Directed seed dispersal: The case of howler monkey latrines

Abstract

Latrines of vertebrates are a good example of directed seed dispersal. Although the main problem for seed dispersal at such sites is the low per capita survival expected, because of the high densities of seeds and saplings, saplings of several species recruit in latrines. The aim of the present study was to determine the mechanisminvolved in the recruitment of saplings in latrines of Alouatta caraya (Primate, Atelidae). To this end, we evaluated the growth and recruitment of saplings of Ocotea diospyrifolia, Eugenia carthagenensis and Psychotria carthagenensis for two years. Our first hypothesis was that latrines are suitable for sapling growth and that saplingsin latrines are larger, grow faster and have higher nutrient concentration than those outside them, whereas our second (and opposite) hypothesis was that sapling recruitment in latrines is only a consequence of a large input of seeds and that the saplings in and outside latrines show similar size, growth rate and nutrient concentration. Considering the variability in the number of saplings recruited among latrines, we predicted the following: based on the first hypothesis, we predicted that the number of saplings recruited in latrines at the end of the study will be positively associated with the per capita survival probability in the latrine and that the survival probability will be related to the particular conditions of each latrine, whereas, based on the second hypothesis, we predicted that the number of saplings surviving in a latrine at the end of the study period will be positively associated with the number of conspecific saplings present at the beginning of our study. Our results showed that therecruitment of saplings in latrines was a consequence not only of the large input of seeds, but also of the fact that latrines are suitable for growth due to the nitrogen availability. However, each species had a different strategy to improve recruitment in latrines: O. diospyrifolia saplings required good conditions for growth, P. carthagenensis saplings required conditions favoring the saturation of mortality factors, and E. punicifolia saplings required either of both conditions. The availability of nitrogen in latrines appeared to be the main factor determining sapling recruitment in them, whereas that of phosphorus contributed by river floods would be the limiting factor for sapling growth.