Mysterious microcephaly in Mazama
Everyone knows that deer belonging to the South and central American genus Mazama have defied conventional classification. Every decade or so another new species is announced. Even within the recognised species, complications keep emerging which do not fit subspeciation or polymorphism (https://www.jstor.org/stable/1381006 and https://www.researchgate.net/publication/43227142_Elucidating_the_Evolution_of_the_Red_Brocket_Deer_Mazama_americana_Complex_Artiodactyla_Cervidae and https://naturalhistory.si.edu/sites/default/files/media/file/2013-rutter-poster.pdf and https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7197991/ and https://www.mdpi.com/2073-4425/12/2/165 and https://pubmed.ncbi.nlm.nih.gov/28486644/).
Mazama is odd in combining conservative phenotypes with advanced genotypes; its generalised, primitive-looking form seems to mask certain subtle evolutionary specialisations.
A particular oddity which seems to have been overlooked is the diminution of the head, relative to the body, in those individuals of the red brocket (Mazama americana) which have achieved the full body-size of the genus.
The red brocket is the approximate ecological counterpart of bushbucks (Tragelaphus spp.) and large duikers (Cephalophus spp.) in Africa. However, there is such an obvious difference in the proportions of the figures that it seems that the red brocket - alone among deer - combines determinate growth of the head (and particularly the antlers) with indeterminate growth of the body.
The following sample of photos of large duikers gives an idea of the proportional head-size to expect in the red brocket: https://zooinstitutes.com/animals/yellow-backed-duiker-tiergarten-nurnberg-18809.html and https://www.shutterstock.com/nb/image-photo/yellow-backed-duiker-cephalophus-silvicultor-1833382282 and https://www.biolib.cz/en/image/id402377/ and https://www.alamy.com/abbotts-duiker-cephalophus-spadix-is-a-small-antelope-native-to-tanzania-specimen-in-captivity-image388009275.html. Ditto for adult males of a species of bushbuck: https://umthilodge.com/kenton-media/bushbuck-ram/ and https://www.knysnaplettherald.com/news/News/General/175466/Bushbuck-dies-despite-rescue-effort.
The following show what the proportional head-size in fully mature specimens of the red brocket is really like. Why do these heads look disproportionately small?
https://www.inaturalist.org/observations/36852319
https://alchetron.com/Red-brocket#red-brocket-b3aa4423-4668-4671-8b3d-c1e6b13baba-resize-750.jpeg
https://www.inaturalist.org/observations/70146616
https://www.inaturalist.org/observations/43199429
https://www.inaturalist.org/observations/17222248
https://www.gettyimages.com.au/detail/photo/red-brocket-deer-royalty-free-image/518869847?adppopup=true
https://www.natureinstock.com/search/preview/red-brocket-deer-mazama-americana-female-atta-lodge-iwokrama-rainforest/0_10065408.html
https://www.researchgate.net/figure/Mazama-americana-zamora-Credit-Aldo-Fernando-Sornoza-Location-Yasuni-National-Park_fig11_337409269
https://www.gettyimages.com.au/detail/news-photo/brazil-mato-grosso-pantanal-area-red-brocket-deer-news-photo/967570846?adppopup=true
https://www.flickr.com/photos/marcospaiva/14641805476
several photos within https://www.nature-myview.com/blog/red-brocket-deer
The general trend among deer is that, as body size increases, the size of the antlers increases disproportionately. Moose (Alces alces) and caribou (Rangifer tarandus) are not just 'small deer writ large'; instead there is a compounding effect in which growth of the body is multiplied in the growth of the antlers (e.g. see https://en.wikipedia.org/wiki/Irish_elk#/media/File:Irish_Elk_Side_(white_background).jpg). This can be seen between species as well as with age of the individual within a given species.
However, the allometric trend (see https://www.researchgate.net/figure/41-Allometry-of-antler-size-against-shoulder-height-in-deer-based-on-data-from_fig5_278696167 and https://www.researchgate.net/publication/227670948_Revisiting_the_allometry_of_antlers_among_deer_species_Male-male_sexual_competition_as_a_driver) fails to apply to Mazama; indeed, it seems to have been reversed. The result is that the largest specimens of the red brocket have antlers of negligible size.
