Artificial enhancement of an extended phenotype signal increases investment in courtship by three-spined sticklebacks

Interactions between the components of a multiple-signal sexual display can be complex, and previous work has shown that alteration of one component can lead to changed investment in either the altered or other display components. Extended phenotype signals (nonbody structures that carry a signalling function) provide an ideal means to manipulate signal quality in a noninvasive manner, to investigate investment patterns in display components. We made three predictions as to how males might alter investment in display components in response to artificial enhancement of an extended phenotype signal, and tested those predictions using three-spined sticklebacks, Gasterosteus aculeatus. The addition of brightly coloured ornaments (threads) to the nests of male sticklebacks led to increased investment in both courtship of females and nest construction. In a field experiment, males offered coloured threads spent increased time engaged in interactions with females, and in the laboratory, they built nests that were neater and more compact (better quality), relative to males offered dull threads. Our findings support a hypothesis based on resource budgeting, and suggest that resources saved by having an artificially enhanced nest are reallocated to other courtship behaviours. The study provides a framework for investigating the interaction between signal components, and demonstrates that manipulation of extended phenotype signals can provide insight into the ways in which animals balance investment in interacting signal components in sexual displays.