Trimorphism in Dung Beetle, Proagoderus Watanabei

When competition between males for mates is intense, some males will adopt alternate tactics to increase their chance of mating success. In our new paper co-authored by Jon Parrett, Rob Knell and Eleanor Slade we found that the dung beetle, Proagoderus watanabei, may employ ‘sneak’ tactics to avoid precopulatory contests with other males. They do this by trading-off weaponry (horn size) and testes mass (sperm competition) with the males of this species having developed into three different morphs: alpha, beta and gamma males.

Figure 1. Frontal and canted view of (a) alpha male, (b) beta male, (c) gamma male and (d) female Copyright: TEELab Ong Xin Rui

In alpha males (figure 1a), long head horns and a pronotal horn are expressed while beta males (figure 1b) have long head horns but no pronotal horn. Gamma males (figure 1c) only have short head horns and resemble females (figure 1d). These males are thought to be ‘sneak’ males, that by resembling females can sneak passed the alpha and beta males to gain access to females, rather than engaging in fighting.

The study also found that the alpha morph with large head and pronotal horns, invest relatively less in testes mass in comparison to beta or gamma males. This is thought to be because they are better able to defend their females and so have reduced sperm competition than beta and gamma males who are more likely to be ousted by rival males. Beta and gamma males therefore trade-off reduced horn size against increased testes mass to increase their chances of fertilisation.

Read more about this study and its exciting results here: https://doi.org/10.1111/jzo.12941