T-Rex Forever LLC has a mating pair of Tyrannosaurus rex. They're a bit shady on how they got them, time travel, cloning...they won't say. Third-party biologists and paleontologists have examined the pair and their first clutch of offspring. They are definitely real.

Somehow, T-Rex Forever goes out of business. As a last act, they release this breeding pair out into the rain forests of Costa Rica.

How long can the T-rex' survive without starving in an environment without significant megafauna?

Never mind that any humans anywhere near a T-rex will be highly inclined to hunt it down and kill it. We aren't worried about diseases (infectious or metabolic) that the T-rex might catch, we are only worried about the predator-prey relationship between a T-rex and the things they eat. No other dinosaurs were released, just these two T-rex'.

My assumption is that without herbivore megafauna to prey on, or with scarce megafauna, it wouldn't be too big of a deal to release a mating pair of T-Rex into the wild. There wouldn't be enough for them to eat and they would starve before too long. (I'm using the "bigger than humans and not domesticated" definition of megafauna.)

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – James
    Commented Dec 13, 2017 at 15:00

9 Answers 9


A young Tyrannosaurus Rex would be eaten

If you've ever looked at collections of dinosaur fossils, you might wonder about why there are really big predators (like T. Rex) and really small ones (like Velociraptor, actually the size of a big turkey) but nothing in between. That is because Tyrannosaurus was the mid-sized predator.

enter image description here

Above is the growth chart for T. Rex. Notice that T. Rex takes about 10 years to grow to the size of grizzly bear; and won't reach full size until it is more like 20. A lion, on the other hand, rarely even lives that long in the wild. Wikipedia gives the average age of a male at 10-14 years. Meanwhile a cub hits sexual maturity in 2-3 years.

So there is the problem. A young tyrannosaur would have to survive for 5 years at least avoiding predators. Just looking at Costa Rica, a baby tyrannosaur would be avoiding otters, racoons, weasels, foxes, grison, tayra, skunks; a predator for every habitat. He wouldn't be safe in the trees, on the ground, or in the water. And then if this tyrannosaur survived to 10kg, he is still potential food for a cougar or jaguar.

You might think, well baby T. Rex survived just fine in the Mesozoic, why is so hard about now? The difference is endothermy in all the creatures that baby T. Rex would be competing with. Warm blooded creatures are much more active than cold blooded. In the modern world, T. Rex isn't keeping away from lizards and snakes and proto-mammals; it is avoiding high metabolism predators that have to eat half their body weight each day. The whole paradigm of 'have a lot of eggs and hope for the best' is literally extinct in today's world for large animals. If you want a small baby to survive to a big animal, you need to take care of your offspring, and you need to make it grow faster. You need to feed it milk, which means that you need to be a mammal. Exceptions in modern ecosystems are strictly limited.

A big Tyrannosaurs Rex would starve

Can dinosaurs run? Not very fast, probably, given their sizes. In the olden days of the Mesozoic, the way to protect yourself was to have lots of offspring, and grow to massive size. After all, we have just gone over how long it takes a T. Rex to get to huge size. Huge T. Rex are rare! Most of the T. Rex that your average hadrosaur encounters on a day to day basis are going to be much smaller; too small, in fact to bother a 6 ton hadrosaur. Herbivores evolved for size (like sauropods) or toughness (ceratopsian or ankylosaurs). Its pretty hard to argue that any of those three dinosaurs was designed to outrun a predator.

Today, if you are a large herbivore, you protect yourself by running.

enter image description here

Now, lets address the T. Rex life cycle. As a newly hatched dino, the T. rex can probably find enough to eat with lizards and rodents and what have you. But as it gets bigger, it starts to have a problem. What does a 25 kg T. Rex eat? Its a bit big to be going after mice. It can't climb trees like the comparably sized ocelot. It can't run as fast (or more importantly, as long) as a coyote to track down small deer in a forest. The best it can do is scavenge.

What does a 100 kg tyrannosaur eat? There is no way it is going chase down a deer. Or even a peccary through thick brush. Those creatures are both designed to evade jaguars. A jaguar will be stealthier, due to its low profile, and much more agile in a chase through the forest, due to having four legs.

