Abstract
Investigating the expression of trade-offs between key life-history functions is central to our understanding of how these functions evolved and are maintained. However, detecting trade-offs can be challenging due to variation in resource availability, which masks trade-offs at the population level. Here, we investigated in the European earwig Forficula auricularia whether (1) weapon size trades off with three key immune parameters – hemocyte concentration, phenoloxidase and prophenoloxidase activity - and whether (2) expression and strength of these trade-offs depend on male body condition (body size) and/or change after an immune challenge. Our results partially confirmed condition dependent trade-offs between weapon size and immunity in male earwigs. Specifically, we found that after an immune challenge, weapon size trades off with hemocyte concentrations in low-condition, but not in good-condition males. Contrastingly, weapon size was independent of pre-challenge hemocyte concentration. We also found no trade-off between weapon size and phenoloxidase activity, independent of body condition and immune challenge. Overall, our study reveals that trade-offs with sexual traits may weaken or disappear in good-condition individuals. Given the importance of weapon size for male reproductive success, our results highlight how low-condition individuals may employ alternative life-history investment strategies to cope with resource limitation.
Highlights
The reversal or removal of trade-offs has been suggested for male sexual traits as part of the handicap principle[19], which aims to explain how male sexual traits evolved and more importantly, why they - in theory - must be honest, i.e. reflect the condition of the bearer
Hemocyte concentration traded off with forceps length in the smaller males, whereas hemocyte concentration increased together with forceps length in the largest males (Fig. 2a). Independent of this effect, post-challenge hemocyte concentration was overall higher in LPS-pricked than in control-pricked males (P = 0.015, Table 1 and Fig. 2b) and the post-challenge and basal levels of hemocyte concentrations were positively correlated in the smaller males only
We investigated whether the forceps size of field-sampled earwig males traded off with their immunocompetence and whether the expression and direction of this trade-off depended on male body size
Summary
The reversal or removal of trade-offs has been suggested for male sexual traits as part of the handicap principle[19], which aims to explain how male sexual traits evolved and more importantly, why they - in theory - must be honest, i.e. reflect the condition of the bearer. In a refined model based on the handicap principle, Grafen proposed in 199020 that marginal costs of advertising sexual traits are higher for individuals in poor condition This suggests that trade-offs involving sexual traits should be less taxing on other life-history traits in high compared to low-condition males and result in an overall positive growth allometry in high condition males only (relative scaling of body parts)[21]. Condition dependency of sexually-selected weapons or ornaments in males has been established in a number of species, such as horn length in the isopod Deto echinata[24], eye span in the stalk-eyed fly Diasemopsis aethiopica[25], or weapon size in the cactus bug Narnia femorata[26] To what degree this condition dependency of sexually-selected traits affects any trade-offs with other, non-sexual traits remains largely unknown. We investigated whether trade-offs between sexual (forceps length) and life-history (immunity) traits are condition-dependent in males of the European earwig Forficula auricularia. If the condition of a male determines the presence and direction of an investment trade-off between weapon size and immunocompetence in the direction predicted by Grafen[20], we expected to detect a trade-off between forceps length and the level of basal immunity and/or immune response in the smallest (i.e. low quality) but not the largest (i.e. high quality) males
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