Increased terrestriality in a Neotropical primate living on islands with reduced predation risk.

An arboreal lifestyle is thought to be central to primate origins, and most extant primate species still live in the trees. Nonetheless, terrestrial locomotion is a widespread adaptation that has arisen repeatedly within the primate lineage. The absence of terrestriality among the New World monkeys (Platyrrhini) is thus notable and raises questions about the ecological pressures that constrain the expansion of platyrrhines into terrestrial niches. Here, we report the results of a natural experiment, comparing patterns of terrestrial behavior in white-faced capuchin monkeys (Cebus capucinus imitator) living on two islands off the Pacific coast of Panama that lack mammalian predators (island sites) with the behavior of capuchins at three sites in central Panama with more intact predator communities (mainland sites). Surveys with camera traps revealed increased terrestriality in island vs. mainland sites. Capuchin detection rates were higher, the range of party sizes observed was larger, and individuals engaged in a wider range of terrestrial behaviors on the islands lacking mammalian predators. Furthermore, females carrying infants were frequently photographed on the ground at the island sites, but never at the mainland sites. These findings support the long-standing hypothesis that predators constrain the exploitation of terrestrial niches by primates. These results are also consistent with the hypothesis that arboreal locomotion imposes costs that primates will avoid by walking on the ground when predation risk is low.

Parallel altitudinal clines reveal trends in adaptive evolution of genome size in Zea mays.

While the vast majority of genome size variation in plants is due to differences in repetitive sequence, we know little about how selection acts on repeat content in natural populations. Here we investigate parallel changes in intraspecific genome size and repeat content of domesticated maize (Zea mays) landraces and their wild relative teosinte across altitudinal gradients in Mesoamerica and South America. We combine genotyping, low coverage whole-genome sequence data, and flow cytometry to test for evidence of selection on genome size and individual repeat abundance. We find that population structure alone cannot explain the observed variation, implying that clinal patterns of genome size are maintained by natural selection. Our modeling additionally provides evidence of selection on individual heterochromatic knob repeats, likely due to their large individual contribution to genome size. To better understand the phenotypes driving selection on genome size, we conducted a growth chamber experiment using a population of highland teosinte exhibiting extensive variation in genome size. We find weak support for a positive correlation between genome size and cell size, but stronger support for a negative correlation between genome size and the rate of cell production. Reanalyzing published data of cell counts in maize shoot apical meristems, we then identify a negative correlation between cell production rate and flowering time. Together, our data suggest a model in which variation in genome size is driven by natural selection on flowering time across altitudinal clines, connecting intraspecific variation in repetitive sequence to important differences in adaptive phenotypes.

A Saki Saga: Dynamic and Disruptive Relationships among Pithecia aequatorialis in Ecuador.

Saki monkeys live in socially monogamous groups and in groups containing more than one same-sex adult. As part of a 10-year study of equatorial sakis (Pithecia aequatorialis) in Ecuador, we documented the immigration of a second adult male into a group containing a resident male-female pair that had associated with one another for seven years and the resident female's two daughters. In the first month after immigration, the resident male spent more time closer to and grooming his putative adult daughter than the resident female, and the two males were seen performing a cooperative territorial display. After two months, the resident male interacted more with the resident female than with his putative adult daughter, while that daughter interacted more with the immigrant male and copulated with him. After three months, the males left the group together and associated with an unfamiliar female, leaving the resident females and a neonate behind. The resident male then paired with a new female, while the immigrant male joined another group, again as a second male. Compared to other socially monogamous primates, sakis appear to have a more variable social system whereby additional males can join established groups and form relationships with putatively unrelated males.