Is there any mention of this small-headedness in the Spanish or Portuguese literature, which I may perchance have missed?
One possible explanation is that the red brocket exceeds any duiker or bushbuck in its adaptation for swimming, both as a way of escaping from the bush dog (Speothos venaticus) and in commuting across the wide rivers typical of the Amazon (see https://www.inaturalist.org/observations/11296409 and https://www.inaturalist.org/observations/37387825). The body is muscular, with particularly propulsive hindquarters, but the head is so light that it can easily be held above water.
It is true that the moose is one of the most amphibious of deer without being particularly small-headed (https://www.gettyimages.com.au/detail/photo/moose-crossing-dirt-road-royalty-free-image/544498088?adppopup=true). However, there are at least three differences. Firstly, the large hooves of the moose, adaptated to deep snow, double as paddles. Secondly, most of the aquatic activity of the moose is diving, to forage underwater. And thirdly, the moose can close its nostrils (e.g. see https://videohive.net/item/cow-moose-underwater/1583181 and https://mashable.com/video/moose-dives-off-cliff-swims-away and https://www.tetonscience.org/can-moose-really-dive/ and https://www.cbc.ca/wildcanadianyear/m_videos/web-series-one/moose-are-one-of-the-last-species-youd-expect-to-be-swimming-for-their-supp#:~:text=Fortunately%2C%20moose%20are%20adept%20swimmers,of%20reach%20for%20other%20species and https://www.wellbeingintlstudiesrepository.org/cgi/viewcontent.cgi?article=1005&context=feebeh).
Might it even be possible - using our wildest imagination - that the red brocket secretly forages along the edges of flooded forest (varzea) by swimming to reach floating fruits?
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Interesting. In general, I think I agree of the body shape of neotropical ungulates like Mazama, but also tapirs and peccaries, as being escape-oriented. Not so much escaping by swimming from bush dogs, jaguars et al, but for escaping by running from these predators through dense vegetation, lianas, etc without being snagged or slowed down by protruding body parts. Could that help explain a compact head in Mazama? And if that explanation made sense, are Congo forest duickers proportionally smaller-head'ed than their relatives who live in grasslands and less dense woodlands?
see Manoeuvrability hypothesis https://onlinelibrary.wiley.com/doi/10.1111/j.0030-1299.2008.16069.x
"As a crucial adaptation impacting on most aspects of a species’ biology, body size is likely to be under multiple selective pressures. This study aims to determine whether the physical structure of dense habitats selects directly for small size in order to reduce resistance to movement from the vegetation and facilitate concealment (Manoeuvrability hypothesis). This is unclear because other selective forces could indirectly cause habitat to covary with body size. Focusing on bovids, I tested the explanatory power of the Manoeuvrability hypothesis in an interspecific comparative analysis controlling for phylogenetic relatedness. Bovids in dense habitats were indeed found to be reduced in size. Confirming the Food choice hypothesis, grazers, who predominantly feed on abundant food in open habitats, were found to be larger than browsers in a multivariate analysis. However, after controlling for diet there remained a residual variation in body size which correlated negatively with habitat density. No support was found for Bergmann's hypothesis as the explanation for this remaining correlation since body size did not correlate with latitude of the species’ range. Likewise, sexual selection on male body size in open habitats due to increased polygyny was an unlikely cause since body size did not correlate with sexual body size dimorphism in multivariate analysis. Rather, the finding that tail length was disproportionately shorter in closed habitats points to dense habitat structure selecting directly for compact body conformation and small size, as predicted by the Manoeuvrability hypothesis."
@muir Many thanks for your thoughts. It does seem likely that ruminants of dense forest and thicket would keep the head adornments small enough to avoid snagging as they flee. And it is true that all duikers have small horns. However, bushbucks and the bongo (https://www.dreamstime.com/stock-images-bongo-antelope-image20033374) retain horns of considerable size. Putting aside the adornments and looking at just the size of the head, there does not seem to be much difference, in duikers or tragelaphins, between species of dense vs open vegetation.
