Males that drop a sexually selected weapon grow larger testes

Abstract

Costly sexually selected weapons are predicted to trade off with postcopulatory traits, such as testes. Although weapons can be important for achieving access to females, individuals of some species can permanently drop (i.e. autotomize) their weapons, without regeneration, to escape danger. We capitalized on this natural behavior to experimentally address whether the loss of a sexually selected weapon leads to increased testes investment in the leaf-footed cactus bug, Narnia femorata Stål (Hemiptera: Coreidae). In a second experiment, we measured offspring production for males that lost a weapon during development. As predicted, males that dropped a hind limb during development grew significantly larger testes than the control treatments. Hindlimb autotomy did not result in the enlargement of other nearby traits. Our results are the first to experimentally demonstrate that males compensate for natural weapon loss by investing more in testes. In a second experiment we found that females paired with males that lost a hind limb had 40% lower egg hatching success than females paired with intact males, perhaps because of lower mating receptivity to males with a lost limb. Importantly, in those cases where viable offspring were produced, males missing a hind limb produced 42% more offspring than males with intact limbs. These results suggest that the loss of a hind-limb weapon can, in some cases, lead to greater fertilization success.

Experimental manipulation reveals a trade‐off between weapons and testes

“…This environmentally induced change in relative testes size was of moderate but significant effect (Cohen's d = 0.70, Somjee et al ., ). In another study, removal of a single hind leg of leaf‐footed cactus bugs during their fourth instar resulted in adults with proportionally larger testes compared to a group where a single mid‐leg was removed (Cohen's d = 0.39, Joseph et al ., ). In comparison, in our study, males that were prevented from developing both hind legs had an increase in testes size approximately one standard deviation larger than insects that developed without mid‐legs (Cohen's d = 1.01).…”

“…To reveal resource allocation trade-offs, it is necessary to perform experimental studies where one of the traits in question can be manipulated in isolation, to reveal effects on other traits (Roff, 2002). To date, exceptionally few studies have used experimental manipulations to examine the relationship between pre-and postcopulatory traits; those that have employed an experimental approach have reported evidence of resource allocation trade-offs between sexually selected pre-and post-copulatory traits (Moczek & Nijhout, 2004;Fry, 2006;Simmons & Emlen, 2006;Joseph et al, 2017). The scarcity of studies is likely a result of the difficulty in manipulating allocation to isolated traits, yet these experimental manipulations remain a powerful tool for uncovering allocation trade-offs among life-history traits (Simmons et al, 2017).…”

The ecology and evolution of autotomy

“…Whether this pattern is mediated by female choice or is a byproduct of male condition is unknown, making this a topic that deserves further investigation. On the other hand, males that autotomized as juveniles (without the ability to regenerate) produced more offspring than intact males in the cactus bug N. femorata (Joseph et al, ).…”

Costs of weaponry: Unarmed males sire more offspring than armed males in a male‐dimorphic mite

“…This trade‐off is exemplified by the leaf‐footed cactus bug, Narnia femorata (Stål, 1870), which, after autotomizing its weaponized legs during development, is able to grow testes before maturity that are larger than those of nonautotomized (control) males (Joseph et al., ). This suggests that the autotomy of its weapons freed up resources that could be invested elsewhere (Joseph et al., ). Similarly, the ablation of genital precursor cells in juvenile male horned scarab beetles Onthophagus taurus (Schreber, 1759) results in the growth of larger horns compared to unablated males (Moczek & Nijhout, ).…”

“…Allen & Levinton, 2007;Basolo & Alcaraz, 2003;Goyens, Dirckx, & Aerts, 2015;López & Martín, 2002;Wilson, James, Bywater, & Seebacher, 2009), require large, energy consuming muscles to operate (e.g. Joseph, Emberts, Sasson, & Miller, 2017;Marden, 1989) and/or increase a male's body volume, which increases somatic maintenance costs (Emlen, 2008;Kooijman & Metz, 1983;Parker, 1983). Post-copulatory attributes such as nuptial gifts (e.g.…”