When does an animal become adult?

First of all: I’ve tried looking for something similar to this in the forum but haven’t been able to find any related thread so far. Sorry if I missed it.

My question is: how do you deal with age annotations when dealing with animals? When would a given individual be considered an adult? Once it can start breeding? Once it looks like adult individuals of the species look?

Some taxa may be more obvious than others. Take butterflies, for instance: if it is not a recently moulted teneral, a buttefly would be an adult, while its caterpillar version would be a larva. However, birds may be trickier: many birds (specially small species) start moulting right after the breeding season, and may get an adult-looking appearance well ahead of the next breeding season.

Is there an official take on this? How do you usually proceed?


I usually mark an animal as an adult if it is at least close to adult size. Some animals, such as gray tree frogs, eastern rat snakes, and North American racers will have different juvenile and adult coloration, in which case I will mark the annotation as whichever it looks closest.


If you break down biology, adult means beeing bodily able to breed… (of course as always, be aware of exceptions)

As you said, in many animals (the taxa I mostly deal with) the adult form can be pretty obvious. In spiders, the genital region (epigyne or male palps) are heavily sclerotized and mark an adult individuum. In almost all insects the adult form and the larval form are quite different from each other, not only if they undergo a complete metamorphosis like in butterflies and beetles, but also if the metamorphosis is incomplete like in orthopterans or heteropterans.

I think it can be pretty difficult in mammals, birds and some fish for example to determine that state. So one uses an estimate. In mammals, many non-adult forms do have the “Kindchenschema” (childlike characteristics… sorry, don´t know if there is a translation for this?). So if they reach a certain size and do not show them anymore, one could assume it must be an adult then. I am not too much into birds, but I know that many change their plumage when becoming adult and at least adult males are often pretty clearly adult. Also, behavioural context can be important. If you observe mating behaviour, you can also assum you observed adults there.

I personally often do not use the age annotations in birds and mammals, unless I am pretty sure (e.g. observing a bird breeding)


If you are not sure, just omit that annotation.


For insects in general it’s pretty easy, holometabolic and heterometabolic adult insects have developed wings and are sexually mature and won’t molt anymore, so in these groups anything without wings or not mature wings must be a larvae or nymph respectively. Of course there’s exceptions like apterygota, very primitive insects or hexapods without wings, and groups that mostly lost their wings during evolution like non-reproductive casts of ants and termites, or fleas (groups where if they’re not larvae they’re adults) and lice (which keep a similar appareance all their life cycle like apterygota). For vertebrates is another story.


For birds, we generally use the term adult to refer to the definitive plumages, both breeding and non-breeding plumages. In other words, the point at which the plumage doesn’t really change from year to year. That doesn’t always refer to the ability to breed - Bald Eagles, for example, take 5 years to reach adult plumage, although they can breed by their third year. (Most don’t get the chance, of course.)
It gets more complicated with some species, like Snail Kites (the females, at least) and the large albatrosses, where their plumage can change from year to year for many years.
In many cases, distinguishing adult birds from young ones is quite tricky, requiring a close look at the wing coverts, etc.


Trying to click the correct descriptor for observations has certainly made me appreciate creatures that molt! That final molt creates a clear adult state. For others, well, I breathe a sigh of relief at January 1st, when I count virtually all birds and small mammals as adult so no problem until the breeding season is well under way.


I used to be an evolutionary demographer, so I’ve been party to far to many hair-splitting discussions on this. What it comes down to is that evolution leads to too many wonderful complexities, so there is no on-size-fits-all answer. Even in humans, it is a hard question to answer. It is unfortunately the case that some humans end up reproducing at an age when we would all agree they are not yet adults. One child giving birth at five years old is horrific, and not an indication that all five year olds, or even that one, are adult. So generally an individual’s biological adulthood is not considered to start until it reaches the age when most members of its population are fully grown and ready to successfully reproduce. Except that definition is full of holes and exceptions. For species that grow indefinitely, “fully grown” is not a good criterion. For species where age is not closely correlated with life stage, it doesn’t work either. Etc. So again, there can be no generally useful definition of adulthood across all taxa.


When it completes the moult into first adult plumage, it’s annotated as an adult.


Thanks! That’s the way I think when adding observations of roe deer, for instance. When you see a young individual next to a parent, the size difference becomes obvious. But I often hesitate and think it may look like an adult but still not be one :sweat_smile:. Males may start developing antlers pretty soon. In the end, I only annotate very young individuals when observed next to their mother, and leave all other potential adults with no annotation whatsoever. No easy answer, I guess ;)


Sounds liek a good approach. Definitely don’t feel the need to annotate something if you’re not sure, there are plenty of other observations to annotate.

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Yours is a perfect summary of the issue, thank you! The behavioural approach is a very interesting one, good point. There are many examples where the way an animal acts can be a good hint as to its actual age.

The Kindchenschema may also be of great help with big mammals, such as the roe deer I mentioned in a previous answer, but it gets very diluted in smaller species or in birds once they’re past the fledging stage!

Thanks for your input!

That’s my default approach :sweat_smile:. I used to add the ‘adult’ annotation to many bird species when the individual showed an adult-like plumage. As I learnt that first-calendar-year birds may already have moulted into it, I started leaving the annotation blank.

Exactly, you’re right. I sometimes struggle with some hemimetabolic species, such as Orthoptera or Hemiptera. I don’t know them well enough for me to tell when a grasshopper has reached its final size and appearance… Odonata on the other side, though hemimetabolic as well, seem to be easier. Otherwise, taking a look at the presence or abscence of wings is usually very helpful, as you say. Thanks!

Yup, birds are tricky :sweat_smile:. Sometimes they may display the main features of an adult plumage, but maybe a closer look —if we were to hold the bird in the hand— would prove that they still retain some juvenile coverts or who knows what :man_shrugging:t4:.

Hahaha, that’s a good one :smile:. I’ll keep that in mind from January the 1st to mid February, when some early birds can already be seen working on their nest around here :sweat_smile:. Thank you, @sedgequeen!

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I’m afraid that’s the kind of answer I expected. There’s no answer! :sweat_smile: Thanks, @dlevitis. It is indeed a very interesting discussion to have.

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That used to be my approach for a while. But then I realized that some birds may start looking as an adult even during their first calendar year. That’s when I started hesitating as to if there would be some kind of consesus around this issue. Thanks, Marina!

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I think the English word is neoteny.



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