Germination of native and exotic seeds dispersed by wild black-and-gold howler monkeys (Alouatta caraya): assessing deinhibition and scarification effects.

Fruit ingestion by frugivores and the subsequent passage of seeds through their digestive tract can influence seed germination patterns. The removal of germination inhibitors contained in the fruit pulp (deinhibition effect), and/or the abrasion of the seed coat (scarification effect), can modify the probability and speed of seed germination. We followed seven groups of black-and-gold howler monkeys (Alouatta caraya) in northeastern Argentina between November 2012 and January 2013 to study the effect of seed passage through the howler monkeys' gut on seed germination. For three native and one exotic species, we compared the proportion of germination and germination times among seeds in three different treatments: gut-passed seeds (GP), manually extracted seeds from fruits (ME), and seeds in intact fruits (IF). Paired comparisons between treatments allowed us to assess the overall effect of seed passage (total effect without distinguishing individual effects, GP vs. IF), deinhibition effect (ME vs. IF), and scarification effect (GP vs. ME). Our results suggest that passage through the howler monkeys' gut enhances seed germination by an increase in the proportion of germinated seeds and/or by a reduction in germination times. We found that deinhibition enhanced germination in three out of the four studied species, while scarification enhanced germination in one and decreased germination in another species. Our work highlights that gut passage may affect seed germination by different mechanisms, and all of these mechanisms should be considered in germination studies. It also emphasizes the importance of A. caraya as a seed disperser in northeastern Argentina, showing the high quality of the treatment provided to seeds.

Who is coordinating collective movements in black and gold howler monkeys?

Decisions about when and where to travel are likely to have a strong influence on the feeding, ecology, and foraging strategies of individual primates living in a cohesive social group. Specifically, given differences in age, sex, reproductive status, or social dominance, particular group members may benefit from remaining at their present location while others may benefit from traveling to another area of their range to feed or rest. In this study, we present data on movement coordination in two groups of wild black and gold howler monkeys inhabiting Isla Brasilera (27º 20'S and 58º 40'W) in northern Argentina. We examine how factors such as sex, age, reproductive status, and dominance affect patterns of group movement coordination at feeding or resting sites, and in the context of intergroup encounters. Two groups were followed five days a month from sunrise to sunset during June to November 2004. Using focal and scan sampling techniques, we recorded 262 group displacements, the identity of the individual initiating and leading displacement, and the identity of the first individual to arrive at feeding, resting, or intergroup encounter sites. We found that overall age was the only factor that influenced group coordination: adults led more often (94.5 %) than immature individuals (5.5 %) in both groups. We did not find differences among adults. However, we found that males lead more often than females at intergroup encounters, consistent with the male-mate defense hypothesis. The distributed leadership pattern among adults observed in this study may suggest that adult individuals make equally shared consensus decisions. This pattern should be further examined using this individual-level approach in other populations of black and gold howlers, other species of howlers, and in other atelines in which within-group social tolerance is the rule rather than the exception.