Lets say that this Tyrannosaur somehow makes it to 500 kg....now what does it eat? It isn't fast enough to catch anything anymore. There is no thing slow enough for it to overpower with size. Maybe 10,000 years ago it would feast on ground sloth and elephants, but those aren't around any more.

The closest thing to proof

When North America and South America come together, a few million years ago, the continents exchanged predators. From the North came the cats, like sabretooths and cougars and jaguars. From the South came the Terror Birds. Weighting up to 150 kgs, they are in many ways an analogue of the juvenile T. Rex in the 5-10 year old range. They could run fast, had excellent vision, and powerful beaks and claws.

They are also extinct. The big cats are not.


I tried to make this as short as possible. It is quite impossible for me to cite every little statement in here, so you will have to do your own research to see if you agree with me. The bottom line is that the way animals were designed and that the ecosystem interacted in the Mesozoic was radically different from how it works today.

In the Mesozoic, there were tons of immature animals of every size, enough to provide a continuous buffet of meals in all weight classes. Today, mammals power grow their children to full size in 2 or three years; there just aren't that many half-sized deer or what have you to catch.

Today, animals are built for the chase. Utilizing the mammals warm blood, every good prey item from rabbits to antelope to horses is designed to run away, fast, and with endurance. T. Rex simply isn't built like that. Even if it were a stealthy stalker (which is dubious), it would not have the agility on two legs to catch the deer or wild pigs it would need.

No, the age of the T. Rex is firmly over.


Question says 'modern ecosystem.' The possibility of T. Rex gorging on cattle is right there, but since the question is whether it would survive in the wild, I'd say it is a resounding no.

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    $\begingroup$ Nowadays, dinosaurs are no longer believed to be cold-blooded actually, but at least intermediate-blooded (mesotherms). Recent findings from studies on dinosaurs' energy rates, energy consumption and teeth suggest that it's even possible, if not probable, that they were warm-blooded like mammals. $\endgroup$
    – not2savvy
    Commented Dec 13, 2017 at 22:21
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    $\begingroup$ I just want to say, this is some of the most interesting stuff I have ever read. Thanks for the read :) $\endgroup$
    – Recelica
    Commented Dec 14, 2017 at 8:47
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    $\begingroup$ I’d also argue that the Terror Bird would be much better than the T-Rex - possibly faster, definitely more nimble in a forest, possibly a better kill method. The lack of Terror Birds (which I’m not especially unhappy about if I’m honest) really does suggest a T-Rex would be at huge disadvantage $\endgroup$
    – Tim
    Commented Dec 14, 2017 at 13:44
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    $\begingroup$ I thought T-rex was supposed to be a scavenger, using its size to drive other predators way from their kills? $\endgroup$
    – Joren
    Commented Dec 15, 2017 at 11:46
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    $\begingroup$ Modern research indicates that the Tyrannosaurus wasn't really fast by any standard. It's top speed was probably around 12 mph at most - which is a light jog for humans. For comparison, Grizzlies get up to around 35 mph. This all feeds into the idea that a grown Tyrannosaurus was much more likely to be a scavenger who would just walk up to food and drive the real predators away with its size. $\endgroup$
    – KAI
    Commented Dec 15, 2017 at 20:39

Randall Munroe has done most of the work for me already here. In summary, a T-rex requires approximately 40,000 calories per day, that's equivalent to approx 40 kg of meat per day.

According to the Costa Rican Travel Agency, T-rex would feast on crocodiles (average 400 kg), tapirs (150–400 kg) giant ant-eaters (up to 40 kg), peccaries (20–40 kg), and jaguars (50–100 kg)(if it could catch one napping). If they can hunt in water, then manatees (200–600 kg) are on the menu too. Jurassic park has some delightful carnage showing that humans (ave. 62 kg) are a valid food source.

