Harnessing ant defence at fruits reduces bruchid seed predation in a symbiotic ant-plant mutualism

Abstract

In horizontally transmitted mutualisms, mutualists disperse separately and reassemble in each generation with partners genetically unrelated to those in the previous generation. Because of this, there should be no selection on either partner to enhance the other's reproductive output directly. In symbiotic ant-plant mutualisms, myrmecophytic plants host defensive ant colonies, and ants defend the plants from herbivores. Plants and ants disperse separately, and, although ant defence can indirectly increase plant reproduction by reducing folivory, it is unclear whether ants can also directly increase plant reproduction by defending seeds. The neotropical tree Cordia alliodora hosts colonies of Azteca pittieri ants. The trees produce domatia where ants nest at stem nodes and also at the node between the peduncle and the rachides of the infloresence. Unlike the stem domatia, these reproductive domatia senesce after the tree fruits each year. In this study, I show that the tree's resident ant colony moves into these ephemeral reproductive domatia, where they tend honeydew-producing scale insects and patrol the nearby developing fruits. The presence of ants significantly reduced pre-dispersal seed predation by Amblycerus bruchid beetles, thereby directly increasing plant reproductive output.

Keywords: Azteca pittieri; Cordia alliodora; by-product benefits; horizontal transmission; partner fidelity feedback.

Figure 1.

(a) Cordia alliodora reproductive domatium with developing fruits and old leaves, pictured in February 2012. Azteca pittieri ants are visible on the surface of the domatium (inset). Photo credit: Enrique Ramírez-García. (b) Reproductive domatium post-senescence, pictured in June 2012. (Online version in colour.)

Figure 2.

(a) Time in minutes (mean + s.e.) for ants to find a termite pinned proximally to the reproductive domatium on a developing fruit or an old leaf (n = 25). Asterisk indicates p < 0.04 by matched-pair Wilcoxon-signed rank. (b) Number of ants (mean ± s.e.) recruiting to the fruit or leaf where the termite was pinned. Ants were counted each minute for 15 min after finding the termite on that structure (n = 14 fruit trials, nine leaf trials; repeated-measure mixed-effect ANOVA: for tree tissue (fruit or leaf), F = 18.58, p < 0.0001; for time, F = 20.33, p < 0.0001; for tissue × time, F = 0.06, p = 0.8).

Figure 3.

Mean proportion + standard error of seeds with bruchid exit holes from trees with and without ants. Inset, bruchid exit hole from a C. alliodora fruit. Asterisk indicates p < 0.02 by a Wilcoxon test. (Online version in colour.)