MALCOLM: How do you know they can't breed?
WU: Because all the animals in Jurassic Park are females.
We engineered them that way.

Why would they engineer them as all-female instead of all-male? It would work better since even if there was one female by nature-finds-a-way accident, it wouldn't reproduce nearly as fast for obvious biological reasons.

I'll accept either in-universe explanation or, lacking that, scientific one (since the book and the movie pretend this is "real world science")

  • 37
    The ancient age-old asinine assumption that the female is the more docile of the two genders?
    – Radhil
    Commented Oct 21, 2015 at 1:17
  • 45
    It's sooo much easier just to copy that X chromosone instead of genning up a Y. Commented Oct 21, 2015 at 1:21
  • 13
    @Radhil I'm not sure if docileness matters that much when it comes to dinosaurs.
    – Saturn
    Commented Oct 21, 2015 at 5:41
  • 37
    "We decided to make it female so it would be more docile and controllable." "More docile and controllable, eh? You guys don't get out much." -- Species Commented Oct 21, 2015 at 9:12
  • 18
    @OrganicMarble They have the ZW female, and ZZ mal system
    – xyz
    Commented Oct 21, 2015 at 11:47

9 Answers 9


This is mentioned in the original novel by Micheal Crichton:

It's easier to manage

“Sex organs vary with the species. It's easy to tell on some, subtle on others. But, to answer your question, the reason we know all the animals are female is that we literally make them that way: we control their chromosomes, and we control the intra-egg developmental environment. From a bioengineering standpoint, females are easier to breed. You probably know that all vertebrate embryos are inherently female. We all start life as females. It takes some kind of added effect-such as a hormone at the right moment during development-to transform the growing embryo into a male. But, left to its own devices, the embryo will naturally become female. So our animals are all female. We tend to refer to some of them as male-such as the Tyrannosaurus rex; we all call it a 'him'-but in fact, they're all female. And, believe me, they can't breed.”

There are also behaviour benefits

“Interesting,” Grant said. “I was just digging up an infant antirrhopus. Are there any full-grown raptors here?”

“Yes,” Ed Regis said without hesitation. “Eight adult females. The females are the real hunters. They're pack hunters, you know.”

  • 7
    Wait, them being more aggressive is considered a benefit? Now I'm wondering if there is anything in the books about the behavior of the males of the species to compare to the females behavior.
    – Reaces
    Commented Oct 21, 2015 at 9:12
  • 24
    @Reaces - It's a zoo. You want the exhibits to be active.
    – Valorum
    Commented Oct 21, 2015 at 9:58
  • 1
    There is some overstatement actually in "vertebrate embryos are inherently female". That is not so strongly true because in females the egg development happens very early on during embryogenesis. Also, there is a difference between being poised for being female and actually being a female. The only ease of making females is that they would be a source of unfertilized eggs that can be used to make more dinosaurs, instead of depending on crocodile or any other distantly related species. Chance of success is higher.
    Commented Oct 21, 2015 at 13:17
  • 1
    Looks like the movie writers took a reasonable explanation, complete with a "such as," and made it into a self-contradictory mess. Commented Oct 21, 2015 at 20:58
  • 4
    @WYSIWYG - AFAIK, that was considered to be true when the novel was written (and when the film was made, for that matter). The SRY gene, which causes testicular development, had just been discovered, which lent weight to the "female is default" hypothesis. It wasn't until a decade later that further discoveries (e.g. WNT4) overturned that.
    – Compro01
    Commented Oct 25, 2015 at 13:26

From the movie, it sounds as though they chose female because it was less work. Since they apparently neglected to consider the possibility of life finding a way the dinosaur's sex spontaneously changing, they assumed that a single-gendered group would work regardless of gender.

Dr. Ian Malcolm: But again, how do you know they're all female? Does somebody go out into the park and pull up the dinosaurs' skirts?

Henry Wu: We control their chromosomes. It's really not that difficult. All vertebrate embryos are inherently female anyway, they just require an extra hormone given at the right developmental stage to make them male. We simply deny them that.

  • Comments are not for extended discussion; this conversation has been moved to chat.
    – user1027
    Commented Oct 23, 2015 at 18:11

First of all, making them all male wouldn't have solved the "life finds a way" problem - because some frogs can go from male to female.

Secondly, cloning females would be easier because an accidental YY gamete would probably not grow, while an XX one does.

Thirdly, in most species females are smaller than males, meaning less resources required for a fully grown adult specimen.

Lastly, Henry Wu wouldn't have considered a the failure of a male specimen being among the all female population because through out the books he is shown to be wildly over confident in his abilities.

