Crested geckos can drop their tails, but don't have the ability to regrow them.  Some individuals are more sensitive about things, and will drop them when simply startled by something, such as a loud noise, and others will only drop them in the case of an attack or injury, and some still hang onto them even then - and the tails have to be manually removed to keep infection from spreading into the body.  I've read that in the wild there are virtually no adult Crested geckos with tails.  The tails separate from the body in a very unique and fascinating way - in a "star shape", which closes over itself to cover the wound.  The tail will continue to wiggle for quite a while later.  I had one gecko lose her tail in my care, and while it was upsetting (she had the most gorgeous white tail) it was also amazing how perfectly they're designed to do just that.

crested geckos drop their tails as a defense when appraoched by potential predators. the 'attacker" goes for the gecko, gets the tail, and the tail comes off. But it still wiggles when it comes off the keep the predator occupied while they run off. They will grow back, however in crested geckos it grows back rather slowly and will not look as good as the original tail. Most geckos have this ability. It dosn't affect the lizard too much.

Gargoyle geckos, and the rest of rhacodactylus regrow their tails (I'm pretty sure) - but ciliatus don't.  Ciliatus can drop them, but just not regrow them.  A lot of times if you look real closely at a crested gecko with a tail, you can kind of see a very faint line across the tail between the row of scales that would stay with the gecko, and the row that would go with the tail.  Mostly they have a little nub left over.  That's why there's so many frogbutts.  This is a link to a thread over on Pangea Forums, with some graphic (but really awesome) pictures of a dropped tail from a Crestie.

http://www.pangeareptile.com/forums/showthread.php?33769-Darn-a-tail-drop-(graphic)

CreepyCrawly is correct, Crested geckos do NOT regrow their tail, once it's gone, it's gone.

They're "froggy butts!" Sometimes cresties are even born without tails (very rare). We have one female adult with a tail, but the rest of our crew (3 adults) have no tails. All our babies still have them, of course, but it's pretty common for them to fall off. In this species they just don't grow back. That's normal. :) Other species of geckos do grow back, but not all of them. hehehe. They're still charming without them.

I don't know if this has already been said, but unlike other gecko species with tails that regenerate, the tail of a crested gecko is prehensile and therefore too complex to regenerate.      -p-

crested geckos do not have prehensile tails.  I did my capstone presentation for college on gecko tails and such and i never came across it. In the scientific community it is unknown why crested geckos can drop but not regenerate.  And to my knowledge they are the only gecko that doesn't regenerate when voluntarily dropped. 

Wikipedia definition of prehensile tail: 

A prehensile tail is the tail of an animal that has adapted to be able to grasp or hold objects. Fully prehensile tails can be used to hold and manipulate objects, and in particular to aid arboreal creatures in finding and eating food in the trees. If the tail cannot be used for this it is considered only partially prehensile - such tails are often used to anchor an animal's body to dangle from a branch, or as an aid for climbing. The term prehensile means "able to grasp" (from the Latin prehendere, to take hold of, to grasp).

Maybe I should have been more specific and said that they have partially prehensile tails... but by this definition only primates have fully prehensile tails. So to say that a crested gecko does not have a prehyensile tail would be to say that a chameleon or a possum doesn't either. Crested geckoos CAN grip with their tail, making it prehensile by definition. Now, what I said before (as I should have also specified) is just a theory that I have heard several times and discussed with others before. I do not have any true scientific research to back up the claim, but it is the logical explanation.      -p-

Oh, and sorry about the highlighted words. I copied it directly from Wiki.

I have other geckos, and all of them have little tails that sort of wrap around things to help them get about. Even the fat little tails of leos do this. I don't think that has anything to do with weather they are regenerated or not. Not to argue the point, just what I have noticed. We really are unsure why cresties don't grow their tails back, and it is true that they are one of the only ones that don't.

Yes, but can any opf those geckos actually hang from a branch by their tail? Monitors can slightly wrap their tails around an object, but the arboreal crocodile monitor can actually grip branches with their tails. That's why they are considered to have a semi-prehensile tail, while other monitors do not.

