Ten days in the life of a mandrill horde in the Lopé Reserve, Gabon.

Mandrills have long been known to form large aggregations in the wild, but it has proved difficult to obtain detailed information on the socioecology of these groups. An unusually large (>600) horde of mandrills was followed for ten days during the 1995 dry season in Central Gabon, and data were collected on group composition and ecology while the mandrills were in an area of forest-savanna mosaic habitat in the north of the Lopé Reserve. Three separate counts of most of the group showed that fully coloured "fatted" adult males were present throughout the horde at a mean ratio to other individuals of 1:21. Paler "non-fatted" adult and sub-adult males were also distributed throughout. Mandrill diet over the ten days consisted mainly of insects, seeds from forest trees, and leaves or stems of understory herbaceous plants. Feeding was extremely selective, with most food items consumed in a much higher proportion than would be predicted from their relative availability. Ranging data also showed that the mandrills foraged preferentially in certain forest types within the forest-savanna mosaic, namely in Marantaceae and Rocky Forest. It is suggested that one reason why mandrills pass through gallery forests and forest-savanna mosaic in the dry season in the Lopé Reserve is because they find fruit there from preferred species, which are no longer fruiting in the main forest block, thus allowing them to maintain the fruit component of their diet at a time of fruit shortage. © 1996 Wiley-Liss, Inc.