Loss of paired weapons leads to larger testes and a lighter load for dispersal

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.