Trade‐offs between weapons and testes do not manifest at high social densities

Abstract

Social conditions can alter the allocation of resources to reproductive traits. For example, an increase in social density during development is frequently associated with an increase in the testes mass of males. Sperm competition theory assumes that increased investment in testes should come at the expense of investing into precopulatory traits, such as sexually selected weaponry. However, much remains unknown about the role of the social context on the concurrent, relative investment in both testes and weapons. We found that the leaf‐footed cactus bug, Narnia femorata (Hemiptera: Coreidae), grew nearly 20% larger testes when raised in high social densities. In addition to manipulating social density, we used autotomy (limb loss) to limit investment in their hindlimb weapon during development. At low densities, we found that those that lost a weapon during development grew larger testes by adulthood, supporting previous work demonstrating a weapons–testes trade‐off. However, at high social densities, males that dropped a hindlimb did not grow larger testes, though testes were already large at this density. These results underscore the importance of the social context to resource allocation patterns within the individual.

The trade‐off between investment in weapons and fertility is mediated through spermatogenesis in the leaf‐footed cactus bug Narnia femorata

“…We took advantage of the wealth of knowledge about a related model insect, O. fasciatus, to explore how loss of a weapon during testis development impacted spermatogenesis in N. femorata. We found an increase in testis size and fertilization advantage in autotomized male N. femorata that has been previously documented (Joseph et al, 2018;Miller et al, 2019;Miller et al, 2021) was associated with an increased rate of mitotic divisions in spermatogonia.…”

“…While hind limbs are crucial for winning fights with other males (Emberts et al, 2016), twelve percent of adult male N. femorata in the wild are missing one or more limbs through the process of autotomy (Emberts et al, 2016). Autotomy is used by N. femorata of all ages to escape entrapment, and autotomy does not reduce their survival in a laboratory setting (Joseph et al, 2018;Miller et al, 2021).…”

“…Instead, autotomized males reallocate resources into postcopulatory reproductive success by growing larger testes (Joseph et al, 2018;Miller et al, 2019;Miller et al, 2021). They also have a fertilization advantage over intact males (Joseph et al, 2018;Cirino et al, 2021).…”

Runaway evolution from male-male competition

“…CC-BY-NC-ND 4.0 International license made available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprint this version posted May 17, 2021. ; https://doi.org/10.1101/2021.05.17.444494 doi: bioRxiv preprint interacting phenotypes because behavioral responses in contests between males are socially contingent (38). When fitness is determined by male-male competition and when aggression is potentially costly, males should be expected to evolve strategies wherein they modulate their behavior in response to phenotypes of their competitors that signal willingness or ability to fight.…”

“…We focus on the role of social context in male-male competition, with the goal of exploring the potential for extreme elaboration evolving through this process alone. We adopt the genetic framework of interacting phenotypes because behavioral responses in contests between males are socially contingent (38). When fitness is determined by male-male competition and when aggression is potentially costly, males should be expected to evolve strategies wherein they modulate their behavior in response to phenotypes of their competitors that signal willingness or ability to fight.…”