Why has there never been a donkey-size jackrabbit? part 1

The main orders of herbivorous terrestrial mammals show a remarkably wide range of body sizes. Why does this not apply to lagomorphs?

Ruminants and herbivorous rodents range from:

These represent a 500-fold and a 5,000-fold range in body mass respectively.

However, the order Lagomorpha (https://en.wikipedia.org/wiki/Lagomorpha and https://www.frontiersin.org/articles/10.3389/fevo.2021.636402/full#B88) does not conform. All hares, rabbits, and pikas have remained small-bodied, weighing less than 20 kilograms, even during the era of gigantism in the Pleistocene.

The largest-bodied extant lagomorphs are hares (Leporidae: Lepus, body mass up to 7 kilograms). These overlap in body size with the most diminutive of ruminants (e.g. https://en.wikipedia.org/wiki/Dik-dik).

How does the basic nature of lagomorphs help to explain this limitation on body size in a herbivorous order of mammals?

A recent attempt to answer this question (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8252017/#:~:text=We%2C%20therefore%2C%20hypothesized%20that%20the,%2C%20depending%20on%20the%20environment) is hard to understand.

Instead, I suggest that the solution lies in hares being different from ungulates and rodents in their combination of specialisations in

  • digestion,
  • vigilance,
  • posture, and
  • locomotion.

The guts, sense organs, neck, and limbs of hares are all specialised in ways suggesting a limitation on body size.


Hares oddly combine repeated digestion with once-off chewing. Would this work at body sizes larger than those of hares?


Hares oddly combine night-eyes with long ears. Would this work at body sizes larger than those of hares?


Hares oddly combine a short neck with long legs. Would this work at body sizes larger than those of hares?


Hares oddly combine soft feet with extremely rapid galloping. Would this work at body sizes larger than those of hares?

Having provided this conceptual framework, I will now expand on each of the above oddities.


Hares ferment food in their hindgut, not their foregut (see https://allfamousbirthday.com/faqs/which-is-better-foregut-or-hindgut-fermentation/ and https://www.researchgate.net/figure/The-digestive-tract-of-a-rabbit-Numerical-values-are-those-observed-in-a-25-kg-New_fig1_275519577).

Hares thus differ from ruminants, including bambis (diminutive ruminants). Instead of chewing the cud, they eat the least-fibrous portion of their feces (https://en.wikipedia.org/wiki/Cecotrope and https://www.sciencedirect.com/science/article/abs/pii/0301622681900543).

This means that hares have to take the time to chew their food thoroughly in the first place as they forage. By contrast, ruminants can swallow food hastily and then retire to a safe place to chew thoroughly.

Herbivorous mammals have four main options for thorough digestion - which requires fermentation so that microbes can release metabolisable energy from otherwise indigestible fibre.

These are:

Cud-chewing (as in bambis) and eating of feces (as in hares) both achieve thorough digestion. However, they are mutually exclusive, i. e. cannot co-occur in the same animal.

A crucial advantage of rumination is that it combines thorough digestion in the stomach and small intestine with an ability to fill the fermentation-chamber initially with hastily-swallowed food. This is important because the chewing of fibrous food is noisy enough to interfere with auditory vigilance.

Foregut-fermentation with cud-chewing therefore allows ruminants to

  • minimise the time spent foraging, while
  • maximising the time spent lying vigilant while chewing the food thoroughly after retiring to a position of relative safety.

Furthermore: as body size increases, the ability of the mouthparts to select the nutrient-richest, least fibrous food-items decreases. Because the large parts of plants tend to be the most fibrous, large-bodied herbivores must spend much of each 24-hour period foraging and chewing.

Few herbivorous eutherian mammals with body masses 10-50 kg have - as far as we know - ever existed In the last of the four categories above (viz. hindgut-fermentation without systematic eating of feces). This suggests an evolutionary barrier to the increase in body size by hares or other lagomorphs, even if they abandoned the caecotrophy that is characteristic of lagomorphs.

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


Hares lack the visual system - including a long neck - of ungulates, that allows wide scanning by both day and night. Instead, they combine eyes specialised for darkness with specialised hearing, as indicated by the length of the ear pinnae.

Hares are, as far as I know, the only animals combining vertically-oriented pupils with eyes placed so laterally (https://www.inaturalist.org/observations/67587308 and https://pixels.com/featured/big-eyes-brown-hare-lepus-europaeus-wonderfulearth.html and https://www.ephotozine.com/photo/back-view-of-hare-36048706 and https://commons.wikimedia.org/wiki/File:Old_hare_copy.jpg and https://www.alamy.com/stock-photo-brown-hare-lepus-capensis-rear-view-of-single-adult-sitting-in-field-139323674.html) that the field of binocular vision (https://en.wikipedia.org/wiki/Binocular_vision) is minimal.