  • 8
    If you note, I explain in the question how it would solve a problem. If 5 go F=>M, you have 100s of females who can breed with those 5 males. If 5 go M=>F, only those 5 can breed. Commented Oct 21, 2015 at 1:25
  • 2
    I've added a clarification as to address that.
    – user20155
    Commented Oct 21, 2015 at 1:27
  • 47
    It's likely dinos had a ZW sex determination like birds and some reptiles, rather than XY like mammals.
    – Viergacht
    Commented Oct 21, 2015 at 1:34
  • 4
    @LewDelport Whaaaa.... I had never heard of that before! I am most probably wrong.
    – user20155
    Commented Oct 21, 2015 at 1:38
  • 9
    It's impossible to know for sure, but they probably didn't have temp. dependent sex determination like some reptiles because of the wide range of environments they lived in, but it would have to be either that or ZW (the main functional difference being the ovum determines the sex, not the sperm). Mammals and birds/reptiles seem to have evolved sex determining chromosomes completely independantly.
    – Viergacht
    Commented Oct 21, 2015 at 6:22

I am addressing this question from a biological standpoint.

With the technology available at the moment, it is not possible to clone a male in the absence of an egg-donor i.e.the female. Most nutrients and other factors required for the early development of the embryo are present in the egg and not the sperm.

Recent research work (Easley et al. 2013, Hayashi & Saitou. 2013) have shown that it is possible to produce gametes in-vitro (i.e. in the lab) using pleuripotent stem cells. However, there has not been any work that has investigated producing eggs using stem cells from a male. I don't say that is absolutely impossible but it would certainly be very challenging to do that.

In comparison, it is quite easy to clone a female (as had been done with Dolly the sheep).

Females can also produce offspring without fertilization by the male gamete, by a process known as parthenogenesis. In the end, this did happen in the Jurassic Park, that explains the flaw in their (Wu and others) initial assumption that an all female population cannot grow naturally.


The XX/XY explanation is not really valid because it is likely that many theropod dinosaurs like T.rex and Velociraptor have the ZW chromosome system, in which the female is the heterogametic sex (ZW, male is ZZ). However WW configuration is not impossible like YY as shown in case of Boa where a female gave birth to viable WW offspring parthenogenetically (Vicoso et al. 2013).

  • 8
    +1 for being only answer to mention the ZW system!
    – Amory
    Commented Oct 21, 2015 at 13:55
  • 3
    I'm tempted to downvote the other answers for not pointing out this flaw. I won't, but it's very tempting.
    – Pharap
    Commented Oct 24, 2015 at 7:08

I wholeheartedly agree with what has been said about aggression and females - unlike males - pairing together. I'd like to add something about gender however.

First off, there are some species that can switch between male and female... there are females in species that may switch between sexual and asexual reproduction - eg. having male and female offspring after sex with a male... or having only female offspring "by herself". In addition there are species - like some earth worms- that are both male and female, and that "battle" during sex. The winner is the one that with one of his three penises manage to impregnate the other - who will be the "female" actually becoming pregnant.

Gender may also be determent by external factors. For some reptiles, the gender is determent by how heated the nest become in the sun. Warmer nests gives most/only females, while a few degrees colder gives most/only males.

For many insects - like bee and wasps - unfertilized eggs become males, while fertilized eggs become female. It's speculated that this happened first for spiecies living of scarce resources in very harsh climate. Imagine a female crawling around the desert, trying to find a suitable dung-heap. When she does - rather that trying to attract a male - she just start laying unfertilized eggs. These became males - males with whom she could mate (yes, incest). Her next batch of eggs - fertilized eggs - all became female For this species, the female would wander, while the males stayed put.

My second point is that not all animals are XX=female and XY=male. Birds - which are closely related to dinosaurs (or rather, is the one group of dinosaur that remains) - have ZW, which is "opposite" of ours... Two identical sex-chromosoms ZZ=males, while two different ZW=female. This is also found in some reptiles.

Some insects (look above) have XX=female and X0=male - ie. the absence of a second X-chromosome - like we would have in unfertilized eggs - gives males.

More at: Sex-determination system (Wikipedia).

So it's not always as obvious as we're used to how gender is determent.


For Jurassic Park, it's also likely that they intended to have some controlled breeding in the future, and since the bottleneck with reproduction are the females, it would make sense to start with - and have most of - them. They wouldn't even need live males (for very long), since they could go for artificial insemination or test-tube dinos.

  • 1
    Another thing... For many species - including humans - the female is the "fittest" of the genders. During harsh times - drought, starvation, illness - the male fetuses and young male children dies-off more easily than the females, causing a surplus of surviving females. Which make sense, since it's the females that are the reproductive bottleneck in quickly increasing the population when the environment improves. One male can impregnate lots of females, but one female can only birth so many children or lay so many eggs. Thus nature protects the females above the males. Commented Oct 21, 2015 at 13:00
  • Note that birds are theropods (T.rex is also one) but even mammals and dinosaurs had a common ancestor. The cytogenetics for different groups of dinosaurs is not really characterized.
    Commented Oct 21, 2015 at 13:03

Another factor: Males of species that don't pair-bond generally try to drive away other males in order to win access to the females. Females, however, have no such sexual need to drive away other females. (They may drive away everyone except their offspring for resources, though.)

Not knowing exactly how the dinosaurs are going to behave it would make more sense to have all females than to have all males.

