Why the chilla is not a Cape fox

Lycalopex griseus (see https://en.wikipedia.org/wiki/South_American_gray_fox) and Vulpes chama (see https://en.wikipedia.org/wiki/Cape_fox) both weigh about 3.5 kg, bigger than a chihuahua but smaller than a toy poodle (e.g. see https://www.inaturalist.org/observations/68228768). And both could have been called 'Cape fox', because they occur to the southernmost capes of South America and southern Africa respectively.

However, this is not a good example of evolutionary convergence, because the chilla is less an alternatively-derived fox and more a miniaturised coyote.

One of the peculiarities of the South American mammal fauna is a theme of diminution, one aspect of which is miniaturisation (exemplified by ruminants in my most recent Post) and another aspect of which is usurpation of ungulate niches by rodents (see my series of Posts on 'The Empty Pampas').

Because body size is more evolutionarily flexible than growth-rate, what miniaturisation means - for both bambis and canids - is relatively slow reproduction.

For example, pudus reproduce faster than large ruminants, but slower than non-miniaturised ruminants of the same body mass. Pudu puda gestates for 40 days longer than the like-size bovid Raphicerus campestris (https://en.wikipedia.org/wiki/Steenbok) of southern Africa, the average periods being respectively 210 days and 170 days.

This functional retardation arises because the southern pudu is a miniaturised deer, but the steenbok is not a miniaturised gazelle, just a small form of antilopin bovid about the same size as the probable ancestor of gazelles.

Just as the odocoileine deer of North America have been miniaturised in some of their descendants in South America, so the genus Canis has been miniaturised to varying degrees in the closely-related genus Lycalopex.

The result: the Cape fox reproduces fast enough (gestation 51-52 days, litter size 3-5) to survive intense predation by a diverse community of larger carnivores in southern Africa, but the chilla probably could not keep up under similar pressure (gestation at least 60 days, litter size 2-4).

The 'miniaturised coyote' is instead adapted to a 'quasi-insular' geographical setting owing to the tapering of the South American landmass towards Cape Horn, and the isolation of its range west of the Cordillera. The pressures exerted on it by the puma (Puma concolor) were far less than those exerted on the Cape fox by lion, leopard, cheetah, African hunting dog, two species of hyenas, caracal and black-backed jackal.

The following show how different the body proportions are in chilla vs Cape fox. The African form has whippet-like legs and noticeably large eyes, indicating speed and vigilance. Also note that the chilla retains the faint horizontal banding on chest and front-of-neck (https://www.inaturalist.org/observations/52836928), typical of the coyote (Canis latrans) and its congeners.


Cape fox

Posted by milewski milewski, August 20, 2021 23:14


The following shows yet another carnivore with which the Cape fox has to contend: https://www.africansafarico.com/blog-post/cape-fox-takes-on-honey-badger/.

Posted by milewski 9 months ago (Flag)

You are aware that you can see species on iNaturalist in picture form?: so
Cape Fox: https://www.inaturalist.org/taxa/42055-Vulpes-chama/browse_photos
Chilla: https://www.inaturalist.org/taxa/332338-Lycalopex-griseus/browse_photos

Curiously, both appear to end up more often than not as Flat Eared Boxes.

Posted by tonyrebelo 9 months ago (Flag)

@tonyrebelo Many thanks again for the guidance.

Posted by milewski 9 months ago (Flag)

The chilla features a dark patch on the outer surface of the upper hindleg, just above the tarsal joint. In this aspect of colouration in particular, the Cape fox is closely convergent, because it features the same dark patch. Compare chilla https://www.inaturalist.org/observations/44748722 with Cape fox https://www.inaturalist.org/observations/18880633.

Posted by milewski 9 months ago (Flag)

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