There's definitely plenty of food there for them, so the only unknown is whether or not the T-rex is capable of hunting in dense rainforest. Unfortunately, the evidence seems to point toward the T-rex living in forested areas, so there seems to be no upside to your plan! (Unless you're planning on writing a sequel)

It might be safer to release them on an island off the coast of Costa Rica:

An island off the coast of costa rica

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    $\begingroup$ "people wandering around (50–100 kg)" $\endgroup$ Commented Dec 11, 2017 at 14:53
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    $\begingroup$ As a Costa Rican... not sure there would be enough tapirs and crocs available for them to eat. Definitely would be gone in a few months, if they were able to catch them. $\endgroup$ Commented Dec 11, 2017 at 18:05
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    $\begingroup$ "According to the Costa Rican Travel Agency, T-rex would feast on..." Why do I find it both very difficult and extremely easy to believe a tourist agency would cheerfully explain what a T-Rex could eat if it visited? $\endgroup$
    – anon
    Commented Dec 12, 2017 at 5:42
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    $\begingroup$ +1. Also worth noting: If the T-Rex was instead released in Colombia, hippos are an invasive species there and could provide an ample food source. $\endgroup$ Commented Dec 12, 2017 at 11:44
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    $\begingroup$ @RoyalCanadianBandit Oh, good - let's solve the hyppo problem with T-Rexes! That smells like a more hardcore version of Dingos on Austrália... $\endgroup$
    – Mermaker
    Commented Dec 12, 2017 at 13:53

Megafauna? costa rican cattle https://lecherialaslapas.com/2013/09/08/purebred-watusi-calves-born-in-costa-rica/

There are really a lot of cattle in Costa Rica.


Nationally, more than 45,000 livestock farms employ at least 12% of the Costa Rican workforce and occupy over 35.5% of the territory.


Americans are not the major factor in the 39 million pounds of beef Costa Rica exports each year to the United States. But they are certainly a growing part of it and the cost of doing busi ness in the United States is the reason. A ranch here that supports 1,000 cattle and costs $250,000 to buy is about half the cost of a comparable ranch in the American Mid dle West. There are no restrictions on the amount of land a foreigner, can buy in Costa Rica.

I think that in Costa Rica your T.Rexes would find cattle (and it would not take long), and eat them. Cattle would be easy picking for a T.Rex. When the cattlemen come to kill them, your T.Rexes will point out your paragraph where it says not to worry about that.

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    $\begingroup$ ", your T.Rexes will point out your paragraph where it says not to worry about that" -- Unfortunately the only means of communication available to a T-rex is biting you in half.. Jurassic Park was all one big misunderstanding $\endgroup$
    – JeffUK
    Commented Dec 10, 2017 at 22:45
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    $\begingroup$ "When the cattlemen come to kill them, your T.Rexes will point out your paragraph where it says not to worry about that." I seriously would love reading a story that directly cites SE. "How do you know this crazy plan will work?!" -- "there's a website called StackExchange which describes our exact predicament; it must work!" $\endgroup$ Commented Dec 11, 2017 at 5:43
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    $\begingroup$ But... How can T-Rex point to it with those sadly short arms? $\endgroup$
    – frarugi87
    Commented Dec 11, 2017 at 8:57
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    $\begingroup$ Your New York Times article is roughly 45 years old. I'm sure the relevant numbers have changed significantly in the intervening years. $\endgroup$
    – Erik
    Commented Dec 11, 2017 at 19:47
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    $\begingroup$ Good answer! I have 2 questions: 1, you assume T Rexes are cold blooded and less active. Isn't there some significant school of thought that they were bird-like and warm blooded? 2, you keep referring to them as slow. I don't see how you can call a biped of that size slow. Even a juvenile is built a lot like an Ostrich as far as locomotion. While a very heavy full size 'rex may accelerate slowly, they would actually move quite fast. I read something about analysis of fossilized dinosaur footprints that backs this up: I think some of the Rex prints put them at around "galloping horse" speed... $\endgroup$
    – JBiggs
    Commented Dec 11, 2017 at 21:03

So Will and JeffUK have covered the what would they eat part, but what happens after that?

You're a little bit sparse on the details (and our knowledge of T.Rex breeding is fairly sparse) but it seems that T.Rex reached their full adult weight around the age of 18, but could quite possibly have started breeding at age 16 so let's assume the pair were 16 when the company first 'acquired' them. Assume another couple of years in captivity before the company went bust and released them and they are roaming the Costa Rican wilderness approx. age 18.

Unfortunately hard data about number of eggs is pretty much non existent. It is speculated that like their descendants (birds) dinosaurs practised parental care, and this is one reason why we don't find many eggs or dead young.

