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Today there is an article in The Guardian that states

Three bison were released in Kent in July but, unknown to the rangers, one had a secret passenger on board. Bison conceal their pregnancies to prevent predators targeting pregnant animals or their offspring.

I got interested in this and tried to find a source that explains how a bison would conceal pregnancy - but I couldn't find any.

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    I don't think there is any actual concealing needed. A pregnant bison doesn't look very different from a slightly more well fed one. So a typical human can't really see the difference. Could you distinguish a pregnant cow from an unpregnant one?
    – quarague
    Commented Oct 21, 2022 at 7:28
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    @quarague I can't, but a ranger (or in case of a cow, a farmer) should be much better at this than I am…
    – gerrit
    Commented Oct 21, 2022 at 7:46
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    @gerrit: That intrigued me. I am not particularly vouching for this source but it says "It can be difficult to spot a pregnant cow among your herd", and suggests the "bump" shows up late - and includes some subtle tell-tales (and, of course, medical tests).
    – Oddthinking ♦
    Commented Oct 21, 2022 at 8:31
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    @quarague, vets detect pregnant cows (cattle) by hand, while wearing a rubber glove that goes all the way up to the shoulders. And people with short arms become pet vets, not farm vets.
    – Ray Butterworth
    Commented Oct 21, 2022 at 13:27
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    "Conceal" implies intent, anthropomorphizing the animals. I guess an objective version of the claim might be simply that humans and/or predators have difficulty determining whether a bison is pregnant.
    – Nate Eldredge
    Commented Oct 22, 2022 at 3:07

1 Answer 1

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The article is about wild bison in the UK - so the European bison (Bison bonasus). It is also known as Żubr in Polish, which comes up later.

I found this question was disputed in an article from 1958 - but gives hints as to where the claim may have originated from.

On p7, Jaczewski writes:

Pregnancy is said to last 9 months (Brincken, 1929; Karcov, 1903; Wróblewski, 1927). The condition of a gravid cow can be recognized by its external appearance (Wróblewski, 1927)2.

So the 1958 author says you can tell that a European Bison is pregnant ("gravid"), and justifies that with an even earlier reference:

Unfortunately, I don't have access to this book, so I don't know what evidence they have to support this claim. [I see this as a weakness to this answer.]

But wait... the original quote from the 1958 source has a footnote:

Karcov claims that a gravid cow cannot be recognized by her appearance, which, however, is not correct.

Oooh! Controversy!

That source is:

  • Karcov G (1903) Belovezhskaya Pushcha. Ee istoricheskii ocherk, sovremennoe okhotniche khozaistvo i vysochaishe okhoty v Pushche [Białowieża Forest. Its historical description, contemporary game management and monarchical hunts in the forest]. A. Marks, St. Petersburg, 414 (in Russian)

Alas, I haven't found the text for this either.

To summarise:

  • 1903: Forest Game management expert says you can't tell if a cow is pregnant.
  • 1927: Żubr expert says you can tell.
  • 1958: Another Żubr expert says (with no justification provided) that the first one is wrong, the second one is right.

I am hoping we can find some more verifiable evidence, but this was sufficient for me to tentatively accept experts can tell, and that there is at least one source in the literature that could explains why people think you can't.

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    None of these mention when during the pregnancy they are talking about. If they are talking about the last month or the first month, the appearance might be noticeably different. But perhaps the first month doesn't look much different from the second-last.
    – Ray Butterworth
    Commented Oct 22, 2022 at 13:56
  • @RayButterworth on top of that, I reckon almost anyone can recognize a bison pregnancy in the last hour or so.
    – phoog
    Commented Oct 24, 2022 at 0:31

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