The role of extragroup encounters in a Neotropical, cooperative breeding primate, the common marmoset: a field playback experiment

Abstract

In cooperatively breeding species, encounters with intruders may serve multiple functions ranging from reaffirming group territory ranges to facilitating assessments for additional breeding opportunities. While these distinctive events offer the opportunity to investigate the delicate balance of these social dimensions within animal societies, their unpredictable occurrence makes witnessing and controlling these events in the wild particularly challenging. Here we used a field playback approach to simulate conspecific territorial incursions in cooperatively breeding common marmosets (Callithrix jacchus) to distinguish between the three following non-mutually exclusive functions of intergroup encounters in this species of New World primate: territorial defense, mate defense, and assessment of breeding opportunities. For these experiments, we systematically broadcast species-typical long-distance contact calls - phees - commonly used in intergroup interactions from the core and periphery of the groups' territories using either male or female vocalizations. Consistent with a territorial defense hypothesis, a group's reaction was independent of the simulated intruder's sex and the response strength was greater when the playback stimulus was broadcast from the core areas of groups' territories relative to stimulus broadcast from periphery areas. However, sex differences in some facets of their responses suggest that this is not the only potential function for these encounters. Mated males and females started to move first in response to simulated intruders of the opposite sex, suggesting that these events offered opportunities to assess extra-pair breeding opportunities, while the occurrence of females' piloerection towards simulated female intruders is suggestive of mate-guarding. These data provide unique experimental evidence for the theory that excursions by conspecific intruders may serve multiple functions in a cooperatively breeding vertebrate and are reflective of the known complexities of common marmoset sociobiology.

Keywords: Callitrichidae; common marmoset; cooperatively breeding; intergroup interactions; mate defense; mate-fidelity; neighbours assessment; sexual conflict; territory defense.

Figure 1.

Spatial representations of groups’ home ranges, significant resources and playback locations. (A) Maps on the left indicate the location of the study site, while the map on the right indicates the location shows an expanded view to indicate the location of each groups’ range at the site. Territorial ranges for Groups 1, 2 and 3 are shown. (B) Maps depict the significant Resources in the territorial range for Groups 1 (left), 2 (middle) and 3 (right). Feeding trees are indicated by black dots while sleeping trees are shown with white dots. (C) Maps depict the location of the playback broadcasts within the territorial ranges of Groups 1, 2 and 3. White dots indicate playback locations in the Core, while black dots indicate playback locations in the Periphery. (B & C) The polygons represent the total area calculated with Minimum Convex Polygon (MCP) using 100% of location points (group 1(A): 11.6 ha, group 2(B): 5.26 ha, group 3(C): 2.11 ha). The gray scale in the bottom graphics represents the utilization distribution (UD) estimated using the adaptive kernel method. The UD grayscale indicates the probability of finding the group in each location, with more frequently used areas in darker colors. The black circles highlight the location of core areas.

Figure 2.

Comparison of group’s reaction to simulated intruders in the Core and Periphery of each group’s home range. (A) Percentage of monkeys that arrived at loudspeaker location. (B) The speed to arrive in loudspeaker location. (C) The time spent in loudspeaker location). The horizontal line shows the median, the bottom and top of the box show the first and third quartiles, respectively, and vertical dashed lines show 1.5 times the interquartile range of the data (approximately 2 standard deviations). The dots beyond the vertical bars represent the outliers.

Figure 3.

Individuals which started to travel towards the loudspeaker. Mosaic plot depicting the association between stimuli sex and speaker location with individuals’ status (Mf: mated female; Mm: mated males; nMm: nonmated males). The width of each cell with respect to its axis indicates the proportional contribution of each variable level to the total. The colors represent the level of the residual (Pearson residuals) for each combination of levels, with the blue color indicating the cells in which the individuals of a specific status started to travel more often than would be expected by chance.

Figure 4.

Piloerection display by individuals’ status to playback stimuli presented from Core and Periphery areas. The mosaic plot depicts the association between stimuli sex and loudspeaker location with individuals’ status (Mf: mated female; Mm: mated males; nMm: nonmated males). The width of each cell with respect to its axis indicates the proportional contribution of each variable level to the total. The colors represent the level of the residual (Pearson residuals) for each combination of levels, with the blue color indicating the cells in which there are more observations of piloerection than would be expected by chance. The bar with circles indicate the trials in which the adults did not reacted with piloerection. While all female stimuli broadcasted from groups center stimulated piloerection in the mated females, it did not stimulate piloerection in males.