Intragroup competition predicts individual foraging specialisation in a group-living mammal

Abstract

Individual foraging specialisation has important ecological implications, but its causes in group-living species are unclear. One of the major consequences of group living is increased intragroup competition for resources. Foraging theory predicts that with increased competition, individuals should add new prey items to their diet, widening their foraging niche ('optimal foraging hypothesis'). However, classic competition theory suggests the opposite: that increased competition leads to niche partitioning and greater individual foraging specialisation ('niche partitioning hypothesis'). We tested these opposing predictions in wild, group-living banded mongooses (Mungos mungo), using stable isotope analysis of banded mongoose whiskers to quantify individual and group foraging niche. Individual foraging niche size declined with increasing group size, despite all groups having a similar overall niche size. Our findings support the prediction that competition promotes niche partitioning within social groups and suggest that individual foraging specialisation may play an important role in the formation of stable social groupings.

Keywords: Mungos mungo; Banded mongoose; competition; foraging behaviour; foraging niche; group-living; social group; specialisation; stable isotope.

Figure 1

Calculating the relative individual niche index (RINI) in banded mongoose groups (Mungos mungo). (a) In each group (group 21 in this example) individual 95% prediction ellipse areas, corrected for sample size (ell95c, Jackson et al. 2011) were overlaid. The outline of the area these overlaid ellipses covered was calculated (b) to create a group niche area (c) which the individual ellipse areas in (a) were expressed as a proportion of. In each plot: colours represents different individuals; points represents carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios obtained from vibrissa samples (in this example 17 samples from four individuals); thin coloured lines show individual's 95% prediction ellipses; the think black line shows the estimated group niche area.

Figure 2

Individual banded mongooses (Mungos mungo) in larger groups (a) gained less weight day‐to‐day and (b) overall were in poorer condition. Points and error bars are the mean and standard errors (n = 12 592 weight records from 264 individuals in 11 groups measured between 2000 and 2016) for each group size and lines are the relationships predicted by our models with all other variables set at their mean.

Figure 3

Banded mongoose (Mungos mungo) vibrissa nitrogen (δ¹⁵N) and carbon (δ¹³C) isotope ratios. The data are divided into social groups by colour. Each point represents one vibrissa sample collected from an individual (760 vibrissa samples from 10 social groups). Ellipses are the 95% prediction ellipses corrected for sample size (ell95c) calculated from these data for each social group.

Figure 4

Individual banded mongooses (Mungos mungo) in larger groups have smaller foraging niches, measured as a proportion of their group's niche (RINI, see Fig. 1). Panel a shows the raw data (points; 315 samples from 64 individuals) and relationship ± SE (line and shaded area) predicted by our model with group size as a continuous variable with all other variables set at their mean. Group size data were bimodal (panel b) and so we divided our data into individuals from small and large groups (denoted by dotted line at 17 individuals in panel b) and refitted our model with group as a categorical. This also showed that individuals in larger groups occupied smaller foraging niches (panel c). The box‐and‐whisker plot in panel c shows the median (thick horizontal line), interquartile range (boxes) and 1.5 times the interquartile range (whiskers) for the data in small and large groups. The points show data that fall outside of this range.