Age- and sex- specific survival trajectories in the endangered Indri (Indri indri) in Maromizaha forest, Madagascar

Indri (Indri indri) is the largest extant lemur and, since 2014, it has been classified by the IUCN as Critically Endangered. The Indri's population size is decreasing and the rate of decline is projected to accelerate over the next three generations due to habitat loss, climate change and illegal hunting. Indri inhabits the eastern rainforests of Madagascar, living exclusively on trees and feeding on fruit and leaves. The animals live in family groups composed by 2-6 individuals (the adult pair and their offspring). The Indri society is generally based on female dominance, which is mainly expressed in feeding contests, with the reproductive female having priority access to resources. Females give birth to a single offspring every 2-3 years and juvenile reach sexual maturity at an age of 7-9 years. Once subordinates become mature they tend to migrate from their natal group. This research was conducted on the Indri wild groups that inhabit the Maromizaha Forest Protected Area. The study is part of a ten-years ongoing project based at the Maromizaha Multipurpose Centre, built in 2010 as a support for researchers and an added value to the reserve activity. Monitoring is essential in the collection of long-term data series and the databases built by researches allow the development of knowledge necessary for population analysis and insights. Eight Indri groups were regularly recorded within the ten-year data collection period, providing a sufficient timeframe for applying a survival analysis. A total of 63 individuals were included in the research. Six parameters were selected with the purpose to conduct a life history study that returns survival and mortality trajectories for this species. The study provides demographic models that include sex and social status as influent factors on an individual life perspective. Data was processed in the open source statistical environment R using the package “Bayesian survival trajectory analysis” (BaSTA), which works under a Bayesian framework. It was found that sex does not significantly influence the survival probability of Indri, whereas dominance is a more influent factor, which is also related to juvenile dispersal. Indeed, individuals leaving their natal group represent a strong influence on the dominant/subordinate survival rates. Moreover, based on the current state of literature on the matter, the general pattern of survival in Indri appears similar to Primates in general, with high infant mortality (mainly due to environmental causes) followed by stability during the juvenile and adult phases and increased mortality in the late stages of life. The subject should be further explored with future research on demographic models of survival. In particular, dominance influence and juvenile dispersal represent valuable aspects to develop, as they would increase the knowledge on Indri's life history and consequently provide positive conservation implications.