Abstract
Sexual selection has driven the evolution of exaggerated traits among diverse animal taxa. The production of exaggerated traits can come at a cost to other traits through trade-offs when resources allocated to trait development are limited. Alternatively some traits can be selected for in parallel to support or compensate for the cost of bearing the exaggerated trait. Male giraffe weevils (Lasiorhynchus barbicornis) display an extremely elongated rostrum used as a weapon during contests for mates. Here we characterise the scaling relationship between rostrum and body size and show that males have a steep positive allometry, but that the slope is non-linear due to a relative reduction in rostrum length for the largest males, suggesting a limitation in resource allocation or a diminishing requirement for large males to invest increasingly into larger rostra. We also measured testes, wings, antennae, fore- and hind-tibia size and found no evidence of a trade-off between these traits and rostrum length when comparing phenotypic correlations. However, the relative length of wings, antennae, fore- and hind-tibia all increased with relative rostrum length suggesting these traits may be under correlational selection. Increased investment in wing and leg length is therefore likely to compensate for the costs of flying with, and wielding the exaggerated rostrum of larger male giraffe weevils. These results provide a first step in identifying the potential for trait compensation and trades-offs, but are phenotypic correlations only and should be interpreted with care in the absence of breeding experiments.
Highlights
Competition between males for mates has led to the evolution of exaggerated traits through sexual selection [1,2]
An initial inspection of a scatterplot of untransformed rostrum length against pronotum width for male L. barbicornis showed a continuous relationship between the two variables, but there appeared to be a decrease in relative rostrum length for the largest males (Figure 2)
The presence of a positive allometry when studying an exaggerated trait in males is generally used as evidence that the trait is under sexual selection, and demonstrates the importance of the trait as a weapon or ornament used during mate acquisition [3]
Summary
Competition between males for mates has led to the evolution of exaggerated traits through sexual selection [1,2]. Recent analyses have shown that these thresholds are not always straightforward, and that some model species such as the scarabaeid beetle Onthophagus taurus show a continuous relationship between weapon and body size and do not show evidence for a reprogramming event between small hornless (minor) and large horned (major) males, supporting instead a non-linear model of exponential horn growth followed by a growth constraint in the largest males due to competition between body parts during resource allocation [7]. In many cases the resulting dimorphism between small and large males is coupled with important differences in behaviour between the two morphs, such as alternative mating tactics [8,9]. These tactics are generally divided at a threshold between minor and major males, and in many cases minors adopt sneaking tactics while majors rely on aggressive mate guarding tactics [10,11,12]
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