A weapons-testes trade-off in males is amplified in female traits

Abstract

Sexually selected weapons are assumed to trade off with traits related to ejaculates, such as testes. However, remarkably little is known about what governs resource allocation and why trade-offs are found in some cases and not others. Often-used models depict competitive allocation occurring within the functional grouping of traits (e.g. reproduction); however, other factors including tissue expense and developmental timing may influence allocation. Experimental comparisons of investment across the sexes have the potential to illuminate allocation rules, because the sexes do not always use traits for the same functions. Here, we capitalize upon a species where females have weapons-testes homologues. We report that a documented trade-off in investment between hind-limb weapons and testes in leaf-footed cactus bugs, Narnia femorata, is even more pronounced in female hind limbs and ovaries. Female hind limbs in this species do not share the clear reproductive function of male hind limbs; therefore, this trade-off spans trait functional groups. Such patterns of investment suggest that future studies of reproductive trade-offs should consider factors such as tissue expense and developmental timing.

Keywords: Y-model; post-copulatory; pre-copulatory; resource allocation; sexual selection.

Figure 1.

(a) Silhouette of Narnia femorata with an autotomized left hind limb, (b) male testes and (c) female reproductive system. (Online version in colour.)

Figure 2.

Both males and females with autotomy showed a positive relationship between the muscle mass of the remaining (right) hind leg and gonadal mass (a), though the slope of this scaling relationship was steeper for females. At the mean overall body size, females (c) received a greater boost in gonadal mass following autotomy than did males (b). In (a), back-transformed raw data (x-axis) and model estimates (y-axis) are displayed, produced from the full (both sexes) model in table 1. We used model estimates of gonadal mass in (a) to adjust for differences in oocyte number within the ovaries which contributed substantially to their mass. In (b) and (c) hollow circles denote estimated marginal means (EMM) ± standard error bars that were generated directly from the same model without back-transformation. EMM produced at the mean body size (log_e pronotal width = 1.4215 mg). Simpler models (right columns in table 1) produced qualitatively the same results as depicted here. (Online version in colour.)