@muir Many thanks for the excerpt re the Manoeuvrability Hypothesis. It seems sure that specialist ungulates of dense vegetation tend to be relatively small. I could point to the forest vs savannah species of African buffaloes as an obvious example. Indeed, the only continent on which any ruminant more massive than the red brocket is specialised for dense forest is Africa (particularly bongo and okapi). However, the intercontinental differences show that the red brocket is not close to a 'limiting size' in this respect. A similar comparison could be made between white-lipped peccary and giant forest hog. So I would argue that it is the extreme morphological modification of the red brocket which goes beyond the explanatory power of the Manoeuvrability Hypothesis. Your further thoughts?
@muir In other words, my rationale would be: a) the red brocket is more extremely small-headed than any other ruminant of dense vegetation, b) it seems unlikely that the vegetation through which the red brocket flees is denser than that through which okapi, bongo and the largest duikers flee (all of these being bigger than the red brocket), c) what instead sets South America apart is the extensiveness of flooded forest and a common adaptation among various clades of mammals for swimming (e.g. compare jaguar with leopard), d) the morphological extremes of the red brocket are therefore more consistent with the fluvial environment than with the vegetal environment. Does this make sense?
Maybe. I'm resistant to giving up on a manoeuvrability-related hypothesis entirely. Having walked around in both, I could believe that the "vegetation through which the red brocket flees is denser than that through which okapi, bongo and the largest duikers flee." African forests have elephants, and buffalo, and gorillas, and hippos, and more things walking through them.
I don't think this is an original thought for me. I have a vague memory of a publication I used to own, I think a 1970s/1980s conference proceedings with a type of comparative descriptive ecology that has gone out of style, and a diagram showing the convergent shape of tapirs/peccaries/paca etc. Sort of slanting front with a head that can go low, and muscular hind quarters to power and burst forward. And even African forest ungulates like bongo can come close to the same shape by positioning their head and neck so their horns lay flatter with their body, no? Regardless, fun to think about.
@muir I know the volume you're thinking of, with the drawings re evolutionarily convergent forms. I do agree that the basic shapes of forest ungulates reflect the problem of obstruction. I also agree that equatorial rainforests on different continents can be surprisingly different in height and density. By the way, do you have any thoughts on my Post (August 16, 2021) comparing the gray brocket with the bush duiker?
@muir What is your take on the video I linked above, i.e. https://mashable.com/video/moose-dives-off-cliff-swims-away. Should we take it at face value or you think there is some extenuating context? Would the moose really voluntarily dive from such a height?
Thanks for flagging that video, I didn't click through on all the links in your journal post. I've certainly never seen something like that before, but I would agree with the quoted U MN prof -- moose probably just found itself on a steep, uneven slope and couldn't quite figure how to get back up, so it had to make the risky decision to keep going down. Happens to people all the time. In terms of extenuating circumstances, I was feeling a little uncomfortable with how close the boat was to a probably stressed animal. Hope it fell on just water and not underwater rocks.
@muir Many thanks, I value your opinion.
A basic reference for the red brocket is file:///C:/Users/Antoni%20Milewski/Downloads/Neotropicalcervidology_15Mazamaamericana.pdf and https://www.researchgate.net/publication/291162163_Red_brocket_deer_Mazama_americana_Erxleben_1777/link/59b01378458515a5b484d339/download. The red brocket is widely sympatric with smaller-bodied congeners: Mazama gouazoubira in the south, replaced in the northern part of South America by Mazama nemorivaga: file:///C:/Users/Antoni%20Milewski/Downloads/Neotropicalcervidology_23.pdf and https://www.researchgate.net/figure/Captive-adult-male-of-the-Amazonian-brown-brocket-Mazama-nemorivaga-from-the-state-of_fig2_323784090. The latter species has a similar diet to that of the red brocket but tends to occur on hilltops unsuitable for the red brocket.
The adult body mass of the red brocket ranges from 12 to 65 kg, the variation usually being explained as a sign that more than one species are lumped under M. americana. However, has anyone investigated the possibility that this species has indeterminate growth (except of the head) in both females and males, which would mean that body mass at sexual maturity could be 15 kg but body mass in full maturity, say at seven years old, could be more than 40 kg?
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