Unique inhibitory cascade pattern of molars in canids contributing to their potential to evolutionary plasticity of diet

Abstract

Developmental origins that guide the evolution of dental morphology and dental formulae are fundamental subjects in mammalian evolution. In a previous study, a developmental model termed the inhibitory cascade model was established. This model could explain variations in relative molar sizes and loss of the lower third molars, which sometimes reflect diet, in murine rodents and other mammals. Here, I investigated the pattern of relative molar sizes (inhibitory cascade pattern) in canids, a taxon exhibiting a wide range of dietary habits. I found that interspecific variation in canid molars suggests a unique inhibitory cascade pattern that differs from that in murine rodents and other previously reported mammals, and that this variation reflects dietary habits. This unique variability in molars was also observed in individual variation in canid species. According to these observations, canid species have greater variability in the relative sizes of first molars (carnassials), which are functionally important for dietary adaptation in the Carnivora. In conclusion, an inhibitory cascade that differs from that in murine rodents and other mammals may have contributed to diverse dietary patterns and to their parallel evolution in canids.

Morphological variation of the maxillary lateral incisor

“…This model explains the relative size of mandibular molars by a balance of inhibitor and activator substances, and predicts evolutionary patterns in the dentition. The model has been tested from both paleontological [51,52] and comparative anatomical [53] perspectives.…”

The integration of quantitative genetics, paleontology, and neontology reveals genetic underpinnings of primate dental evolution

“…The concept was soon applied to other tripartite skeletal phenotypes (13). However, the extent of the predictive power of the IC for taxa with less similar (14) and more heterodont (15)(16)(17)(18) dentitions suggests that either the IC mechanism does not characterize these other organisms' dental patterning as specifically or the characterization of the mechanistic output (the phenotype) is not accurately assessed. Despite empirical evidence for how primate dental patterning differs from dental patterning of the mouse (19,20), proponents of the mouse model continue to adhere tightly to it.…”

The origin of the lower fourth molar in canids, inferred by individual variation

“…Bat-eared foxes are primarily insectivorous (Sillero-Zubiri, 2009), with a molar morphology comprising an undeveloped carnassial blade, equally sized molars (in relation to the other canids) and increased number of molars, which are attributable to an adaptation to an insectivorous diet (Wang & Tedford, 2008; Asahara, 2013; Asahara et al, 2016). It has been proposed that this dentition is suitable to a diet of insects that are small relative to the body size of the bat-eared fox, with a larger molar row grinding surface that enables greater chewing efficiency (Asahara, 2013; Asahara et al, 2016).…”

“…It has been proposed that this dentition is suitable to a diet of insects that are small relative to the body size of the bat-eared fox, with a larger molar row grinding surface that enables greater chewing efficiency (Asahara, 2013; Asahara et al, 2016). However, this does not explain the presence of the fourth molar, the developmental origin of which remains unclear.…”

“…According to the balance of these two factors, the relative size of M 1 , M 2 , and M 3 (typically shown numerically as M 2 /M 1 and M 3 /M 1 size ratios; the size is defined as projected tooth area from occlusal view) result in a pattern of M 1 > M 2 > M 3 , M 1 = M 2 = M 3 , or M 1 < M 2 < M 3 (Kavanagh, Evans & Jernvall, 2007). This model has previously been applied to explain dental variation in Carnivora (Polly, 2007; Halliday & Goswami, 2013; Asahara, 2013; Asahara et al, 2016). …”