4. Discussion
4.1. Sexual size dimorphism (SSD) varied among populations
According to the differential equilibrium model, SSD arises from the equilibrium of three major evolutionary forces that acts differently on body size – sexual selection that selects for larger body size in males, fecundity selection that selects for larger body size in females and viability selection (e.g. natural selection) that select against large body size in both sexes, within general constraints (e.g. genetic, physiological etc.) (Blanckenhorn, 2005). In addition, Rensch’s rule predicts that in species that exhibit male-biased SSD, the differences between the sexes increases with average body size and this is a well-supported phenomenon in vertebrates (Rensch, 1959).
Unlike birds and mammals, most insects exhibit female-biased SSD, where females have a larger body size (Blanckenhorn, 2005; Esperk et al. , 2007; Rudoy & Ribera, 2017; Stillwell et al. , 2010). In beetles, only 9% of the reported species exhibit male-biased SSD while 72% exhibit female-biased SSD (Stillwell et al. , 2010). However, more specifically among the Onthophagus species, most do not display sexual size dimorphism (Pomfret & Knell, 2006). In fact, a study of six Southeast Asian Onthophagus species, includingO. Babirussa (from Peninsula Malaysia), reported no sexual size dimorphism (Goh & Hashim, 2020). In our study, we documented significant male-biased SSD in both Singapore populations (CCNR and Pulau Ubin) and Langkawi (Figure 8 and appendix 4). Populations from Peninsula Malaysia had larger males but the difference between the sexes was not significant, thus replicating the results reported in Goh & Hashim (2020).
Theory suggests that strong precopulatory sexual selection drives male-biased SSD in insects as larger body size in males have been widely documented to increase mating success due to female choice or male-male competition (Blanckenhorn, 2005; Puniamoorthy, Schäfer, & Blanckenhorn, 2012; Stillwell et al. , 2010). In many Onthophagus dung beetles and related taxa, males compete to gain access to females and body size is a predominant factor in determining fighting success (Emlen, 1997; Moczek & Emlen, 1999). However, the intensity of sexual acting male body size is not necessarily stronger than the fecundity selection acting on female body size. In beetles, larger females are generally able to produce larger and more offspring, thus accounting for the female-biased SSD observed in most species (Stillwell et al. , 2010). As such, the male-biased SSD in O. barbirussa is likely a derived trait that can be due to a relative increase in the intensity of sexual selection on male body size in this species.
Based on the Rensch’s rule, one may predict that the SDI index or the difference between the sexes is most pronounced in the populations where body size is the largest i.e. the Malaysia populations. Instead, we document that SSD is most pronounced in the Singapore populations (CCNR and Pulau Ubin) where overall body size is the smaller. This is unlikely due to stronger sexual selection on males as the body size of males did not differ significantly between males of Singapore and Malaysia populations. Rather, larger SSD in Singapore was more likely due to weaker fecundity selection and/or stronger viability selection on females in Singapore as females were significantly smaller in Singapore than in Malaysian populations.
In Singapore, approximately 95% of forests were cleared over the last 200 years due to urbanisation, causing high local extinctions of fauna such as birds and mammals in forest habitats (Bickford et al. , 2010; Brook, Sodhl, & Ng, 2003). Singapore’s remaining forests are mostly degraded, highly fragmented and often subjected to high levels of disturbances, leading to a decrease in the general abundance of mammals (Bickford et al. , 2010; Lee et al. , 2009). Thus, there are fewer food and brood resources, leading to lesser opportunities for oviposition in female dung beetles in Singapore where the main sources of dung are likely from long-tailed macaques (Macaca fascicularis ) and wild boars (Sus scrofa ) (Culot et al. , 2013). In contrast, the sites surveyed in Malaysia is located within larger stretches of forests that serve as a refuge for larger mammals not found in Singapore, such as Asian elephant (Elephas maximus ) and Malayan Tapir (Tapirus indicus ), that are likely able to provide more food and brood resources for oviposition opportunities for female (Qie et al. , 2011; Rufino et al. , 2010). Hence, lesser food resources in Singapore suggests that there could be a stronger viability selection on females in Singapore compared to Malaysia, leading to smaller female body size and male-biased SSD in Singapore populations. In addition, there could also be weaker fecundity selection on Singapore females because of lesser brood resources.
4.2 Investment in precopulatory and postcopulatory traits varied among populations
Sexual selection can occur before copulation, where males invest in precopulatory traits to increase mating opportunities and after copulation, where males invest in postcopulatory traits to increase chances of fertilising the ova of females (Birkhead & Pizzari, 2002; Eberhard et al. , 2018). Our results show that all four populations showed strong positive static allometry for horn length where horns are disproportionately longer in larger individuals. In dung beetles, horns are weapons used in male-male combat to gain access to breeding females, strong precopulatory sexual selection on horns could explain the strong positive static allometry in male O. babirussa(Emlen, Lavine, & Ewen-Campen, 2007; Simmons & Ridsdill-Smith, 2011). Furthermore, compared to the allometric coefficient of classic case studies of sexually selected traits such as deer antlers (β=1.86) and rhinoceros beetle horns (β=4.15), the allometric coefficient for maleO. babirussa horns was approximately twofold (Figure 9a, β =8.552 – 9.685), further suggesting the presence of strong precopulatory sexual selection on horns (Kodric-Brown, Sibly & Brown, 2006).
Despite the importance of possessing larger horns in gaining access to females, males with small body sizes and small horns were still regularly sampled and seem to persist in wild populations (Figure 4a). In many Onthophagus species, males with small horns utilise alternative mating strategies in which they masquerade as females to sneak past guarding males with larger horns to gain access to breeding females (Simmons & Ridsdill-Smith, 2011). Perhaps such an alternative mating strategy exists in O. babirussa , which could explain the phenotypic variation in horn length observed in wild-caught populations (Moczek & Emlen, 2000).
Due to limited resources for growth and development, there may potentially be trade-offs in the investment of precopulatory and postcopulatory traits (Moczek & Nijhout, 2004). As there was a high relative investment in horn length, a precopulatory trait, we hypothesized that there would be a low relative investment in postcopulatory traits such as testes weight and sperm length. We would also then expect a lower allometric coefficient compared to horn length allometry. As predicted, our results show a negative allometric relationship for sperm length (β<1) for all populations, suggesting that there is a lower relative investment in sperm length than horns which could be due to weaker postcopulatory selection in maleO. babirussa. To test this, further studies would be needed to identify the rates of polyandry in wild-caught O.babirussa populations by determining paternity estimates of offspring to determine the intensity of postcopulatory sexual selection via sperm competition (McCullough, Buzatto, & Simmons, 2017).
Interestingly, a positive allometric relationship for testes weight (β= 2.977) was found in Central Peninsula MY while the rest of the populations had a negative allometric relationship with testes weight (β<1). Males from Central Peninsula MY have similar body sizes to those from Langkawi and Singapore, but with larger testes. This suggests that there is a higher relative investment in postcopulatory traits in Central Peninsula MY. This could be due to stronger postcopulatory selection arising from sperm competition, as many studies have shown that species under stronger selection often have larger testes (Simmons & García-González, 2008). To test this, further studies would be needed to assess the level of sperm competition between males to determine if males in Central Peninsula MY have a competitive fertilisation advantage compared to the rest of the populations (Simmons & García-González, 2008).