Chameleons in the genera of Brookesia and Rampholeon have a very slight grip to the tips if their stumpy tails, but they are not considered prehensile like that of true chameleons- Chamaeleo sp., Furcifer sp., Trioceros sp., Bradypodion sp., Calumma, etc.

All snakes can grip things with their tails to a certain extent, but the tail of Naja sp. (cobras) or Lampropeltis sp. (kings and milks) is not considered prehensile like that of certain arboreal snakes like Morelia sp. (GTPs, carpets, scrubs, etc) or Corallus sp. (ATBs, ETBs, Basins, etc.).

Do you see the pattern I am establishing here and the point I'm trying to make? Crested geckos DO by definition have prehensile tails (I lovingly call them monkey geckos for this very reason... and because of the way they jump). Whether or not this is why they cannot regenerate their tails, I do not know for a fact. But it is a sound theory based on logic and a basic understanding of anatomy.

A spider can regenerate a lost leg. And octopus can regenerate a lost tentacle. But a bird cannot regenerate a wing nor an elephants it's trunk. The reason for this is quite simple- an avian wing, a mammalian limb, these body parts are far more complex than leg of an arthropod or the tentacle of a cephalopod. It stands to reason that the same principle applies to gecko tails.

Does anyone know if Naultinus sp. can regenerate their tails? Just like crested geckos, they are arboreal, and they have long, narrow prehensile tails. The only information I can find is that they can shed their tails, but they are very reluctant to do so. The theory is that this is because the use of the tail when climbing is very important to them. I would also think it is related to engergy conservation. Naultinus sp. are endemic to New Zealand, and they have a very low activity level due to the low temperatures. Anyway, I am interested to know if they can regenerate their tails, since they are similar in structure and function to those of R. ciliatus.      -p- 

Gargoyle geckos can also do that, hang from branches, and their tails can regenerate multiple times in their life. They are in the same Rhac. family, so I would assume that their tail structure is very similar. They even have the little sticky pads on the end. I have had my leos "hold on" with their tails before. Because they are so fat they can't wrap them around as easily as a thin crestie tail, and they don't have sticky pads on the ends like cresties do but it's very much the same. ;) We don't know the reason cresties don't regenerate, it's true, but I personally don't think it's because their tails are too complex.

I don't have much experience with Gargoyles, but don't cresties have tails that are much stronger and more dexterous? Just out of curiosity, why don't you believe it hjas anything to do with physical complexity? That's how it works elsewhere in the animal kingdom. I am not at all trying to argue, but this has intrigued me just enough to be willing to have a constructive debate on the subject.

I am open to any other logical suggestions as to why this phenomenon occurs. There is a reason for it, and I find it very hard to believe that no one knows. In just a few minutes via Google (while trying to find out whether or not Naultinus can regenerate) I found a plethora of VERY specific information about the structures and mechanisms that allow geckos (and other lizards and herps) to both drop and regenerate their tails. I am willing to bet someone out there knows exactly why cresties don't regenerate.      -p-

Well I have one question then, why on earth would a crestie need a more complex tail when if it dropped wouldn't grow back? It's a complete waste of evolution. Most cresties in the wild don't even have tails. So what would be the point, evolutionarily speaking, to have a more complex tail, and to drop it and then never have it again? Cresties can lose tails quite easily if startled, or even from a cage mate nipping it. Garg and crestie tails are very similar, I would say damn near identical. Cresties just don't grow back. I wouldn't say that Crestie tails are more strong and dexterous. But, then again I haven't done any research on it, and haven't googled it. I honestly don't care about the debate side of it haha. Too tiring with what I have going on right now (snake awareness day event coming up soon and I'm hosting it) I just gave my two cents, and what I observed owning these two species and conversing with other owners of the species. If someone more knowledgeable about the scientific facts on crestie vs garg tails or what evolutionarily happened to crestie tails wants a debate, then go for it. :)

The question here is, can crested geckos voluntarily move their tails to grip something, or do they react to stimulus only? From my experience, they only grip when in contact with something, same as gargoyles, snakes, etc. Definitely not the same thing as, say, a spider monkey who deliberately moves its tail to grip branches around it. Yes, monkeys have prehensile tails. Chahoua, another Rhac family gecko, can voluntarily curl their tails into a ball. Crested geckos cannot do that. They have a sticky pad at the end of their tails to aid in gripping, but they don't actively move their tails in a way that can truly be defined as prehensile.

