Caleonic colouration in the caribou, part 2: Rangifer tarandus pearyi in context

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...continued from https://www.inaturalist.org/journal/milewski/67529-caleonic-colouration-in-the-caribou-part-1-rangifer-tarandus-terranovae#

The following show what seems to be caleonic colouration in Rangifer tarandus pearyi:
https://www.change.org/p/world-wildlife-fund-help-raise-awareness-on-the-endangered-peary-caribou-6869eb8d-b1ec-41eb-9722-1b5897a374f1 and https://www.arcticphoto.com/polar-info/info10/peary-caribou/jk0006-00.htm.

Given that two geographically and latitudinally separated subspecies of R. tarandus possess caleonic colouration, how has this adaptive pattern arisen?

As I see it, the major differences among the main three main forms of R. tarandus in North America are as follows.

I focus on fully mature males in autumn. Differences can be seen in antler form and patterns of colouration. 
 
Migratory (barren-ground) type (typically R. t. groenlandicus and R. t. granti):

https://photos.alaskaphotographics.com/img-show/I0000pqZ2U_D_aI8 and https://www.alamy.com/stock-photo-sideview-of-an-adult-bull-caribou-walking-along-a-ridgetop-near-highway-40002640.html?imageid=B658E192-A8DF-42E2-8FEC-856427CAFA0B&p=228470&pn=6&searchId=d45a0b079e0a44d6df19e063f123082f&searchtype=0 and https://www.alamy.com/stock-photo-a-bull-and-cow-caribou-in-the-alaskan-range-mountains-during-the-autumn-126175283.html?imageid=FCD3A92E-5377-4F5B-A54A-A1E9CA2572AB&p=194525&pn=1&searchId=26929b68aec462c7654cb43d9dd2f7a4&searchtype=0 and https://www.alamy.com/stock-photo-a-bull-and-cow-caribou-in-the-alaskan-range-mountains-during-the-autumn-126175284.html?imageid=4928F00C-F791-45EC-93BC-1A700551D8D6&p=194525&pn=1&searchId=26929b68aec462c7654cb43d9dd2f7a4&searchtype=0 and https://www.alamy.com/stock-photo-a-bull-caribou-follows-his-harem-in-the-alaska-tundra-during-the-autumn-102726416.html?imageid=51D937F6-E643-48FC-B5B3-89DE79BD20A6&p=194525&pn=1&searchId=26929b68aec462c7654cb43d9dd2f7a4&searchtype=0

  • posterior parts of antler emphasised, with minimal palmation
  • colouration pied; overall, tonally balanced (approximately equal areas of dark and pale, in profile)
  • flank-banding maximal
  • pale feature near elbow

Insular type:

R. t. pearyi:
https://www.canada.ca/en/environment-climate-change/services/species-risk-public-registry/cosewic-assessments-status-reports/peary-caribou-barren-ground/chapter-4.html

R. t. terranovae:
https://saltscapes.com/roots-folks/3205-caribou-country.html and scroll Inn https://trophyhunts.com/listing/steel-mountain-lodge-newfoundland-canada-fly-in-hunting-lodge-for-moose-and-caribou/ and https://mobile.twitter.com/ducboreal/status/1369313314820521990 and https://m.facebook.com/NewfoundlandLabradorTourism/photos/a.111088443781/111092763781/?type=3 and https://www.alamy.com/stock-photo-large-bull-caribou-with-shedding-velvet-antlers-walks-through-crimson-75290365.html?imageid=3A3FE78C-D4D3-4C1F-93D3-567A2543B15E&p=228679&pn=25&searchId=9892403e79eeaa448727cb13a1e36c01&searchtype=0

  • antler form moderate
  • colouration pallid
  • flank-banding minimal (by means of pallour)
  • pale extension on haunch

Woodland/mountain type (typically R. t. caribou sensu stricto):

https://www.alamy.com/stock-image-male-woodland-caribou-rangifer-tarandus-caribou-central-british-columbia-162739550.html?imageid=705471E7-CC68-4E93-BE30-678AC46531C4&p=281926&pn=4&searchId=d557ba6763acd5a93532c35ee5eb7dff&searchtype=0 and https://www.alamy.com/stock-photo-woodland-caribou-rangifer-tarandus-caribou-bull-northern-british-columbia-125476288.html?imageid=B8781D2E-37BA-4870-8F53-467D9A3A15BD&p=360763&pn=19&searchId=ca9b65ac9c231055a4ff7c926e1bb949&searchtype=0 and https://market.newfoundlandcanvas.com/bradjames/woodland-caribou-1 and https://kidadl.com/facts/animals/woodland-caribou-facts and https://www.tbnewswatch.com/local-news/first-nations-criticize-caribou-protection-plan-692585

  • anterior parts of antler emphasised, with maximal palmation
  • colouration dark (even the neck failing to become white in autumn)
  • flank-banding minimal (owing to darkening)
  • pale feature near elbow

If these three major types of R. tarandus in North America are valid, then the question arises:
Which environmental differences have produced this differentiation?

