I recently posted this observation of a young white-tailed deer, but am unsure of how to mark it for age in annotations. According to my friend who lives where the picture was taken, this deer was born last year, which checks out with its mostly faded spots. It was still very near its mother and noticably smaller than her, but probably about ready to go off on its own soon. So would it be best to mark it as a juvenile, adult, or just leave the annotation field blank? And in general, what’s the best practice for marking age on animals like this that don’t have a clear transition point between juvenile and adult?
I think in general, an adult would be an individual that is sexually mature. Which is sometimes difficult to determine (although I don’t know much about deer).
It’s often subjective, and it depends on your knowledge of the specific species.
With the primates I work with we break them into Infant, Juvenile, Subadult, & Adult, but even with all the experience we have at our conservation organization we still sometimes have to discuss whether an individual falls into one category or another.
@tiwane’s suggestion that ‘adult’ should be when the individual is sexually mature is generally correct, but as he said that can be difficult to tell. Another way is to consider it ‘adult’ when it has complete adult coloration, but that also can be complicated in some species, particularly birds that change coloration depending on mating season.
In the case of this deer, I’d call it a sub-adult, but there is no annotation for that, so I’d probably call it a juvenile based on coloration, and make a note that it’s nearly out of the juvenile stage. That’s my personal, subjective perspective though.
Mammals are difficult in that regard and in many cases it might be better to just leave the annotation blank. In your example it could be nice to add a comment with the information you explained here.
There are other taxa, where the cut-off-points are much more clear, like most insects and spiders (if you see the right features in the photo).
While there are often complications with figuring out the age of birds, the adult vs. immature terminology is usually quite clear. The difference between seasonal plumages isn’t a consideration – we simply accept that there are two different adult plumages. Where there is a real issue, it’s with species like Snail Kites (females, anyways) or Wandering Albatross, where rather than a definitive adult plumage, they keep changing a bit every year for most of their lives.
Incidentally, the “sexual maturity” standard that works so well for other species is more problematic for birds (well, it would work if we chose to use it). There are quite a few species of birds that achieve sexual maturity well before they reach adult plumage. Scarlet Tanagers and Bald Eagles are both examples.
