Long-tailed macaques are primarily arboreal (and are quite elusive in our site). Tracking them in the wild to observe defecation events requires A LOT of input. I'm fully aware of the huge difference between captive and wild populations but would it still be acceptable to do so?
I think it would depend how similar the diets of captive and wild macaques are. Fiber content plays a large role in passage time in the gut. Are there other macaque species where defecation rate has been assessed in multiple populations? That might give you some insight into how much variation there is as a result of diet differences.
You might struggle to publish the results unless you can validate that captive defecation rates match those in the wild (and that rates of detection in captivity and the wild are somewhat equivalent).
Before you go any further, I guess you should clarify what your aims are. Do you simply want to track the over-time variation in population size, or get reliable estimate of population size? In the first case, you may consider to use simpler population indexes (faecal group count -FGC- can be really time-consuming!). If population abundance is what you need, then FGC might be an option, but:
- as Kimberly said, its use largely depends upon the reliability of defecation rate (you should provide more details about your captive population: how about the size and environmental features of the enclosure, are they in any way comparable to the wild? Are captive animal fed ad libitum or not? Does defecation rate vary according to season?...). If you think the captive population live in conditions somewhat comparable to the wild one, then you may -with some caution- use that defecation rate (lots of studies simply assumed defecation rates based on other studies, sometimes on different species!);
- even if you can obtain reliable estimates of defecation rate, FGC may be affected by several other sampling issues, so pay attention to your sampling protocol! (normally you need a lot of plots to get reliable estimates, and this can make fieldwork really really tedious! A good sampling strategies, for example, could be to divide you area in grids and pick a random plot within each grid -is your area fully accessible? Otherwise you may risk to bias your estimate. Plus, are you 100% sure you can find all scats in the field? I am not a primatologist, but you talked about elusive species in a forest environment...that sounds tricky =)
To sum up, FGC is mathematically sound, but there are a lot of practical sampling issues to take into account. As said before, if you simply want to track the over-time variation, I would definitely not bother using FGC. If your aim is to obtain estimates of population size, you may use it, though I wonder if have the chance to try other strategies (mark-resight? camera traps? distance sampling? They also have flaws to be aware of, but at least the fieldwork required is much less than FGC.
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