If parental care is practiced then you would assume only one clutch of eggs per year (or possibly even one clutch every other year) and that each clutch is small. Eagles lay an average of 1 - 3 eggs but ostriches lay 7 - 10. Let's err on the side of caution and assume eagle like, which is an average of two eggs.

If we assume that the pair lays two eggs per year till they die aged 30 that's 24 individuals, but by the time the first pair are dead the oldest offspring will probably not even have reached breeding age. Even if we assume no human interaction at all and they live their lives completely unmolested you'd probably have 40 - 60 years before you'd have any real danger of a population explosion and serious effects on the ecosystem.

When you take into account the complete lack of genetic diversity too it's far more likely the T.Rexs will either die off (again) or survive in very small numbers and cause little trouble for the ecosystem.
Who knows, they might even help it...

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    $\begingroup$ +1 just for noticing the genetic diversity, Its not a deal breaker, it has happened before in island species, but that is onse severe bottleneck. $\endgroup$
    – John
    Commented Dec 12, 2017 at 22:00
  • $\begingroup$ one thing we do know about dinosaur breeding, they lay eggs in pairs, so even numbers at least. nests often show eggs a paired pattern we see in animals with paired oviducts. $\endgroup$
    – John
    Commented Dec 18, 2017 at 2:47

Besides all the other "they'd all die" reasons... https://en.wikipedia.org/wiki/Minimum_viable_population

Minimum viable population is usually estimated as the population size necessary to ensure between 90 and 95 percent probability of survival between 100 and 1,000 years into the future. The MVP can be estimated using computer simulations for population viability analyses (PVA). PVA models populations using demographic and environmental information to project future population dynamics. The probability assigned to a PVA is arrived at after repeating the environmental simulation thousands of times.

MVP does not take human intervention into account. Thus, it is useful for conservation managers and environmentalists; a population may be increased above the MVP using a captive breeding program, or by bringing other members of the species in from other reserves.


While many of the answers cover much of the ground, I think the biggest unknown is how, exactly T-Rex lived and worked in family groups. I have seen some speculation that the adults would nest and then nurture the T-Rex chicks. As they reached adolescence, the "gangly" teenagers would have proportionally longer limbs and lighter bodies than the adults, leading to speculation that family groups would send the adolescents to "drive" the prey to the lurking adults waiting in an ambush position. This sort of hunting strategy actually makes sense considering many of the potential prey dinosaurs were heavily armed and armoured (and probably sporting nasty dispositions).

enter image description here

Reconstruction of a T-Rex chick. The proportionately longer limbs would make it an efficient "driver" for the hunt

This sort of hunting strategy, if it was how a flock of T-Rex hunted, doesn't really translate well to either the dense rainforest environments of Costa Rica, nor the more open pastures cattlemen would have their herds on (more because cattle would react differently to the presence of hunting T-Rex than the T-Rex has evolved to deal with).

The other issue is related to the genetic variability of the T-Rex being released into the wild. Small family groupings like this exist even within modern hunting mammals, but generally when the young males reach sexual maturity, they either leave on their own or are driven away to stake out new territories and seek mates outside the family group. Given there is only one breeding pair in existence, this could lead to a situation several years down the line where young male T-Rex's are staking out territory across large swaths of Central America, but have no viable mates. Any who remain with the initial brood (or move back to take over) will be mating with their sisters, with pretty predictable long term results.

Assuming there is a moratorium on hunting and killing the T-Rex flock for ecological or other reasons, the new result may be to have another team of scientists either try to recreate more T-Rex's via the initial process, or capture some of the existing flock and then do some genetic engineering of their own to modify the DNA and release "2nd cousins" into the wild to ensure viability of the free range T-Rex community. Of course if you are planning to go that far, then recreating the rest of the Cretaceous ecosystem is probably quite doable as well (although the entire menagerie will have to be imported back to the American West Coast to live in the Douglas Fir forests, which is probably much closer to their ecosystem than anything else these days. Tourists will be thrilled in the moments before their cars are kicked off the road and they are eaten....)