@Rachelnjason- now you are asking a totally different question. The question is not whether or not the crested geckos tail is more complex. It is obviously more complex. More complex structures are required for a prehensile tail than for one that is not so. It is that simple. The question is whether or not this is the reason that the tail does not regenerate.

And who knows, maybe cresties are evolving away from a prehensile tail. According to the literature I found last night, most lizards and some snakes used to have far more complex structures for the purpose of tail regeneration (which I assume means that the regenerated tails were more functional than those seen on today's lizards).  

However, through evolution these structures have either been replaced with less complex systems (resulting in the types of regenerated tails we see now- no scales and no bone, just smooth skin and cartilage), or they have evolved to not drop their tails at all (like chameleons, monitors and most agamids). Though where crested geckos fall into place along these lines, I do not know. But again, that is not the question.      -p-  

@Ihkura- that is also NOT the question. If you will read what I posted above, you will see that what you are talking about is the difference between "partially" prehensile tails and "fully" prehensile tails. The difference between reptiles and monkeys. No reptile has a fully prehensile tail, regardless of it's dexterity. Have you ever seen a chameleon or look back and reach up to grab a branch with its tail? No. Because only mammals can do that, and probably only primates (possums seem to just have reactive tails).

The pattern I'm seeing here- the difference between my side of this debate and the other side is that I am backing up my theory (a theory that is shared by others) and also my other claims with facts, not my opinions. Google the word "prehensile". Look at the definition of the word on Wikipedia. You will see that the standards I am referring to strictly adhere to the actual accepted definition of this word, not my opinion about what it means.

And I am not at all saying that my theory is correct. It's just a logical possibility. Let's simplify this and take it back to the beginning before it strays any further. The phenomenon at question is the fact that crested geckos do not regenerate their tails. My theory, one that I have discussed with others who agree, is that maybe they do not regenerate their tails because they are prehensile and therefore more complex. The question is NOT whether or not they actually have prehensile tails. THEY DO. By definition. And the question is NOT whether or not a crested geckos tail is more structurally complex that that of a house gecko or a leopard gecko. IT IS inherently more so because IT IS prehensile.

So, I will ask one last time (because I really don't have the time to participate in a debate that is going nowhere)- Can anyone offer any other logical possible reason as to why crested geckos do not regenerate their tails? Thank you for your time.   

DIISCLAIMER: It is often difficult (if not impossible) to convey ones true meaning, emotion, intent, etc. through written word. So I will say it- it is not my intent to argue, to be negative, to offend. I hope that I don't come off as arrogant, elitist or a "know it all". I just love herps, and learning and sharing my knowledge and experiences with other herps is one of my favorite aspectys of the hobby. I love a good constructive debate. In a good debate, everyone learns something. Some may be right, some may be wrong, but everyone learns different perspectives, different ways of seeing things, different paths to certain truths.

The only reason I have committed this much time to this particular debate is because I was hoping to answer a question I have had for some time and to see if my theory was correct. And just FYI, I have kept and bred crested geckos (and cared for MANY at various jobs), and we have some now. They just aren't on our page yet. A large percentage of our herps are not on our page yet. Anyways, I just don't want anybody to be upset. It's sad that I even have to say any of this, but I find myself doing so often because of the way some people react. The moment it stops being fun is the moment I am either done with it or trying to fix it.       -p-

P.S. The first part about "written word" and "So I will say it" was supposed to be a subtle joke (since I am still obviously typing), but I realize now that it doesn't read well... thus proving my point about interpreting written word.       -p-

Another possibility as to why they can't regenerate their tails: some weird founder effect in the population; or for some reason the cresteds/ancestor that were able to drop their tails and not regenerate them simply had better fitness than those who regrew their tails; or perhaps a complexity reason like you proposed.  

My problem with the complexity explanation (that you ignored on Ihkura's response) is that is doesn't really have any good support, as chahoua and gargoyle tails (I'm not sure about leachies) can manipulate themelves the same way as crested tails, yet still retain the ability to regrow them after tail loss.