Loss of Paired Weapons Leads to Larger Testes and a Lighter Load for Dispersal

TL;DR

Male cactus bugs that lose their leg weapons grow larger testes and may fly farther due to reduced weight.

Contribution

The study reveals a direct, quantity-specific trade-off between weapon loss and testes growth in cactus bugs.

Findings

Testes size increased by 9% with one lost hindleg and 20% with two lost hindlegs.

Males without weapons did not increase flight muscle investment but may disperse farther due to reduced weight.

Abstract

Reproduction is often costly for males, as it may require the growth of structural traits that aid in dispersal to find females, competition over mating opportunities, and ejaculate production. The growth of such traits can be energetically demanding, and these demands often arise concurrently during development. As such, these traits may be especially prone to resource allocation trade‐offs. Yet, such traits are rarely studied in tandem. We designed a study to improve understanding of investment dynamics in flight muscle, a dispersal trait; a sexually selected weapon used in mate competition; and testes used for sperm production. We used the leaf‐footed cactus bug,
Narnia femorata
(Hemiptera: Coreidae), a species where males use their hindleg as weapons to compete for matings. Males can naturally drop their limbs, and when hindlegs are lost during development, adult males do not grow a weapon. Existing studies have revealed that testes growth increases when investment in weapons ceases. Yet, this work only examined responses to the loss of a single hindleg and limited the scope of traits to testes. Here, we examined weapon loss at two levels and investigated a third trait: dispersal. We found that testes size increased stepwise with limb loss; the loss of one hindleg weapon increased testes mass by around 9%, and two legs increased it by 20%. This intriguing pattern suggests a direct, quantity‐specific trade‐off in tissue development across traits. We also detected only a limited increase in dispersal investment when males did not grow weapons. Yet, dispersal may still be enhanced for those that drop hind legs; those without the substantial weight of hind limbs may have the potential to disperse farther.

We addressed a weapons testes trade‐off using natural limb loss as an experimental manipulation of weaponized legs in the leaf‐footed cactus bug. We found that individuals missing weapons invested in their testes proportional to the number of weaponized legs they lost. We also investigated a third trait, dispersal, and found that individuals missing weapons did not alter their level of investment in flight muscle mass but should be capable of flying further due to their lack of bulky weapons.

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