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    $\begingroup$ Yeah, well, fortunately for the T-Rex's there's an almost endless supply of tourists. Can't keep the buggers away! "Don't go into the forest", we tells 'em. "Don't go walking around alone at night!" "Don't drive down that road with the big sign saying 'All-You-Can-Eat T-Rex MegaBuffet This Way!'". But will they listen..? Nooooo... $\endgroup$ Commented Dec 13, 2017 at 12:34

As others have pointed out there is megafauna in costa rica although most of if is domesticated (35% of the land area used for ranching) or invasive (hippos) but the rexes will not care about that, if anything having them in nice convenience pens they can't get out of will make it easier. there is also middle sized stuff which they can eat, wolves can live of mice if they have to for a while after all.

Costa rica is not however a great environment for them, t-rex was believed to live in a savannah like environment, so they will likely be drawn to the human areas with more open space and more importantly easy food. They will become a major danger long before they starve. Africa may be a "better" for them at least, choice.

The real issue is costa rica is covered in their two closest living relatives birds and crocodilians so the chances of them catching a disease is high although they may be just as likely to be resistant to everything around since their physiology is different enough. Parasite may be a bigger issue as their immune systems will not be ready for modern ones, you might be able to hand wave this, crocodiles do not get many digestive parasites after all.


During the megafauna era oxygen concentration was rather high (that's what led to such big animals/plants in the first place) your T-rex would have some troubles breathing normally which would lead it to have troubles catching prey.

Ignoring the obvious oxygen thing and considering the amount of food one of this must ingest i'd say it would die in more or less ~10ish days because it would use most of it's energy to catch prey that would be far less than the energy used.

Edited: To clarify a bit the my second point, the daily caloric intake of a t-Rex can't be sustained for long periods of time with the current fauna we have . Even if it would feed mostly on elephants or try to (gl being sneaky at that size) the energy needed to fight one is rather high don't forget that the target would be adults not the infant ones (more mass=more calories) and as ferocious as a T-Rex might be or look if you smack it hard enough it would tip over like a cow .

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    $\begingroup$ Oxygen level in the cretaceous was only about 150% of what it is today, i.e. breathing at sea level today would be similar to breathing at 3400 m in the cretaceous. It's a valid point, but I don't think it's too unrealistic to ignore that bit. Did you perhaps mean the carboniferious? That's where O₂ level was three times as high, making giant insects possible. — More relevant might be the 600% CO₂ concentration, which made the cretaceous very warm. (And somewhat counteracts the oxygen argument, because hemoglobin adheres to CO₂ more easily than O₂). $\endgroup$ Commented Dec 11, 2017 at 17:53
  • $\begingroup$ @leftaroundabout tx for the info i thought the cretaceous had almost the same O2 concentration.Ignoring the first part the second one still stands , the prey/food source wouldn't be enough for it to sustain itself(according to the current scientific theories about it's daily calorie intake) $\endgroup$
    – Kaotis
    Commented Dec 12, 2017 at 7:52
  • $\begingroup$ @Kaotis fyi your reasoning should be in the answer, not hidden in a comment. $\endgroup$
    – user41674
    Commented Dec 12, 2017 at 17:49
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    $\begingroup$ @AytAyt tx for the input , edited the original post $\endgroup$
    – Kaotis
    Commented Dec 14, 2017 at 8:26

We do have megafauna today, the question is if your T-Rex is able to go fishing?

Think about whales and other large sea-animals. Now, a T-Rex probably can't swim, but depending on the situation, stranded whales would fill it up for a while. The rest would be cattle, I assume.

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    $\begingroup$ The problem with whales it that they 1. strand very rarely and unreliably 2. when they strand, they rot rather quickly. A t-rex could eat on one for a day, maybe longer if it turns out it's a specialized carrion eater, but stranded whales are not a reliable source of nutrients. $\endgroup$
    – Philipp
    Commented Dec 11, 2017 at 12:46
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    $\begingroup$ Plus, at 40,000 kCal/day, that's just enough to feed 20 people. Not so much food, really. Being cold-blooded (for all we know), a T-Rex would eat a lot less than a comparably-sized mammal. If a T-Rex ate 3 meals a day (unlikely, but bear with me here), one meal might be the size of a couple of buckets from KFC. $\endgroup$
    – Jeffiekins
    Commented Dec 11, 2017 at 17:58
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    – Secespitus
    Commented Dec 12, 2017 at 11:14

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