186 T. O. W. Kamgaing et al.
areas. Secondly,wemonitored dung piles in natural settings, whereas van Vliet et al. (2009) collected fresh dung pellets from animal intestines and installed the pellets in experi- mental settings. This method is suitable to measure precise dung survival time but many fresh dung piles deposited simultaneously in a given site could attract more beetles. Thirdly, the rapid removal of dung piles by beetles results in an overestimation of dung survival time, particularly for the major dry season (see below). The rapid removal of fresh dung piles by beetles has an-
other implication: including dung piles detected when they are older may overestimate dung survival time (Fig. 2). As beetles prefer to remove fresh dung, piles detected at an older age were those that had not been collected by beetles and were likely to remain for a longer time. To minimize this bias and to limit the risk of underestimating duiker den- sities, we discarded dung piles with an age at detection older than 6 hours from all analyses. Mean and median estimates of survival time of dung detected within 3 hours of deposition were similar to that of dung detected within 6 hours, except for that of the blue duiker in the major dry season. Ideally, therefore, we should have used only dung detected within 3 hours of deposition, but to have a sample size sufficient for analyses we included dung piles detected within 6 hours. Nevertheless, we may have missed some dung piles removed by beetles immediately after deposition. Especially in the dry season, therefore, the proportion of dung piles removed by beetles may be higher than our estimates suggest, which would cause an overesti- mation of mean dung survival time. Additionally, the rapid removal of fresh dung by beetles
suggests there are interactions between duikers and beetles. Low dung density does not necessarily mean low duiker density. An area containing many duikers is likely to har- bour many dung beetles, which remove dung as soon as it is deposited. It is at least possible there are different degrees of abundance and activity of beetles in different habitats. Future studies should therefore investigate dung survival time across habitats, including logged forests. Our estimate of dung survival time ismuch shorter (2-to
15-fold for the blue duiker, 2-to 14-fold for red duikers) than values commonly used to convert dung density into duiker population density in Central Africa. Provided that defaeca- tion rates of duikers are comparable between areas, lower values of dung survival time will result in higher estimates of duiker density. We found the survival time of duiker dung was substan-
tially shortened by beetle activity. The abundance and activ- ity of beetles probably affect the variance of survival time and its seasonality. Ideally, to estimate density of duikers from dung counts, dung survival time should be estimated in the same locality. The age of dung used to estimate survival time should be considered. Dung decay studies have generally used dung
piles up to 1 day old but this may overlook rapid removal of fresh dung by beetles and hence overestimate mean sur- vival time. To minimize the bias that results from this, we recommend that only dung ,3 hours old should be mon- itored for survival time, with revisits within a few hours and no longer than 1 day. To estimate duiker densities as accu- rately as possible, surveys of dung survival time should be carried out in the wet season, when variance is lower, and sufficient samples need to be collected for estimation of mean dung survival time and duiker densities.
Acknowledgements This work was supported by the Japan Science and Technology Agency–Japan International Cooperation/Science and Technology Research Partnership for Sustainable Development (Projet Coméca, JPMJSA1702), Japan, and Institute for Agronomic Research and Rural Development, Cameroon. We thank the Ministry of Scientific Research and Innovation and the Ministry of Forestry andWildlife, Cameroon for granting permission to carry out fieldwork in Nki National Park, the people of the study area for their support during fieldwork, and two anonymous reviewers for their critiques.
Author contributions Study design: TOWK, MT, HY; fieldwork: TOWK, ZCBD, NCBD; data analysis, writing: TOWK, HY.
Conflicts of interest None.
Ethical standards This research abided by the Oryx guidelines on ethical standards, and did not involve collection of animal specimens.
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