In animals, male weapons such as antlers, horns, spurs, fangs, and tusks typically provide advantages in male contests and increase access to females, thereby enhancing reproductive success. However, such large and extravagant morphological structures are expected to come at a cost, potentially imposing trade-offs with life history traits, physiological functions, or certain behaviors (Emlen, 2001; Emlen, 2008). These costs should be manageable only by males in the best condition. The present study by Blackwell et al. (2024) examines this assumption through a comprehensive study on the European earwig, where males possess forceps-like cerci that vary widely in size within populations.

In the European earwig (Forficula auricularia), male forceps are used in male-male contests as weapons to deter competitors prior to mating (Styrsky & Rhein, 1999) or to interrupt mating pairs by non-copulating males (Forslund, 2000; Walker & Fell, 2001). Despite providing benefits in terms of mating success (Eberhard & Gutierrez, 1991; Tomkins & Brown, 2004), it remains unknown whether long or short forceps are associated with other important life-history traits.

In this laboratory study, Blackwell et al. (2024) investigated two European earwig populations, each divided into two subpopulations: one with the shortest forceps and one with the longest forceps. They examined the potential costs of long forceps on six different traits: one reproductive trait (sperm storage); three non-reproductive behavioral traits such as locomotor performance (involved in search for resources), fleeing reaction face to a risk (long forceps are supposed to be correlated with boldness), and aggregation behavior (European earwigs are facultative group-living organisms); and survival (when deprived of food and subsequently when exposed to an entomopathogenic fungus).

As males in the best condition are supposed to be those that can afford to develop large forceps, Blackwell et al. (2024) predicted that males with long forceps would perform better than those with short forceps across the investigated traits. However, their predictions were not validated, as no correlation between weapon size and male quality was detected in either population. Although the sample size is sometimes limited, the consistency of these results across different populations adds robustness to their conclusions.

By demonstrating that forceps length in the European earwig does not reliably indicate male quality, this paper challenges existing theories and highlights the complexity of evolutionary processes shaping morphological traits. Furthermore, the study raises important questions about the evolutionary mechanisms maintaining weapon size diversity, providing a fresh perspective that could stimulate further research and debate in the field, notably the search for other traits where costs might be incurred.

References

Blackwell, S.E.M., Pasquier, L., Dupont, S., Devers, S., Lécureuil, C. & Meunier, J. (2024). Relationship between weapon size and six key behavioural and physiological traits in males of the European earwig. bioRxiv, ver. 3 peer-reviewed and recommended by Peer Community in Zoology. https://doi.org/10.1101/2024.03.20.585871

Eberhard, W.G., & Gutierrez, E.E. (1991). Male dimorphisms in beetles and earwigs and the question of developmental constraints. Evolution, 45(1), 18–28. https://doi.org/10.2307/2409478

Emlen, D.J. (2001). Costs and the diversification of exaggerated animal structures. Science, 291(5508), 1534–1536. https://doi.org/10.1126/science.1056607

Emlen, D.J. (2008). The evolution of animal weapons. Annual Review of Ecology, Evolution, and Systematics, 39(1), 387–413. https://doi.org/10.1146/annurev.ecolsys.39.110707.173502

Forslund, P. (2000). Male-male competition and large size mating advantage in European earwigs, Forficula auricularia. Animal Behaviour, 59(4), 753–762. https://doi.org/10.1006/anbe.1999.1359

Styrsky, J.D., & Rhein, S.V. (1999). Forceps size does not determine fighting success in European earwigs. Journal of Insect Behavior, 12(4), 475–482. https://doi.org/10.1023/A:1020962606724

Tomkins, J.L., & Brown, G.S. (2004). Population density drives the local evolution of a threshold dimorphism. Nature, 431, 1099–1103. https://doi.org/10.1038/nature02936.1.

Walker, K.A., & Fell, R.D. (2001). Courtship roles of male and female European earwigs, Forficula auricularia L. (Dermaptera: Forficulidae), and sexual use of forceps. Journal of Insect Behavior, 14(1), 1–17. https://doi.org/10.1023/A:1007843227591