Producers and scroungers: feeding-type composition changes with group size in a socially foraging spider

Abstract

In groups of socially foraging animals, feeding behaviour may change with group size in response to varying cost-benefit trade-offs. Numerous studies have described group-size effects on group-average feeding behaviour, particularly emphasizing an increase in scrounging incidence for larger groups, where individuals (scroungers) feed from the food sources others (producers) discovered. However, individual variation in feeding behaviour remains unconsidered in the vast majority of these studies even though theoretical models predict individuals to specialize in feeding tactic and anticipate higher scrounger-type frequencies in larger groups. We combined group-level and individual-level analyses of group-size effects on social foraging in the subsocial spider Australomisidia ergandros Lending novel experimental support to model predictions, we found that individuals specialize in feeding tactic and that higher scrounging and lower producing incidence in larger groups were mediated through shifts in the ratio of feeding types. Further, feeding-type specialization was not explained by innate individual differences in hunting ability as all feeding types were equally efficient in prey capture when foraging alone. Context adaptivity of feeding behaviour might allow this subsocial species to succeed under varying socioecological conditions.

Keywords: behavioural type; feeding tactics; group size; scrounging; social foraging.

Figure 1.

Stacked percentage bars show the relative differences in the proportion of feeding time spent producing, feeding alone and scrounging (given as mean ± s.e.) between the group-size treatments.

Figure 2.

(a) Silhouette plot for cluster validation; it shows how well each individual lies within its assigned cluster. Each grouping of bars represents a cluster, each bar stands for the membership level (s_i) of an individual to its assigned cluster. N_c denotes the number of individuals within the cluster, s_c the average silhouette width of the cluster. (b) The pie charts illustrate the percentage of individuals assigned to cluster 1 (light grey), 2 (grey) and 3 (dark grey) per group-size treatment.

Figure 3.

Barplots describe cluster characteristics. Barplot (a) shows differences between the individuals assigned to clusters 1, 2 or 3 in the proportions of feeding time they spent producing, feeding alone and scrounging (time measurements, table 1). Analogously, barplot (b) shows differences in the producing, feeding alone and scrounging proportion of feeding frequency (frequency measurements, table 1).