  • 6
    Females do drive other females, so they can have a better pick. You may observe the human species as an example.
    – xyz
    Commented Oct 21, 2015 at 11:45
  • 7
    In many species, female is the aggressive one. Dinosaurs are a huge group with diverse behavioral differences among species. This is not really a strong point. It's like expecting that penguins should behave like ostriches.
    Commented Oct 21, 2015 at 13:20
  • @prakharsingh95 They'll drive other females away from their mate but there are no mates to protect in this scenario. Commented Oct 22, 2015 at 4:57
  • 2
    @LorenPechtel I was countering your argument regarding males. If you go by this logic, there are no females to win access to.
    – xyz
    Commented Oct 22, 2015 at 5:10

As others have said, males could be expected to fight each other more (from mammalian analogy). And the females likely were the larger sex, thus more impressive to visitors.

It should be noted however, that everything else is speculation, especially answers referring to X and Y chromosomes. Dinosaurs followed ZW sex-determination, as modern birds do.
Here you have Z and W chromosomes instead, with male (ZZ) being the norm instead of female (ZW).

Some dinosaurs may have followed a temperature-dependent sex determination, which would make engineering them all that much harder.

In the end, I suspect the trope of the softer, easier controlled female is the reason.

  • 3
    That trope is very simply irrelevant to many species. There are many cases where the female is the larger of the two and even more where the female is territorial and hunts. To take an extreme example, consider the angler fish.
    – terdon
    Commented Oct 21, 2015 at 14:30
  • 1
    indeed. But combined with wrong sex determination and other examples of wonky science in the movie, it seems to be a more likely reason. All this just shows, why exactly the Jurassic Park was a horrible idea in the first place.
    – Chieron
    Commented Oct 21, 2015 at 14:50
  • You cannot be sure that all dinosaurs followed ZW system. Birds belong to a group of dinosaurs called theropods.
    Commented Oct 23, 2015 at 6:46
  • 3
    While we can not be sure, all closer relatives of the dinosaurs follow either the ZW-system or are temperature-determined (as I wrote in my answer). The XY-system is far removed from their branch.
    – Chieron
    Commented Oct 23, 2015 at 11:26
  • @WYSIWYG: On the other hand XY only occurs in mammals and not in the platypus.
    – Joshua
    Commented Mar 26, 2018 at 18:13

Another possibility is that if they had based the park on an safari type model they may have been assuming that, from the perspective of tourism the most 'picturesque' species tend to be those which form herds/packs/prides, where the majority of the social group are female eg lions, deer etc and from the perspective of creating a spectacle for tourists that is the best approach as opposed to animals which form mating pairs which tend to be more elusive and harder to find on a short tour.


Many of the currently well documented cases of parthenogenesis (females of species that usually include male insemination in the impregnation process nevertheless producing viable eggs and offspring despite never having been exposed to any male) had not been observed, documented, and wouldn't have been available in the literature so while that would have been relevant, the scientists making those decisions may not have had that data. Some cases were in the literature, but scientists continued to doubt the validity of some of the recorded cases for quite some time. Had all of the dinosaurs been male, parthenogenesis could not occur. There is another potential case of all female populations reproducing, however, which should have been available to them: South-western whip tail lizards are all female. This has been known since the 1960s. There are no males in the species. They have extra sets of female chromosomes and pair the least related sister chromosomes to achieve genetic diversity without the inclusion of the Y chromosome. Should the more easily occurring mistake of duplicate chromosomes have been made, this phenomenon could also have led to all-female reproduction.

  • while this is largely correct, DVK is looking for an in-universe reason.
    – phantom42
    Commented Oct 23, 2015 at 16:51
  • If you remove the remarks from this answer that are more of a rant and do not fit the format of this site, you may have a solid answer here.
    – Firebat
    Commented Oct 23, 2015 at 16:52
  • 1
    I am somewhat confused by the writing style of this answer. "Many of the currently well documented cases (...) had not been observed [or] documented" - what does that mean? At the time of writing the book, or of making the movie? Or in the Jurassic Park universe, at the time of preparing the park? Or they had not been documented well back in the day, but would be well documented if they happened today? I think your answer would benefit if that part of the text were clarified with some context. Commented Oct 24, 2015 at 18:53
  • "Currently" means current to posing the question. "Had not been..." means when the decision was made in the universe (1990s book which seems to have been set about then). If information now available makes a decision less reasonable, the fact that it was not known then helps explain the decision. By "literature", I mean the scientific literature of course, not the fictional literature. I could rewrite it, but doesn't that leave your comment reading oddly? I wondered about that when I rewrote this the first time to bring it into the univ. as per previous, nicer, request.
    – emma
    Commented Oct 25, 2015 at 22:37
  • By "nicer" I mean requests that were made without giving me negative points for writing style vs. actual content value; I may be over-reacting to that - in reading about site etiquette in an attempt to meet requirements, a couple of comments said down-voting (apparently regardless of reason which Doesn't make sense to me) is considered rude, so I feel rudely handled. This is off-topic. Hopefully someone can remove this and instruct me how to edit without leaving the comments looking unhinged, which seems unfair to the commenters, and if there is Anywhere newbies can ask Procedural questions.
    – emma
    Commented Oct 26, 2015 at 0:22

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