#khe130

After a recent mammal Monday, I was informed that pikas have a single psuedocloacal opening for reproduction and waste compared, which is very unique. Most other mammals that typically have at least two (urogenital for reproduction and urine, and an anus for feces). I was also asked whether this trait is shared by hares. This seems like it should have been a reasonably easy answer to determine (what did early naturalists like better than cutting up an animal and describing it???) but proved surprisingly challenging actually, with a lot of misinformation out there (looking at you the University of Edinburgh…). So please enjoy this overly long explanation with too many digs at the University of Edinburgh!

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Screenshot from University of Edinburgh page on Lagomorphs with a list of “special adaptations to help obtain nutrients from a diet comprising large quantities of grasses.” The last bullet point is the relevant highlighted text “lagomorphs have only one opening – the cloaca – for both the anus and the urnio genital tract.”

First to make sure we’re all on the same page, what is a pika? What do they have to do with Hares?

Pikas are a small alpine lagomorph. Lagomorphs are the order of mammals that contain pikas, rabbits, and hares. None of these animals are rodents, though lagomorphs are closely related to rodents and together lagomorphs and rodents form the grandorder: Glires.

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Diagram of a taxonomic tree showing three orders of mammals: Scandentia (treeshrews), Lagomorpha (rabbits, hares, pikas), and Rodentia (rodents). Also shows that Lagomorpha and Rodentia are sister taxa forming the grandorder: Glires.

Many probably learned the basic taxonomic levels in biology:

Kingdom (ex. Animalia)

Phylum (ex. Chordates or things with a notochord/precursor to spinal cords but basically vertebrates plus a few other things)

Class (ex. mammalia or mammals)

Order (ex. Lagomorpha)

Family (ex. Leporidae or rabbits and hares)

Genus (ex. Lepus or “true hares”)

Species (ex. L. californicus or Black-tailed jackrabbit)

But there are lots of extra levels like infraclasses, grandorders, and mirorders as scientists determine the more nuanced relationships between species (though classifications often bounce around and scientists argue about what goes where so make sure your taxonomy sources are up to date... yes... I typically use Wikipedia... sue me, but recommend birds of the world for bird species).

Second, what’s a cloaca and why is that interesting in a mammal?

Many vertebrates have one opening that serves for reproductive, digestive, and urinary tracts called a cloaca. This opening is called a cloaca and all amphibians, reptiles, birds, and cartilaginous fish have one. Notably most mammals do not have one, except for a few basal mammals (like monotremes: platypus and echidnas). So, it is at the very least wacky and fun that pikas have one (sort of…).

It’s important to note that they don’t have a true cloaca. There’s one external hole but this quickly divides into the urogenital opening (reproductive and urinary tracts) and anus (digestive tract) a couple millimeters inside the opening; so instead of a true cloaca this is typically referred to as a psuedocloacal opening in articles (except for the University of Edinburgh apparently…sorry, the University of Edinburgh has deeply disappointed me).

Description from the Mammalian Species Pages on the American Pika (Ochotona princeps)

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Males do not possess a scrotum or a baculum; females do not possess a vulva (Duke, 1951). In both sexes, the rectal canal and urogenital canal are separated by a septum terminating 2 or 3 mm from the single "pseudocloacal" oepning, which must be everted to expose the penis (long and pointed) or the clitoris (broad, with a shallow groove; Duke, 1951; Krear, 1965). Females have three... (next line cut off)

So, what about leporids (rabbits & hares)???

Well, domestic/European rabbits are very well documented to have two openings (urogenital and anus), but despite this there are a few websites out there that report that ALL lagomorphs have cloacas (see above... and a final FIGHT ME University of Edinburgh). This cannot be true since we know domestic rabbits do not, but what about other rabbits and what about hares?

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Image of urogenital opening and anus of a domestic rabbit from Medirabbit labels added by myself. The urogenital opening & expressed penis, anus, and two anal glands on either side are labeled with red arrows and text.

Well it was difficult to find sources that directly said that leporids in general DO NOT have pseudocloacal openings. However, I did find sources that didn’t mention that they did directly after stating that pikas did, like this article on lagomorphs as a model species. So I would assume that the lack of indication that leporids have one, implies to me that they DO NOT HAVE ONE.

