Caleonic colouration in the caribou, part 2

(writing in progress)

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

I focus on the fully mature male in autumn. Differences can be seen in antler form and colouration patterns. 
Barren-ground type (typically groenlandicus and granti, and and and and

  • 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 (pearyi and terranovae and scroll Inn and and and

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

Woodland type (typically caribou sensu stricto, and and and and

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

If there are three major types of Rangifer tarandus in North America, 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-spp. communities, cervid spp. tend to compete/indirectly interfere with each other to the effect that only one sp. 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 sspp. (mainly groenlandicus and granti) coexist partly with Alces alces and Ovis dalli but tend to spend a crucial time of year (summer) in a remote extreme environment free of other ruminants (and with limted predation).

The forms found on islands (sspp. pearyi and 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 performed.

The woodland and mountain sspp. (e.g. caribou sensu stricto) were the most subject to coexistence with other ruminants, of all the three types. 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, which affects population-related phenomena such as rutting behaviour the 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 form of R. tarandus on it is so distinctive (and so similar to the remote Arctic R. t. pearyi, which lives at a far lower 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) types: pied colouration; free of other ruminants in summer; relatively free of predation in summer/Island types: pallid colouration; permanently free of other ruminants; predatory pressure set by R. tarandus itself

Woodland/mountain types: dark colouration; permanently subject to coexistence with at least one other sp. of ruminant; 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 different patterns of colouration potentially adaptive to the different 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)

Woodland/mountain types: somewhat inconspicuous year-round, including winter when the background consists of not only snow but also trees and shrubs.

(writing in progress)

Posted by milewski milewski, June 23, 2022 00:07


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