Biotic seed dispersal is a key process maintaining biodiversity in tropical forests where most trees produce vertebrate‐dispersed seeds. Existing meta‐analyses suggest an overall positive effect of vertebrate gut passage on seed germination, but no significant effects for non‐flying mammals. However, previous meta‐analyses combined rodents (seed predators) and primates (seed dispersers) into the non‐flying mammals category, which may confound specific effects of each group on seed germination. However positive effects of monkeys on germination had previously been found in some studies. Here we disentangle the role of Neotropical primates as contributors to seed dispersal in tropical forests by running a meta‐analysis to determine the overall magnitude of gut passage effects on seed germination percentage and mean time to germination. We also compare effect sizes as a function of different feeding guilds, gut complexities, and seed size. Our results show a strong, positive effect of primates on seed germination percentage and on the number of days to first germination. Strictly frugivorous monkeys, the group most threatened by extinction, showed the highest dispersal quality, increasing germination percentage by 75%. Primates that include insects in their diets had no average effect on germination percentage or time. Gut passage had different outcomes on seeds with different sizes; both large and small seeds showed similar increases in germination percentages after gut passage, but only large seeds germinated faster than control seeds after gut passage. Our results show a relevant role for primates in providing high seed dispersal quality and as drivers of forest regeneration. The combined effects of defaunation and forest fragmentation may result in decreased regeneration of trees, which has the potential to affect negatively both forest structure and ecosystem processes. Finally, we provide general guidelines for standardizing research on seed dispersal by primates.

Hybridization is continually documented in primates, but effects of natural and anthropogenic hybridization on biodiversity are still unclear and differentiating between these contexts remains challenging in regards to primate evolution and conservation. Here, we examine hybridization effects on the mitochondrial DNA (mtDNA) control region of Callithrix marmosets, which provide a unique glimpse into interspecific mating under distinct anthropogenic and natural conditions. DNA was sampled from 40 marmosets along a 50-km transect from a previously uncharacterized hybrid zone in NE Brazil between the ranges of Callithrix jacchus and Callithrix penicillata. DNA was also collected from 46 marmosets along a 30-km transect in a hybrid zone in Rio de Janeiro state, Brazil, where exotic marmosets appeared in the 1980s. Combining Callithrix DNA sampled inside and outside of these hybrid zones, phylogenetic and network analyses show C. jacchus and C. penicillata being parental species to sampled hybrids. We expand limited Callithrix population genetics work by describing mtDNA diversity and demographic history of these parental species. We show ancient population expansion in C. jacchus and historically constant population size in C. penicillata, with the latter being more genetically diverse than the former. The natural hybrid zone contained higher genetic diversity relative to the anthropogenic zone. While our data suggest hybrid swarm formation within the anthropogenic zone due to removed physical reproductive barriers, this pattern is not seen in the natural hybrid zone. These results suggest different genetic dynamics within natural and anthropogenic hybridization contexts that carry important implications for primate evolution and conservation.

Animal hybridization is well documented, but evolutionary outcomes and conservation priorities often differ for natural and anthropogenic hybrids. Among primates, an order with many endangered species, the two contexts can be hard to disentangle from one another, which carries important conservation implications. Callithrix marmosets give us a unique glimpse of genetic hybridization effects under distinct natural and human-induced contexts. Here, we use a 44 autosomal microsatellite marker panel to examine genome-wide admixture levels and introgression at a natural C. jacchus and C. penicillata species border along the São Francisco River in NE Brazil and in an area of Rio de Janeiro state where humans introduced these species exotically. Additionally, we describe for the first time autosomal genetic diversity in wild C. penicillata and expand previous C. jacchus genetic data. We characterize admixture within the natural zone as bimodal where hybrid ancestry is biased toward one parental species or the other. We also show evidence that São Francisco River islands are gateways for bidirectional gene flow across the species border. In the anthropogenic zone, marmosets essentially form a hybrid swarm with intermediate levels of admixture, likely from the absence of strong physical barriers to interspecific breeding. Our data show that while hybridization can occur naturally, the presence of physical, even if leaky, barriers to hybridization is important for maintaining species genetic integrity. Thus, we suggest further study of hybridization under different contexts to set well informed conservation guidelines for hybrid populations that often fit somewhere between “natural” and “man-made.”

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