Why has there never been a donkey-size jackrabbit? part 2

continuing from https://www.inaturalist.org/journal/milewski/64019-why-has-there-never-been-a-donkey-size-jackrabbit-part-1#


Hares have a postural and locomotory syndrome in which:

The more massive the body, the harder it is to keep the limbs bent, let alone maintain a crouched posture. Large-bodied ungulates stand with the limbs minimally bent, as well as having necks long enough to avoid resorting to bipedal standing, which for normally quadrupedal animals becomes less and less efficient the larger the body size.

Hares have a short neck (http://museu.ms/collection/object/253558/lepus-europaeus-pallas-1778 and https://www.facebook.com/522456097810699/photos/same-species-different-ages-nope-on-the-left-you-see-a-lynx-skeleton-and-on-the-/1105375776185392/ and https://www.alamy.com/stock-photo-brown-hare-lepus-europaeus-stretching-warwickshire-136611136.html and https://www.alamy.com/brown-hare-stretching-in-a-frozen-meadow-in-winter-gb-image151883820.html and https://www.alamy.com/brown-hare-lepus-europaeus-stretching-after-resting-in-form-in-grass-image2983291.html and https://www.alamy.com/stock-photo-european-hare-lepus-europaeus-adult-stretching-in-grass-field-suffolk-49033927.html.).

The neck of even the lankiest hare, Lepus alleni (body mass about 4 kg, https://www.inaturalist.org/observations/7840174 and https://www.alamy.com/stock-photo-antelope-jackrabbit-lepus-alleni-oracle-pinal-county-arizona-united-13554693.html), remains shorter than that of like-size bambis (https://www.freepik.com/premium-photo/small-antelope-dik-dik-walking-african-savannah_8659869.htm and https://en.wikipedia.org/wiki/G%C3%BCnther%27s_dik-dik#/media/File:Guenther's_Dik-dik_(Madoqua_guentheri_smithii)_(7662529270).jpg and https://www.alamy.com/guenthers-dik-dik-madoqua-guentheri-male-walking-in-dry-grass-samburu-national-reserve-kenya-image462234785.html and https://www.sciencephoto.com/media/873025/view).

In compensation for a short neck, hares are able to

  • forage on small plants by means of crouching, and
  • maintain vigilance by means of bipedal standing.

The bipedal postures used by hares for vigilance are:

Neither of the above bipedal postures are used for vigilance in bambis or any other any ungulates. This is because ruminants

The postural specialisation of hares may thus preclude much increase in body size.


Hares have soft soles, but can run as speedily - and with as much endurance - as any animal of their body size.

Hares are unique among cursorial (https://en.wikipedia.org/wiki/Cursorial) mammals in possessing soles that are fully furred (https://www.alamy.com/hare-stretching-itself-and-preparing-to-run-away-image363826064.html and https://www.gettyimages.com.au/detail/photo/european-brown-hare-royalty-free-image/169837141?adppopup=true and https://www.agefotostock.com/age/en/details-photo/european-hare-lepus-europaeus-running-away-back-side/BWI-BLW028037 and https://www.gettyimages.com.au/detail/news-photo/hunting-bird-chases-and-catches-its-prey-a-hare-during-a-news-photo/106369083?adppopup=true and http://www.arthurgrosset.com/mammals/photos/lepeur13951.jpg and https://naturallycuriouswithmaryholland.wordpress.com/2015/02/27/snowshoe-hare-hind-feet/).

When galloping at full speed, hares maintain traction by means of the four claws on each hind foot, which are not enlarged or otherwise specialised. All known mammals achieving a combination of speed and endurance, at body masses exceeding 50 kg, have modified claws - the most extreme of which are hooves.

The locomotory specialisation of hares may thus preclude much increase in body size.

This conceptual framework allows comparison of hares with ungulates in a fairly categorical way. In the case of rodents, comparisons are complicated because this order shares several of the features, described above, of lagomorphs.

For example, various rodents

Why have the constraints, referred to above, not limited all herbivorous rodents in increasing their body size?

The key lies in explaining why, although certain large-bodied herbivorous rodents have evolved, none has rivalled the ecological versatility of ungulates or the intercontinental success of hares.

It is noteworthy that:

Given that hares overlap with ungulates in body size, and are similar in various ways to rodents, why have they evolved in the first place? What is the overall niche of hares, and how have they proven competitively superior to bambis and the rodents most resembling hares?

Instead of evolving in directions that would lead to competition with large-bodied herbivores, hares have capitalised on ungulates by fitting into the interstices of the herbivorous guild (https://en.wikipedia.org/wiki/Guild_(ecology)). Hares generally depend on large-bodied mammals to keep vegetation open and to create patches temporarily overutilised to the degree that the quantity of food available is small.

Partly because hares have the most efficient digestion known in the category of hindgut-fermentation, they rival the reproductive rates of the most fecund rodents while exceeding all rodents in cursoriality. And such fecundity and fleeing is necessary for the survival of hares, because they are subject to the various predators supported by all the ungulates and rodents sharing the same guild.

In conclusion, hares may be better-adapted for certain niches than are either ungulates or rodents. However, their syndrome of specialisations in digestion, vigilance, posture, and locomotion is likely to lose its competitive edge beyond the range in body masses currently seen in hares, viz. 1-6 kg.

And another way of summarising these findings is that perhaps the overarching specialisation for lagomorphs is a specialisation in body size itself.

Posted by milewski milewski, April 24, 2022 09:06


Posted by milewski about 1 month ago (Flag)

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