This may be partly because hares are more strictly nocturnal than bambis. All hares (except for Lepus arcticus in the brief Arctic summer) are active mainly by night.

It is often assumed that bambis are also nocturnal. However, several species are actually active mainly by day (e.g. see https://en.wikipedia.org/wiki/Blue_duiker and https://en.wikipedia.org/wiki/G%C3%BCnther%27s_dik-dik). Furthermore, species venturing into the open (e.g. https://en.wikipedia.org/wiki/Royal_antelope) only by night may nonetheless be active by day under cover of shrubs and trees.

The shape of the pupils in hares is shown in: https://twitter.com/GarrityPete/status/1131838112060104704/photo/4 and https://www.gettyimages.com.au/detail/photo/african-savanna-hare-in-masai-mara-national-reserve-royalty-free-image/520578266?adppopup=true and https://www.alamy.com/stock-photo-single-adult-brown-hare-portrait-close-up-of-eyes-and-face-81611479.html and https://www.gettyimages.com.au/detail/photo/head-shot-of-a-beautiful-brown-hare-lepus-europaeus-royalty-free-image/1193234080?adppopup=true and https://www.agefotostock.com/age/en/details-photo/brown-hare-european-hare-feldhase-lepus-europaeus-lying-resting-in-meadow-relaxed-very-detailed-close-up-wildlife-europe/VX1-3358923 and http://wildlifephotographic.blogspot.com/2014/12/just-hare-neccesities.html and https://www.gettyimages.com.au/detail/photo/scrub-hare-kasane-moremi-game-reserve-botswana-royalty-free-image/999492980?adppopup=true).

The above contrasts with ruminants, which have horizontally-oriented pupils (https://www.gettyimages.com.au/detail/photo/close-up-of-deer-looking-away-royalty-free-image/1083779544?adppopup=true and https://es.123rf.com/photo_107041593_close-up-of-the-eye-of-a-red-deer-cervus-elaphus-.html and https://www.quora.com/Do-deer-and-antelopes-have-different-vision-during-nighttime) and retinas.

The long ear pinnae of hares can be interpreted partly as compensating for the limitations on visual vigilance by day, and the noise of chewing while exposed to danger during foraging by night.

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

to be continued...
(see https://www.inaturalist.org/journal/milewski/64506-why-has-there-never-been-a-donkey-size-jackrabbit-part-2#)

Posted on April 15, 2022 01:22 AM by milewski milewski


This video shows the posture, locomotion, and position of the tail in Lepus europaeus during sexual activity: https://www.youtube.com/watch?v=OYNGPnbgKzI

Posted by milewski over 2 years ago

Hares differ from bambis (diminutive ruminants) in that the tail is hidden in an 'erect', not lax, position. This is because of the squatting and crouching postures used habitually by hares. When ruminants flee, it is common for the tail to be erected. When hares flee, the tail is usually kept more or less horizontal, including in those species possessing dark-pale contrast on the tail that constitutes a caudal flag. This video (https://www.youtube.com/watch?v=CZOzdd3HUYY) shows how the tail is 'erect' in the normal crouch but lowered when the rump begins to be raised, in the normal bound-walk used while foraging. Compare this with the comparably black-and-white tail in hare-size Philantomba caerulea: https://www.youtube.com/watch?v=LmAAmbI5qBY and https://www.youtube.com/watch?v=PCxdcTsXsJo.

Another corollary aspect of the postural peculiarities in hares is in the mammary anatomy. Instead of an inguinal udder, hares have teats spread out along the abdomen, with no pendulous engorgement during lactation but instead a single, elongated mammary gland on each side (left and right), served by small, non-projecting nipples. Juveniles can suckle while the mother sits upright, in a way never seen in ungulates.

Posted by milewski about 2 years ago

The ecological distribution of hares was different during the Pleistocene, but there is no evidence that any extinct species of Lepus was any larger-bodies than the species remaining today:

Posted by milewski about 2 years ago

A short neck is the only obvious aberration in the skeleton of hares, at least relative to felids (https://www.facebook.com/522456097810699/photos/same-species-different-ages-nope-on-the-left-you-see-a-lynx-skeleton-and-on-the-/1105375776185392/).

Posted by milewski about 2 years ago
Posted by milewski about 2 years ago

The following shows that the spine flexes so much in Lepus europaeus that the fore feet juxtapose the tail in full stride during galloping: https://www.gettyimages.com.au/detail/news-photo/greyhound-races-after-a-hare-at-the-annual-waterloo-cup-news-photo/3010162?adppopup=true.

Posted by milewski about 2 years ago

Very good point about how chewing interferes with auditory vigilance!

