# How to estimate size of ketamin dose to sedate a T. rex.?

To mark the anniversary of our company we have decided to introduce to our VIP guests a once in a lifetime opportunity to get intimate with the dinosaurs, we will put the animal to sleep so the guests can get near to them and molest it however they wish.

The thing is dose makes the poison so I must make sure we don't want the animal preferably a T. rex. to go into a cardiac arrest during the encounter in the late Cretaceous period, also I want to limit the duration of this expedition to 2 hours top.

My concern is how to determine what is a good dose of ketamin to tranq shot a T. rex.? We don't know for sure the first T. rex. on sight will be an adult or a juvenile but with such tight deadline there is no time to be picky.

• If there is no time to be picky, why are you so worried about the right dose?
– L.Dutch
Mar 31 at 7:25
• Any answer would include the math for a per-kilogram ketamine dose and your people would need to bring either a variety of darts for different weight class estimates or be able to change the dose that will be given on the spot. With 2 hours of sleep you might need a tranq dart that also keeps up a dosage over a period of time after the initial dose. Mar 31 at 8:09
• im interested in the answer, it's for a friend
– shas
Mar 31 at 8:51
• This sounds familiar. Anyone else read A Bird In The Hand by Charles Stross? Its a pretty short story, will only take you a few minutes... Mar 31 at 12:24
• While this estimate is possible, it would be mostly a guess and can be wildly wrong (similar to how it happened in "The Name of the Wind"). Mar 31 at 17:27

Ostriches can be dosed using medetomidine (80 μg/kg IM) and ketamine (2 mg/kg IM) for sedation within 15 minutes.

Worth considering seeing as birds are supposedly therapod descendents.

https://www.jstor.org/stable/30133182

• NB "Molest" has a secondary definition that simply means "to bother or annoy". Though considered dated, this definition is still in common use. Mar 31 at 18:15
• I realise that and normally deplore the smuttiness of our public discourse. But in this case, given the corporate VIP encounter and amount of post-Epstein elite sex abuser stories, I decided I'd read it that way. Plus: it's funny. Mar 31 at 18:53
• @Dragonfang An old bilingual coworker preferred this usage. In Spanish, "Molestar" translates "To Bother", so I imagine it was most similar for him. Mar 31 at 21:25
• You guys make it sound like you want to sedate the T-Rex so you can molest it. But if it's sedated why would it get annoyed? Apr 1 at 15:16
• @Dragonfang the OP uses both the term "intimate" and "molest". One mistranslated word might be considered an accident. Two, used together, that work in combination to convey a prurient tone, looks a little less accidental. Apr 1 at 17:46

I think the closest equivalent in present times would be a crocodile and/or a Komodo dragon.

What I was able to find online mentions:

For crocodiles:

Ketamine at 18 to 45 mg/kg in Nile crocodiles had little effect, whereas doses at 59 to 110 mg/kg caused death in three animals. (source)

It is also mentioned that other chemicals might work better

Diazepam followed by succinylcholine chloride was administered intramuscularly to 26 healthy mature female alligators on two occasions. The mean diazepam dosage was 0.37 mg/kg (0.28 mg/kg to 0.62 rng/kg) and the mean succinylcholine chloride dosage was 0.24 mg/kg (0.14 mg/kg to 0.37 mq/kg). This drug combination reduced stress and allowed adequate immobilization for restraint and handling. The reduced drug volume, low dosage of succinylcholine chloride required, short induction period, maintenance of respiration, and adequate degree of immobilization make this drug combination a good alternative to the use of muscle relaxants alone in the chemical restraint of alligators. (source)

• The effect of ketamine in lizards is related to dose, species and the individual. Lizards require lower doses than other reptiles. It is useful for sedation or induction of anaesthesia for intubation. Sedation may require as little as 10 mg/kg while anaesthesia may require up to 50 mg/kg. Induction may take 10 to 30 minutes with recovery over 24 hours.
• Muscle relaxation and analgesia may be marginal
• Prolonged recovery with higher doses
• Larger reptiles require lower dose
• Painful at injection site
• Questionable safety in debilitated animals
• Avoid use with renal impairment
• Lizards require lower dose than other reptiles
• 10-30 mg/kg IM as a sedative, facilitates intubation
• Useful in large lizards (11.6 mg/kg) in combination with midazolam (0.34-0.35 mg/kg)

In principle you can use a dart which can be loaded on the spot based on the estimate of the size of the sample you have spotted. You can compile a table computing the estimated weight based on the estimated height of the specimen. Which dose to use between the two values listed above it's up to you: do you value more the safety of the animal or the safety of your guests?

