Monkey Forest Tales: Why counting monkeys in the same place over many years?

Colombian squirrel monkeys (titi o fraile; Saimiri cassiquiarensis albigena)

In past days when talking with local people, from farm workers to landowners, we were asked several times why we are counting the monkeys year after year? What is the purpose of that? In today’s post, we are going to answer these questions.
Like us monkeys live for several decades, therefore like us every year there are many events happening in their lives, there are some babies born, some old animals die, some accidents can happens that kill some individuals, there are also some disease that also kill or make some individuals more prone to be kill or eaten by other animals.
So when we visit a forest fragment to count individuals, we register every event and observation that can indicate the health state of an individual. We also register every individual we found, their sex and approximated age. All data collected during a year is used to compare it with previous years data and that information give us information about how the population is in each fragment.
As the monkeys in the study area lives for more than 30 years, if we only visit the fragments one year we cannot say much about how the population is doing, we need to sample for more than one decade to know what is happening.
For example, thanks to the sampling we have done since 2004, we know the deaths of some males of Colombian squirrel and red howler monkeys in the area. We also have learned that some old females of Colombian squirrel monkeys seems to stop reproducing after certain age.
This information also help us now that for now the population of all the species we have been monitoring since 2004 in the study area is stable and not decrease despite the habitat fragmentation present in the area. However, 16 years is still not so long time if we have in mind that monkeys species in the study area lives around 30 years or more. So we still need to continue monitoring these populations for at least two more decades…
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