Coprophagy probably allows hares to retire to seclusion for that secondary digestive process, so it may have some value in protecting them from the eyes of predators.

Very interesting things to think about here.

Posted by beartracker about 2 years ago


Could I please run the following by you to see if it makes sense?

It is true that eating of caecotropes, like the chewing of cud, can be done in safety. However, a difference is that lagomorphs, when eating their faces, do not re-chew them. Instead, the sorting mechanism in the caecum/colon ensures that the soft, nutrient-rich fecal pellets eaten are already composed of fine particles = paste. What this means is that although both hares and ruminants re-ingest food in safety, only the ruminants spend most of their (noisy) chewing time in safety.

Expressing this difference between hare and ruminant relative to digestive efficiency:

Imagine a single leaf eaten.

In the case of the hare, this leaf is chewed in only one bout, fairly thoroughly. The larger fragments, i.e. the fragments that failed to be ground into paste, are defecated as the normal feces one finds in the field. The fraction consisting of paste gets side-tracked to the caecum, and is then separately defecated and immediately eaten. Let is imagine this as 10 chews per leaf, leading to about 75% of the leaf being digested and absorbed and 25% of the same leaf being eventually (via two different pathways) defecated.

The crucial ratio is 10 chews for 75% digestive efficiency (= 'digestibility').

In the case of the ruminant, the leaf is chewed repeatedly, first while foraging (say 2 chews) and then as cud (say 20 chews cumulatively, because parts of the leaf will be brought up in cud again and again). Again, 75% is digested and absorbed and 25% defecated, in this case all as one kind of fecal pellet.

Here we have a total of 22 chews, as opposed to the 10 of the hare, for the same digestibility.

The hare is more efficient than the ruminant in terms of the ratio of chews to digestibility. But the ruminant is safer than the hare relative to the same digestibility.

The important point, for vigilance, is that the number of chews needed during risky foraging is 10 in the hare vs 2 in the ruminant.

If it were the case that the hare defecated coarse particles and paste all together, and chewed its feces in the same way that ruminants chew the cud, then there would be a reduction of the risk brought by noisy chewing. But because caecotrophy is more about separation of coarse particles from paste than about reducing coarse particles to paste, the hare gets no benefit in terms of safety and has, overall, a net disadvantage in that it has to risk 8 extra chews per leaf in the exposed activity of foraging.

The hare gets away with this because, at its body size, it does not need to spend many hours foraging. However, the larger-bodied it were to become, the more the risk would tend to outweigh the benefits of its digestive mode. Meanwhile, the risk-minimisation of the ruminant mode applies more or less regardless of body size.

Equids do survive by chewing all their food while foraging. However, caecotrophy is impracticable for them because it would require even more noisy chewing, in order to produce any paste worth separating and re-digesting. So equids have abandoned the eating of feces and have compromised their digestive efficiency, achieving digestibilities of only about 50%, if memory serves. And even so they are less successful than like-size ruminants.

Your thoughts?

Posted by milewski about 2 years ago

OK, that clarification makes sense. I see how this would be a limitation as the body size of the animal gets larger. The hare gets the advantage of needing less food intake, so it has an advantage due to its smaller size. The larger equids rely on their size to be somewhat protective of predators since only the largest would be a threat to them in most cases. Deer being in between in size have a higher risk and need that ability to retire to a quiet place to chew the cud. Some very interesting thoughts here and I need to ponder this some more.

Posted by beartracker about 2 years ago

@beartracker I apologise, it looks like you were right and I was wrong. In https://books.google.com.au/books/about/Grzimek_s_Encyclopedia_of_Mammals.html?id=A7cRAQAAMAAJ&redir_esc=y, , vol. 4, page 247, Eberhard Schneider describes a difference between Oryctolagus cuniculus and Lepus europaeus by stating"...the mucus-coated 'stomach pills' that are found in the stomach of the rabbit...during the day. These stomach pills originate in the contents of the appendix; they are clearly smaller than the firm fecal balls that are so characteristic of hares and rabbits. The rabbit swallows these soft balls without chewing them. In the case of the European brown hare, on the other hand, the cecotrope is not formed in the anus. It is ingested in portions as a semi-solid chyme, then swallowed after being clearly chewed."

Posted by milewski about 2 years ago

Interesting. Thank you.

Posted by beartracker about 2 years ago


Here is a reference to the caecotrophic differences between Oryctolagus cuniculus and Lepus europaeus: https://core.ac.uk/download/pdf/29287121.pdf.

The following may also be worth looking at: https://zslpublications.onlinelibrary.wiley.com/doi/10.1111/j.1469-7998.1983.tb05107.x.

Posted by milewski about 2 years ago

Thank you for those links. I downloaded the articles to read later.

Posted by beartracker about 2 years ago

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