We know that Tribbles are born pregnant. From The Trouble with Tribbles:

McCOY: "Well, the nearest thing I can figure out is that they're born pregnant. Which seems to be quite a time saver."

KIRK: "I know, but really..."

McCOY: "And from my observations, it seems they're bisexual, reproducing at will. And, brother, have they got a lot of will."

Do we know anything more than that, either from statements in another episode, a Star Trek book, or a statement from Roddenberry, Gerrold, a cast member or other similarly knowledgeable person?

Consider: You have a Tribble on your lap; call it (her ??) T-zero. Fairly soon, T-zero produces a Tribble kit, T-one. We know T-one is born pregnant with T-two. But when does T-two become pregnant?

I've ignored litters, but the question is easily generalizable to litters.

  • @rand al'thor Thanks for the edit. How do you do the highlighting?
    – user48960
    Nov 8, 2015 at 22:40
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    Review the edit history in "side by side markdown". Use >. Do this on any post you want to learn the formatting for.
    – Mazura
    Nov 8, 2015 at 22:41
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    The nearest terran equivalent would be aphids, which are also born pregnant and don't do anything other than eat and reproduce (but a lot less cuddly). en.wikipedia.org/wiki/Aphid#Reproduction Oct 7, 2017 at 17:43

2 Answers 2


Having read through the Memory Alpha article, there isn't really much useful information in this regard, only providing the same information as is in the quote above.

Let's take a bit of a scientific journey to answer this question. Now, we know that Tribbles reproduce at a rapid rate, and we also know that they are capable of reproducing without a mate - therefore they are asexual (unless they use some other method of reproduction that we don't know about).

Let's proceed with the assumption that they are asexual; this means they only need themselves to reproduce. Consequently, this means that indeed they are born with the capacity to impregnate themselves whenever they like.

Based on the quote provided above where McCoy explains that they are 'born pregnant', I think he's being serious: sometime during the pregnancy period, the tribble's sexual organs have developed sufficiently for them to impregnate itself, thereby making it born pregnant.

This biological feature is called 'Telescoping generations' which is described as:

This occurrence is characterized by a viviparous female having a daughter growing inside her that is also parthenogenetically pregnant with a daughter cell.

So, the short answer to your question is that tribbles become pregnant during their parents' pregnancy!

With regards to your specific scenario that you present, when T-one is born, T-two is also in existence as a cell in T-one. And by the time T-two is born, T-three already exists as a fertilised egg!

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    Being asexual doesn't necessarily follow from being able to reproduce non-sexually. Check out Komodo dragons: a female can give virgin birth (parthenogenesis), although sexual reproduction is still the norm.
    – Rand al'Thor
    Nov 8, 2015 at 23:51
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    @randal'thor IDK about you, but have you ever seen a male tribble? Let alone a female one? Also, McCoy states that he thinks they are bisexual which lends itself to asexual reproduction IMHO Nov 9, 2015 at 1:45
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    "have you ever seen a male tribble?" I don't know - what does a male look like when he's tribbling? ;-)
    – Rand al'Thor
    Nov 9, 2015 at 1:47
  • @OftenRight - I don’t think McCoy means “bisexual” the way a biologist or gender theorist today might use the word. I think he means what we’d likely now call intersex in humans, or hermaphroditic in animals - having both male and female sex organs.
    – Adamant
    Jan 5, 2018 at 4:34

The Klingons were correct when calling the Tribbles an invasive species. They do nothing but eat and reproduce. As Dr. McCoy remarked they are pregnant from birth. So lets say one Tribble is smuggled to Earth, that’s all it would take for it to become an ecological disaster world-wide. Half of a Tribble’s metabolism is devoted to reproduction. A single Tribble with a sufficient food supply can quickly increase it’s population through asexual reproduction. One Tribble on average can have a litter every ten to twelve hours. Tribbles are hermaphrodites, having both male and female reproduction organs, which allows them to self fertilize at will. A single Tribble with enough food supply could give rise to a progeny numbering over one million in less then four days.

In just as little as a year or two they would breed non stop eventually raising their population to tens of billions and strip the planet of anything edible. On the Tribble’s home world there was a large number of reptilian predators that kept the Tribble’s numbers in check. Due to the way Tribbles multiple, I doubt Earth predators could manage that, they would be overwhelmed. Also Tribbles are slow and unable to defend themselves, so predators would focus on them as an easy meal, leaving Earth’s indigenous prey species populations to also grow out of control. The Tribbles would do physical harm to indigenous species that rely on food that the planet produces and their sheer numbers would take over the planet. They would be virtually everywhere on Earth. Eventually Earthlings would get over the cuteness and how cuddly they are, and move on to hunting them down and eradicating the ecological menace. Just like the Klingons had done at Iota Gerinorum IV; the Tribbles’ home planet. Good Riddance, Qapla!

  • Hello, Galactic Criminal, welcome to Science Fiction & Fantasy. Thanks for your post, but it doesn't really answer the question, when (how soon) a Tribble becomes pregnant. Perhaps you can edit your answer to address this. Also, please take the tour to learn about this site.
    – SQB
    Oct 7, 2017 at 11:30

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