Geometry explains the benefits of division of labour in a leafcutter ant

Abstract

Many ant species have morphologically distinct worker sub-castes. This presumably increases colony efficiency and is thought to be optimized by natural selection. Optimality arguments are, however, often lacking in detail. In ants, the benefits of having workers in a range of sizes have rarely been explained mechanistically. In Atta leafcutter ants, large workers specialize in defence and also cut fruit. Fruit is soft and can be cut by smaller workers. Why, therefore, are large workers involved? According to the geometry hypothesis, cutting large pieces from three-dimensional objects like fruit is enhanced by longer mandibles. By contrast, long mandibles are not needed to cut leaves that are effectively two-dimensional. Our results from Atta laevigata support three predictions from the geometry hypothesis. First, larger workers cut larger fruit pieces. Second, the effect of large size is greater in cutting fruit than leaves. Third, the size of fruit pieces cut increases approximately in proportion to the cube of mandible length. Our results are a novel mechanistic example of how size variation among worker ants enhances division of labour.

Figure 1

(a) Large workers of Atta laevigata cutting mango. (b) Heads of a small and a large worker, showing the difference in head size and shape. The arrow in the right panel shows the distance between apical tooth tip and basal angle, which was used as the measure of mandible length.

Figure 2

Size distributions of workers cutting mango (white bars), cutting leaf (black bars) and workers on a trail (hatched bars). First Kruskal–Wallis ANOVA's are between mango and leaf cutters and second among all three classes for the two colonies where a trail sample was obtained. (a) Colony 1: KW=8.1, d.f.=1, p=0.004; (b) colony 2: KW=19.7, d.f.=1, p<0.001; KW=71.7, d.f.=2, p<0.001; (c) colony 3: KW=20.3, d.f.=1, p<0.001; KW=161.3, d.f.=2, p<0.001; (d) colony 4: KW=17.4, d.f.=1, p<0.001.

Figure 3

Regressions of mass (log transformed) of the item cut against mass of the cutting worker (log transformed). Slopes are given with 95% CI. (a) Mango pieces: (i) colony 1: slope=1.46 (1.06–1.87); (ii) colony 2: slope=1.02 (0.75–1.28); (iii) colony 3: slope=1.06 (0.83–1.29); (iv) colony 4: slope=1.18 (0.87–1.50). (b) Leaf pieces: (i) colony 1: slope=−0.02 (−0.36 to 0.33); (ii) colony 2: slope=0.56 (0.30–0.81); (iii) colony 3: slope=0.38 (0.24–0.51); (iv) colony 4: slope=0.17 (−0.09 to 0.42).

Figure 4

Regressions of mass of the mango piece cut (log transformed) against mandible length (log transformed) of the cutting worker. Slopes are given with 95% CI. (a) Mango pieces: (i) colony 1: slope=3.91 (3.10–4.72); (ii) colony 2: slope=4.33 (3.17–5.49); (iii) colony 3: slope=3.95 (2.87–5.03); (iv) colony 4: slope=3.89 (2.64–5.14). (b) Leaf pieces: (i) colony 1: slope=−0.02 (−1.18 to 1.23); (ii) colony 2: slope=2.02 (1.10–2.94); (iii) colony 3: slope=1.63 (0.54–2.69); (iv) colony 4: slope=0.74 (−0.21 to 1.72).