A test of evolutionary convergence: bush duiker vs gray brocket, part 1

The principle of biological adaptation is that organisms are shaped by selective pressures. Given similar environments and ecological niches, unrelated animals on different continents should, theoretically, come to look similar.

This idea can be tested by comparing two species of small ruminants, the bush duiker (Sylvicapra grimmia) in Africa and the gray brocket (Mazama gouazoubira) in South America. These are ecological counterparts: both have body mass about 15 kg, tend to hide from view, are widespread in various forms of savanna, and eat mainly greens and fruits.

Comparing the appearance of these two species is particularly interesting because they belong to different families: Bovidae vs Cervidae. And one of the immediately noticeable convergences is that in the gray brocket the antlers are so modified to short spikes that they resemble horns.

However, in other respects the differences remain more noteworthy than the similarities. The African species looks more specialised than the South American species in various ways.

The following photo-pair shows that in the bush duiker the eyes are larger and placed more laterally on the head than is the case for the gray brocket: bush duiker https://www.robertharding.com/preview/764-6047/common-duiker-grey-duiker-bushduiker-sylvicapragrimmia-kruger-national/ vs gray brocket https://www.inaturalist.org/observations/91193009. This suggests that the predatory pressures are greater, and/or that the habitat is more open, in African than in South American savannas.

Compared to the African species, the South American species looks primitive and undifferentiated, similar to how I imagine an ancestral odocoileine to have looked: bush duiker https://animalia.us-east-1.linodeobjects.com/animals/photos/full/original/common-duiker-sylvicapra-grimmia-just-before-the-light-was-gone.jpg vs gray brocket
https://www.inaturalist.org/observations/61562922 and https://www.inaturalist.org/observations/67207093 and https://www.inaturalist.org/observations/71846673.

The muzzles and ears of the two species have different shapes: bush duiker https://www.euronews.com/green/2021/04/07/first-ever-images-of-mysterious-walter-s-duiker-captured-in-the-wild vs gray brocket https://www.inaturalist.org/observations/90656839 and https://www.inaturalist.org/observations/73497350.

Both species have a bare, moist rhinarium, but the form of the nose and mouth seems more specialised in the African species. There is a long preorbital gland in the African species, whereas there is no obvious facial gland in the South American species: bush duiker https://en.wikipedia.org/wiki/Common_duiker#/media/File:Common_Duiker_(Sylvicapra_grimmia)_(6011670419).jpg vs gray brocket https://www.inaturalist.org/observations/83213332.

The African species possesses a pedal flag by virtue of the dark/pale contrast on the feet. There is no such feature in the South American species: bush duiker http://janesjaunts.com/wp-content/uploads/2016/10/Grey-duiker.jpg vs gray brocket https://www.inaturalist.org/observations/9072964.

Both species have small tails with only modest potential for flagging. However, the tails are hardly convergent. The African species flags the dark/pale contrast on the narrow tail by flicking it intermittently while walking (https://www.inaturalist.org/observations/91165836), but does not flag the tail while fleeing. The South American species shows the white undersurface of the tail to some extent, but this lacks dark/pale contrast and any flagging of the tail while fleeing is limited by the shortness of the tail: bush duiker https://mldlect2rr41.i.optimole.com/vNyw-1Y-XKT679hT/w:auto/h:auto/q:auto/https://wildmoz.com/wp-content/uploads/2013/11/Duiker-Ewe.jpg and https://www.artranked.com/topic/Duiker#&gid=1&pid=22 vs gray brocket https://www.inaturalist.org/observations/72941220 and https://www.inaturalist.org/observations/48054069 and https://www.inaturalist.org/observations/7036758.

The following provide additional illustrations of the points made above: bush duiker https://www.destinationuganda.com/travel-guide/mammals/gray-duiker/ vs gray brocket https://www.inaturalist.org/observations/59383130 and https://www.inaturalist.org/observations/65598410.

Are these failures to converge owing to 'phylogenetic inertia', i.e. the inherent genetic limitations of being a bovid vs a cervid, or to the more intense predatory regime in Africa than in South America? In a thought-experiment in which the large carnivores in South America were as numerous and diverse as in Africa, would the gray brocket have evolved to resemble the bush duiker more closely?

To be continued...

הועלה ב-אוגוסט 16, 2021 10:53 לפנה"צ על ידי milewski