Coevolution of social and communicative complexity in lemurs

Abstract

The endemic lemurs of Madagascar (Lemuriformes: Primates) exhibit great social and communicative diversity. Given their independent evolutionary history, lemurs provide an excellent opportunity to identify fundamental principles in the coevolution of social and communicative traits. We conducted comparative phylogenetic analyses to examine patterns of interspecific variation among measures of social complexity and repertoire sizes in the vocal, olfactory and visual modality, while controlling for environmental factors such as habitat and number of sympatric species. We also examined potential trade-offs in signal evolution as well as coevolution between body mass or brain size and communicative complexity. Repertoire sizes in the vocal, olfactory and visual modality correlated positively with group size, but not with environmental factors. Evolutionary changes in social complexity presumably antedated corresponding changes in communicative complexity. There was no trade-off in the evolution of signals in different modalities and neither body mass nor brain size correlated with any repertoire size. Hence, communicative complexity coevolved with social complexity across different modalities, possibly to service social relationships flexibly and effectively in pair- and group-living species. Our analyses shed light on the requirements and adaptive possibilities in the coevolution of core elements of social organization and social structure in a basal primate lineage. This article is part of the theme issue 'Cognition, communication and social bonds in primates'.

Keywords: communicative complexity; olfaction; primates; social complexity; visual signals; vocalizations.

Figure 1.

Relationships between communicative and social complexity across modalities. (a) Mean repertoire size as function of group size. (b) Photos depict a black-and-white ruffed lemur and an indri vocalizing. (c) Number of sources for olfactory signals as a function of group size. (d) A male Verreaux's sifakas with a chest gland (brown spot on the chest) and a redfronted lemur depositing a scent-mark with saliva while chewing on a branch, wearing a neck collar to facilitate individual recognition. (e) Number of visual signals as a function of group size. (f) A redtailed sportive lemur displaying the faint-to-cuff gesture, wearing a radio-collar and two Coquerel's sifakas displaying the open-mouth facial expression while playing. Lines indicate regression lines of (a) model table 1b, (c) model table 2b and (e) model table 3b. Social organization is indicated by colours, with solitary species = blue, pair-living species = yellow and group-living species = red. X-axis labels are back-transformed to original values.

Figure 2.

Relationship between (a) mean vocal repertoire size and the number olfactory sources and (b) the number of visual signals as well as (c) the number olfactory sources and visual signals. Lines indicate the regression lines. Activity patterns are indicated by colours, with blue indicating solitary species, yellow indicating pair-living species and red indicating group-living species.