• You could also prepare a few darts of known quantities, and then select the ones that add up closest to the value computed. The great thing about this is that it could provide a better spread, instead of dumping everything in the same spot. The bad part is that you need multiple hits. (or a group shooting simultaneously). Mar 31 at 10:07
• You can't do a test run on some other dinosaur? You absolutely have to go for sending a VIP molesting a T-Rex with untested technology? Mar 31 at 11:48
• Important note: don't go use succinylcholine chloride to knock things out. It is a short term paralytic: get the dose wrong and the victim will fall over, stop breathing and asphyxiate in short order. Unless you have a t-rex sized ventilator with you, of course. Mar 31 at 12:21
• "The closest equivalent in present times would be a crocodile and/or a Komodo dragon": actually T. rex is more closely related to the house sparrow Passer domesticus then to any crocodile; and very much more closely related to birds and crocodiles than to any squamate. At least, both the sparrow and the T-rex are dinosaurs; and both dinosaurs and crocodiles are archosaurs, whereas squamates are lepidosaurs. The most recent common ancestor of V. komodoensis and T. rex or P. domesticus lived about 250 million years ago, at the end of the Permian or the beginning of the Triassic. Mar 31 at 13:15
• @AlexP +1 for the cladistics. I would look carefully at crocodiles/alligators as well as birds, though, because T. Rex metabolism should be partway between the two, so even though crocs are less related they still have useful information to impart. (Slower metabolism should mean larger dose needed, slower effectiveness, as well as more risk of overdose due to slower uptake.) Mar 31 at 17:02

Logarithmic Dartgun

My concern is how to determine what is a good dose of ketamin to tranq shot a T-rex? We don't know for sure the first T-rex on sight will be an adult or a juvenile but with such tight deadline there is no time to be picky.

Your hunters know how long it takes for ketamine to effect a crocodile. Let's say it takes 5 minutes. They suspect a T-rex has a faster metabolism than a crocodile -- they are closer to warm blooded birds than cold blooded crocodiles after all -- and so the correct dose should take hold in less than 5 minutes.

The first T-rex you see, check is he bigger than a crocodile. If not let him go, he's too small to be any fun. If he is big enough he gets a crocodile-sized dart. Follow him for 5 minutes.

If the T-rex is asleep scoop him up. Otherwise shoot another dart to double the dose.

Follow for another 5 minutes. Is he asleep yet? If not then shoot two more darts (or one dart with double the dose). Now he has 4 servings of Ketamine in his system.

Keep doubling the dose and waiting. Eventually when he falls asleep he has no more than twice the minimum dose.

Edit: L. Dutch's answer suggests the lethal dose for crocodiles is only about twice the minimum sedative. That's bad news for our hunters, since the above method might accidentally kill the t-rex.

To fix the method replace "double" with "multiply by 1.1". That way you need more darts but when the T-rex falls asleep you have used no more than 1.1 times the minimum sedative. The doses look like this:

The blue graph is the total amount of ketamine after $$n$$ shots. The red graph is the amount to put in shot number $$n$$.

• crocodile-sized dart makes me think of a dart that's literally the size of a crocodile. I don't think a T-rex that receives that is going to get up any time soon Apr 1 at 15:17
• @Blueriver He would have a big hole in him too. Apr 1 at 15:19

## 0.5-10 mg/kg

this is the minimum and maximum safe dosage for elephants.

Now you say why elephants, because that is the closest thing in size to a t-rex we have, and size matters most.

birds and mammals of similar size differ in dose by much less than across difference masses the change is orders of magnitude. A safe dose for a rat is 80-100 mg/kg while an elephant 5-10mg/kg is considered high. while for a small bird the safe dose is 10-40mg/kg and around 8mg/kg max for an ostrich. mote birds need less than mammals on the small size but not significantly less as size increases.

So in all likelihood you should use less for a t-rex than an elephant based on phylogeny, but the temperament of a t-rex is likely worse than a elephant so you need to use more, but at the same time a t-rex is larger than an elephant so allometry means less.

bigger animals have lower metabolic rates so such drugs are more effective, but things like temperament also matter a lot.

Your margin of error is huge, your best bet is to use multiple smaller doses and work your way up and re-dose as needed. lowering the risk of killing the rex means the animal being down for shorter periods of time.

https://www.mcgill.ca/research/files/research/121-_bird_anesthesia_-_jan_2021.pdf

http://elephantcare.org/resources/formulary/drug-index/ketamine-hcl/

https://animal.research.uiowa.edu/iacuc-guidelines-anesthesia

• Could you run those figures again? Doesn't "A safe dose for a rat is 80-100 mg/kg while an elephant 5-10mg/kg is considered high" sound backwards? Apr 2 at 22:24
• @RobbieGoodwin I gave my sources you can check my numbers, it is not in anyway backwards, smaller animals need more per unit body mas, a lot more. because their metabolism is so much higher. one of the big advantages of large size is lower metabolic demand for the same activity level.
– John
Apr 3 at 4:24
• Thanks and if only you'd said that! Apr 3 at 18:16