Description from "Lagomorpha as a Model Morphological System"

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[Text in image starts with irrelevant citation] Rabbits and hares have a short tail and lack or have minimal sexual dimorphism (e.g., Chapman and Ceballos, 1990; Fa and Bell, 1990; though see Orr, 1940 on sexual dimorphism in Sylvilagus); pikas lack an external tail and both sexes have a cloaca-like structure and lack sexual dimorphism (Graham, 2015). Most leporid males have testes located in a scrotum in front of the penis and females possess two to five pairs of mammary glands. Embryonically, the mammary glands develop from the mammary line (ridge), as is typical... (line is cut off)

But… we can also look at some dead animals!

That's what you're here for right?

So, I did a second Black-tailed Jackrabbit (which is a species of hare... because common names are stupid) this week and can confirm that she had two openings (a urogenital opening and an anus, not a psuedocloacal opening. Here’s a photo of the female jackrabbit’s multiple openings.

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Image of urogenital opening and anus of a female Black-tailed Jackrabbit. The urogenital opening, anus, and two anal glands on either side are labeled with red arrows and text.

Here’s a comparison image of the urogenital and anus openings on a male pygmy rabbit (Sylvilagus idahoensis, same genus as Cottontails).

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Image of urogenital opening and anus of a male Pygmy Rabbit. The urogenital opening, anus, and two anal glands on either side are labeled with red arrows and text.

Can we be sure that all leporids (rabbits & hares) don’t have a psuedocloacal opening?

(Warning: some speculation)

Well no… but we can probably assume that they don’t because parsimony (I am not a taxonomist and this is based on my understanding of what makes the most sense from college biodiversity courses and discussions with my mother the mink mammalogist). Parsimony is the concept that we can logically decide how species are related to each other by looking at the number of differences between groups, “the optimal tree will minimize the amount of homoplasy (i.e., convergent evolution, parallel evolution, and evolutionary reversals)” (Wikipedia). So in other words while these things happen (octopuses and vertebrates convergently evolved camera eyes; and many birds lost flight and reptiles lost limbs independently of each other), if there’s no convincing reason that two groups evolved something independently of each other, it’s probably best to assume they share a common ancestor that had this trait.

In this situation, the common ancestor of pikas (ochotonids) and rabbits/hares (leporids) had openings similar to rabbits/hares or maybe more similar to rodents, but pikas independently developed a pseudocloacal opening (this might be related to them being alpine and wanting to minimize heat loss?) while leporids didn’t. It would be less parsimonious to assume that the common lagomorph ancestor developed a psuedocloacal opening and then leporids (or some leporids) re-evolved two openings. So I would assume that all leporids have two openings until someone demonstrates otherwise (let me know if you dissect some cool bunnies!).

Side note: how different are rabbits and hares?

I also want to point out that while we say lagomorphs are pikas, rabbits, and hares, these are not three equally distinct groups. Pikas are their own family but hares are just part of the family leporids with rabbits. All leporids used to be classified in the genus Lepus with today’s “true hares” but rabbits have been split up more as biologists have better understood the nuanced relationships between different species. Today there are about 70-80 species of leporid across 11 genera: 1 genus is Lepus or hares, the other 10 genera are all considered “rabbits.” But these are not two distinct groups!!! Hares are nested in the middle of the leporid tree.

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Taxonomic tree of Lagomorphs from Wikipedia. Added by myself are a box around the members of the family leporids, showing the distinction between them and pikas. Also added highlighting the hare genus Lepus to show how it's in the middle of the family rather than sister to all "rabbits."

So even though there’s a lot of sites out there that talk about the differences between “rabbits” and “hares” and treat them like too distinct groups, most of these sites are comparing between hares and well-studied rabbit species like Cottontails in the United States and European Rabbits (Why do you think this is? I'm guessing something to do with colonialism and the domination of western voices in science... but that's kind of outside of the scope of this post), so there’s a lot of REALLY different rabbits out there that are more poorly studied. Look at these cuties!

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Four "rabbit" species [clockwise from top left] Volcano Rabbit (Romerolagus diazi), Sumatran Striped Rabbit (Nesolagus netscheri), Jameson’s Red Rock Hare (Pronolagus randensis, and not actually a true hare), and Amani Rabbit (Pentalagus furnessi)

So, I’d be willing to bet that a lot of these species have very different adaptations to better understood “rabbits.” Maybe I’ll go on a side quest to become a leporid expert… but anyways, this is just to point out that hares aren’t THAT special and distinct! They are special and cool but it would be very strange if they somehow had psuedocloacal openings and other leporids didn’t because to the best of our knowledge, they are smack dab in the middle of leporids.