marca)

Both the goitred gazelle and the springbok have conspicuously pale faces, and in both species the paleness on the face develops as the animal grows. However, the pale face is not as consistent among adult individuals in the goitred gazelle as in the springbok. In the goitred gazelle the pale face is associated more with the male than with the female, and even in the mature male (as shown in http://shutterstock.puzzlepix.hu/kep/132032954 and http://shutterstock.puzzlepix.hu/kep/132032945) some individuals retain the inconspicuous pattern on the face.

Publicado por milewski hace alrededor de 1 año (Marca)

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