60% of primates in the world are threatened with extinction, while 75% have declining populations. The biggest threats to primates are the result of human activity. In this study, I have been looking at the primate Spix’s Saddleback tamarin (Leontocebus fuscicollis) and tried to estimate the home range size of four different study groups, while also trying to assess if there are differences in the type of trees in the areas where the tamarins are present compared to where they are absent. I have also compared the behaviour of the four different study groups. The data was collected in the Tambopata nature reserve in the Madre de Dios region in Peru during the summer of 2022, with the help and support from the research organisation Fauna Forever. To complement the observations, previously collected data from 2019-2021 was also used. The tamarins were followed using a group follow method, and the behaviour was noted every 10 minutes using an instantaneous scan sampling method. The tree composition data was gathered from previously created tree plots, made by the Fauna Forever staff. I found that the home range sizes of the four study groups varied between 2.6 ha and 17.9 ha. The data from the tree plots showed no statistically significant difference in where the tamarins chose to live. There was also no statistically significant difference between the four study groups in behaviour, except for how high up in the trees they groups spent their time. There was a lack of good data in this study, due to time- and material constraints, but overall the home ranges were of close to similar size as earlier studies, and the behaviour of the tamarins was in general also consistent with earlier studies. More research would be needed to answer the further questions that arose during this study concerning the ecological constraints of the tamarins home range.