Capricious subspeciation in the plains zebra, part 1

The plains zebra (Equus quagga, https://en.wikipedia.org/wiki/Plains_zebra) occurs from temperate South Africa, across the equator, to Ethiopia, a distance of about 5000 km (https://upload.wikimedia.org/wikipedia/commons/b/b0/Plains_Zebra_area.png).

This is one of the most deeply subspeciated animals on Earth. The various subspecies look like different species, partly because their adaptive colouration seems to vary not just in degree but in kind. And there is no general conformity with geographical clines.

In this Post I focus on a particular puzzle in this variation: the lack of latitudinal correlation.

A reason to expect a latitudinal pattern in the pelage of the plains zebra is that this species does change in body size in the expected way: body mass was up to perhaps 400 kg farthest away from the equator, compared with perhaps 200 kg near the equator (in western Somalia). However, in other respects the variation seems to make little sense.

At first glance, the striping is least distinct in the southernmost subspecies and becomes progressively more distinct northwards. However, this breaks down on closer scrutiny.

Consider, in no particular order:

Two northern subspecies, in northern Uganda (E. q. borensis) and eastern Kenya (E. q. isabella), have short manes (https://www.safari.com/news/wild-earth/dazzling-zebras-in-need-of-conservation/attachment/maneless-zebra-peter-prokosch/#prettyPhoto/0/). A central subspecies (E. q. crawshayi) has a long mane (https://www.naturepl.com/stock-photo-crawshay-39-s-zebra-equus-quagga-crawshayi-portrait--south-luangwa-nature-image01644673.html). However, there is no latitudinal pattern because moderately short manes occur in both the southernmost subspecies of temperate climates (E. q. quagga: https://www.flickr.com/photos/randomrupert/6770292831/) and the main equatorial subspecies (E. q. boehmi).

Shadow striping occurs in E. q. boehmi from central Kenya to Ethiopia (https://www.inaturalist.org/observations/67006190 and https://www.youtube.com/watch?v=eLFiamuI43A). It virtually disappears in E. q. boehmi in Tanzania to Zambia (https://commons.wikimedia.org/wiki/File:Equus_quagga_boehmi_(skin).jpg), and then becomes extremely well-developed in E. q. chapmani of southern Africa (https://www.dreamstime.com/royalty-free-stock-image-zebra-skin-background-image21209876). In the southernmost subspecies (E. q. quagga), shadow striping cannot be distinguished from a general darkening.

Intensity of striping reaches its acme in central and northern Mozambique. However, the variation defies latitudes: going east to west about latitude 15 degrees South from Mozambique through eastern Zambia (https://parody.fandom.com/wiki/Crawshay%27s_Zebra?file=Zebra%252C_Crawshay%2527s.jpg) and western Zambia (https://upload.wikimedia.org/wikipedia/commons/6/60/Equus_quagga.jpg) to south-central Angola (https://www.allposters.com/-sp/Burchell-s-Zebra-Okavango-Delta-Botswana-Posters_i6222094_.htm) and finally southwestern Angola (https://myloview.com/wall-mural-burchell-s-zebra-is-a-southern-subspecies-of-the-plains-zebra-no-A3D2088) takes us in at least four stages from narrow, black-and-white stripes to wide stripes on a fawn ground-colour and with shadow stripes.

In the Gorongosa-Marromeu area, just west of the mouth of the Zambezi River, there is a switch over a short distance from subspecies chapmani, with its wide and incomplete stripes, to the population with the most intense striping of all, that of the Rift Valley in Gorongosa National Park (see https://www.youtube.com/watch?v=mVrqbm0Ylgs and scroll within https://eowilsonfoundation.org/gorongosa-field-notes-joshua-daskin-hunting-for-history/). These forms are so different that they look like different species. If we accept that they are both subspecies of the plains zebra, it is mystifying that, despite living so near to each other, they have avoided intergradation.

Given all this capricious variation, one begins to see the true complications in any attempt to retrieve the phenotype of the extinct quagga (see my latest Post) by the selective breeding of the adjacent, surviving subspecies.

Equus quagga quagga (https://animalia.bio/quagga) had gleamingly unstriped legs. However, in other respects it was not just an extrapolation of the trend established from Equus quagga chapmani (https://www.alamy.com/stock-photo-burchells-zebra-kruger-park-south-africa-25534098.html) to Equus quagga burchellii (https://www.alamy.com/stock-photo-plains-zebra-in-phinda-game-reserve-south-africa-53987011.html).

The ways in which E. q. quagga was unprecedented included: inexplicably short in the mane, and inexplicably dark on the neck and torso. Furthermore, this darkness was not merely the acme of a southwards trend towards shadow striping. Instead, on the torso it was achieved by an obscure blend of darkened ground-colour and anastomosis of the striping (https://perissodactyla.files.wordpress.com/2013/11/quagga1.jpg). And on the neck it was achieved by an unprecedented widening of the dark stripes, to the point that - unique among the subspecies - the ground-colour was relegated to narrow 'unstripes' (see illustrations in my last Post).

to be continued...

פורסם על-ידי milewski milewski, ספטמבר 08, 2021 05:08 אחה"צ

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Shadow-striping in the plains zebra in Ethiopia can be seen by scrolling to two photos in http://www.cokesmithphototravel.com/expedition-to-wild-ethiopia.html.

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