Why has no deer ever weighed more than a tonne?

The largest extinct deer (Cervalces latifrons, https://prehistoric-fauna.com/Cervalces-latifrons and https://en.wikipedia.org/wiki/Cervalces_latifrons) approached one tonne of body mass.

This is only about half of the maximum body mass of the largest extinct bovids, namely species of Bison (https://en.wikipedia.org/wiki/Bison_latifrons) and Syncerus (https://en.wikipedia.org/wiki/Pelorovis and https://en.wikipedia.org/wiki/Syncerus_antiquus), and also considerably less than that of extinct species of Bos (https://www.researchgate.net/profile/Jussi-Eronen/publication/308079231_Patterns_of_diet_and_body_mass_of_large_ungulates_from_the_Pleistocene_of_Western_Europe_and_their_relation_to_vegetation/links/57d95c2808ae601b39b152d6/Patterns-of-diet-and-body-mass-of-large-ungulates-from-the-Pleistocene-of-Western-Europe-and-their-relation-to-vegetation.pdf and https://en.wikipedia.org/wiki/Aurochs#Size).

In the living fauna likewise, the largest deer is less massive than its bovid counterparts. The moose (https://en.wikipedia.org/wiki/Moose) weighs less than the wild water buffalo (https://en.wikipedia.org/wiki/Wild_water_buffalo) or the gaur (https://en.wikipedia.org/wiki/Gaur).

Cervalces, the only genus of deer which has ever approached a tonne in body mass, was associated with glacial climates. By contrast the four genera of bovids which have surpassed -or even doubled - this value lived in the tropics (Syncerus, Bos, Bubalus) as well as the cold North (Bison, Bos).

So deer fall well short of bovids in both maximum body mass (one tonne vs two tonnes) and the number of genera which have surpassed one tonne (four vs zero).

How can this shortfall be explained?

Deer and bovids have different life-strategies: shedders vs amassers. Although this generalisation is complicated and disguised by many other factors in the various genera and species, it becomes more obvious the greater the body size.

Whereas males of large species of deer tend to stop growing their bodies and then concentrate their further growth in sheddable antlers, bovids tend to continue to amass bone and brawn for years after sexual maturity (e.g. see https://www.researchgate.net/figure/Growth-of-body-mass-kg-of-a-Rangifer-tarandus-reindeer-caribou-and-b-Ovibos_fig1_253519742).

The largest of living deer conforms to this pattern. Both males and females of the moose continually lose much of what they produce, in males via antlers and in females via milk.

During their lifetimes, males of the moose - which grow a new pair of antlers each summer - may shed about as much bone mass as they retain in their skeleton.

Females of the moose grow no antlers, but instead differ from large bovids in being able to bear twins regularly (see https://www.inaturalist.org/observations/90229385 and http://www.adfg.alaska.gov/static/education/educators/curricula/alaskawildlifecurriculum/pdfs/how_fast_can_a_moose_population_grow.pdf). Although there is a correspondingly greater death-rate of juveniles, it seems likely that the amount of milk wasted per lifetime exceeds that in bovids, relative to maternal body mass. Because the nutrient tally in milk matches that in antlers, females may in a sense 'shed' a greater proportion of their skeletons over their lifetime in deer than in bovids.

The shedder/amasser distinction pervades other facets of life-strategy. For example, deer tend to carry far less bulk of food in their stomachs than is the case for bovids. Partly because of this difference, the dressed carcasses of moose vs domestic bovines are about 60% vs about 50% of live weight (see https://www.dpi.nsw.gov.au/__data/assets/pdf_file/0006/103992/dressing-percentages-for-cattle.pdf and file:///C:/Users/Antoni%20Milewski/Downloads/140499-Article%20Text-374455-1-10-20160726.PDF). And what this means is that the body mass of an extinct giant bison can be discounted by several hundred kg of gut fill to be truly comparable with an extinct large deer.

It is more than coincidence that the moose is leggy and light on its feet (https://www.gettyimages.com.au/detail/photo/moose-crossing-dirt-road-royalty-free-image/544498088?adppopup=true). The associated agility allows the moose to be particularly selective of nutrient-rich, minimally fibrous foods, by selecting shoots high above ground or aquatic greens deep below water-level. Relative to this, large bovines have to forage indiscriminately - routinely accepting nutrient-poor grass - in order to keep their capacious stomachs full.

Anti-predator tactics differ along the same lines. The moose tends to choose environments cluttered with fallen trees, allowing it to hide and then to shed pursuers simply by high-stepping the obstacles at a rapid trot. By contrast, the largest bovines tend to be gregarious, forgoing attempts to hide. They are relatively slow even at a gallop and instead rely on the amassed bulk of the group to ward off attacks. Mothers of both the moose and large bovines defend their offspring. However, a light, leggy build allows females of the moose to be the most proficient of all ungulates in defensive kicking. By contrast, females of large bovines are clumsy with their horns, reluctant to kick, and cowardly as individuals.

The difference in life-strategies between deer and bovids is subtle in small members of these groups. However, here too the tendency is for the newborns of deer to be smaller than those of bovids of similar adult body mass. For example, the water deer (Hydropotes inermis, adult body mass about 10 kg) can bear up to seven infants at a time, whereas the maximum for like-size bovids is two.

The shedder/amasser distinction may reflect an even more basic evolutionary difference. Deer have opted towards dynamism, not only in the repeated shedding of their head adornments but also in the rate at which food passes through their bodies and the flux in their populations. By contrast, bovids have opted towards the inertia embodied by the beefiest bosts, buffaloes and bisons.

Posted on Αύγουστος 31, 2021 0101 ΠΜ by milewski