The following occurs to me, bearing in mind that for cervids one of the most important aspects of the environment is avoidance of other ruminants of similar body-size.

In general, the pattern for large (>20 kg) cervids everywhere in the temperate to polar Northern Hemisphere, plus tropical to temperate South America, is that only one species occurs in a given area. Cervids tend to be mutually exclusive in habitat, although there are situations of coexistence of a large species with a small species.

Whereas a theme among bovids is coexistence in multi-species communities, cervid species tend to compete/indirectly interfere with each other, to the effect that only one species can succeed in any given area.

Indeed, this is part of the reason for the decline in the true woodland caribou (R. t. caribou sensu stricto) in the western part of the boreal zone of North America. With logging, the habitat of this form has supported an increase in Alces alces or forms of Odocoileus, or both. These forms, even if they do not compete with R. tarandus for food, tend to support too many predators for the populations of R. t. caribou to sustain the losses in the longer term.

What occurs to me about the three major forms of R. tarandus in North America is the following:

The barren-ground subspecies (mainly R. t. groenlandicus and R. t. granti) coexist partly with Alces alces and Ovis dalli. However, they tend to spend a crucial time of year (summer) in a remote, extreme environment, free of other ruminants (and with limited predation).

The forms found on islands (R. t. pearyi and R. t. terranovae) were (until the introduction of Alces alces to Newfoundland) essentially free from coexistence with other ruminants. Coexistence was irrelevant to any longer-range movements they undertook.

The woodland and mountain subspecies (e.g. R. t. caribou sensu stricto), of all the three types, were the most subject to coexistence with other ruminants. The ruminants concerned included Alces alces, several forms of Odocoileus, and forms of Ovis; and the coexistence potentially occurred throughout the seasonal cycle.

This means, inter alia, that the natural densities of populations of the woodland and mountain forms of R. tarandus were everywhere limited. This affects population-related phenomena, such as rutting behaviour and sexual displays.

The point of this conceptual framework, with particular reference to R. t. terranovae of Newfoundland, is the following.

It may seem surprising, given that Newfoundland is a large island and not remote from the mainland, that the local form of R. tarandus is different from that of the nearby mainland in Labrador, and similar to R. t. pearyi of the remote Arctic - which lives at a far higher latitude with a far more extreme climate.

However, R. t. terranovae was unusual, among all the forms of R. tarandus at its latitude, in having its whole habitat to itself, w.r.t. other ungulates.

I offer the following summary of the ideas introduced here:

Migratory (barren-ground) type:

  • pied colouration,
  • free of other ruminants in summer, and
  • relatively free of predation in summer

Island type:

  • pallid colouration,
  • permanently free of other ruminants, and
  • predatory pressure set by R. tarandus itself

Woodland/mountain type:

  • dark colouration,
  • permanently subject to coexistence with at least one other species of ruminant, and
  • predatory pressure potentially disproportionate, year-round.

Continuing this line of thinking, we may go on to ask:

Given that the colouration of all forms of R. tarandus has conspicuous aspects and inconspicuous aspects, how are the various patterns of colouration potentially adaptive to the various predatory regimes?

I offer the following tentative answer:

  • migratory types: extremely conspicuous at the season of relative freedom from predation, somewhat inconspicuous at the season of relative intensity of predation;
  • island type on Newfoundland: extremely conspicuous in summer, inconspicuous in winter (when its extreme pallor blends in with snow); and
  • woodland/mountain types: somewhat inconspicuous year-round - including winter, when the background consists of not only snow but also trees and shrubs.
Posted on June 23, 2022 12:07 AM by milewski milewski

Comments

The following js an unusually clear photo of Rangifer tarandus pearyi:
https://www.shutterstock.com/image-photo/peary-caribou-buck-running-snow-1232287474

Posted by milewski about 1 year ago

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