Experimental manipulation reveals a trade‐off between weapons and testes

Abstract

Theory predicts a trade-off between sexually selected weapons used to secure mates and post-copulatory traits used to maximize fertilization success. However, individuals that have a greater capacity to acquire resources from the environment may invest more in both pre-and post-copulatory traits, and trade-offs may not be readily apparent. Here, we manipulate the phenotype of developing individuals to examine allocation trade-offs between weapons and testes in Mictis profana (Hemiptera: Coreidae), a species where the hind legs are sexually selected weapons used in contests over access to females. We experimentally prevented males from developing weapons by inducing them to autotomize their hind legs before the final moult to adulthood. We compared trait expression in this group to males where autotomy was induced in the mid-legs, which are presumably not under sexual selection to the same extent. We found males without weapons invested proportionally more in testes mass than those with their midlegs removed. Males that developed to adulthood without weapons did not differ from the mid-leg removal group in other traits potentially under precopulatory sexual selection, other post-copulatory traits or naturally selected traits. In addition, a sample of adult males from the same population in the wild revealed a positive correlation between investment in testes and weapons. Our study presents a critical contribution to a growing body of literature suggesting the allocation of resources to pre-and post-copulatory sexual traits is influenced by a resource allocation trade-off and that this trade-off may only be revealed with experimental manipulation.

Males that drop a sexually selected weapon grow larger testes

“…Thus, these results challenge the idea that morphological tradeoffs might be limited to closed systems, such as the pupae of holometabolous species (Moczek and Nijhout 2004;Tomkins et al 2005). In a study conducted shortly after this one, Somjee et al (2017) found evidence of a similar weapons-testes trade-off in another hemimetabolous insect in the same family (Coreidae). It is possible that resource allocation trade-offs are as prevalent in hemimetabolous species as the Holometabola.…”

“…In a study conducted shortly after this one, Somjee et al. () found evidence of a similar weapons‐testes trade‐off in another hemimetabolous insect in the same family (Coreidae). It is possible that resource allocation trade‐offs are as prevalent in hemimetabolous species as the Holometabola.…”

“…Our research provides a rare experimental example of a resource allocation trade‐off shared between pre‐ and postcopulatory sexually selected traits (Moczek and Nijhout ; Fry ; Simmons and Emlen ; Somjee et al. ).…”

“…We examined fertilization consequences of losing a hind-limb weapon and found that, when offspring were produced, males missing a hind limb had a fertilization advantage. Our research provides a rare experimental example of a resource allocation trade-off shared between pre-and postcopulatory sexually selected traits (Moczek and Nijhout 2004;Fry 2006;Simmons and Emlen 2006;Somjee et al 2017).…”

Stabilizing selection on sperm number revealed by artificial selection and experimental evolution

“…Typically, ejaculate investment is negatively correlated with traits associated with mate monopolization (e.g., Somjee et al. ) and, to a lesser extent, mate attraction (for a review see Simmons et al. ).…”