Canine sexual dimorphism in Ardipithecus ramidus was nearly human-like

Body and canine size dimorphism in fossils inform sociobehavioral hypotheses on human evolution and have been of interest since Darwin’s famous reflections on the subject. Here, we assemble a large dataset of fossil canines of the human clade, including all available Ardipithecus ramidus fossils recovered from the Middle Awash and Gona research areas in Ethiopia, and systematically examine canine dimorphism through evolutionary time. In particular, we apply a Bayesian probabilistic method that reduces bias when estimating weak and moderate levels of dimorphism. Our results show that Ar. ramidus canine dimorphism was significantly weaker than in the bonobo, the least dimorphic and behaviorally least aggressive among extant great apes. Average male-to-female size ratios of the canine in Ar. ramidus are estimated as 1.06 and 1.13 in the upper and lower canines, respectively, within modern human population ranges of variation. The slightly greater magnitude of canine size dimorphism in the lower than in the upper canines of Ar. ramidus appears to be shared with early Australopithecus, suggesting that male canine reduction was initially more advanced in the behaviorally important upper canine. The available fossil evidence suggests a drastic size reduction of the male canine prior to Ar. ramidus and the earliest known members of the human clade, with little change in canine dimorphism levels thereafter. This evolutionary pattern indicates a profound behavioral shift associated with comparatively weak levels of male aggression early in human evolution, a pattern that was subsequently shared by Australopithecus and Homo.

Large male proboscis monkeys have larger noses but smaller canines

The uniquely enlarged noses of male proboscis monkeys are prominent adornments, and a sexually selected male trait. A recent study showed significant correlations among nose, body, and testis sizes and clear associations between nose size and the number of females in a male’s harem. However, to date, the analyses of other common male traits, i.e., canines, are lacking. Whereas male nose size had a positive correlation with body size, we unexpectedly found a negative correlation between body and canine sizes. We explain this by an interaction between sexual and natural selection. Larger noses in males may interfere with the use of canines, thereby reducing their effectiveness as weapons. Additionally, longer canines are opposed by natural selection because the larger gape it imposes upon its bearer reduces foraging efficiency, particularly in folivores. This unique case of decoupling of body and canine size reveals that large canines carry an ecological cost.

Large male proboscis monkeys have larger noses but smaller canines

The uniquely enlarged noses of male proboscis monkeys are prominent adornments, and a sexually selected male trait. A recent study showed significant correlations among nose, body, and testis sizes and clear associations between nose size and the number of females in a male’s harem. However, to date, the analyses of other common male traits, i.e., canines, are lacking. Whereas male nose size had a positive correlation with body size, we unexpectedly found a negative correlation between body and canine sizes. We explain this by an interaction between sexual and natural selection. Larger noses in males may interfere with the use of canines, thereby reducing their effectiveness as weapons. Additionally, longer canines are opposed by natural selection because the larger gape it imposes upon its bearer reduces foraging efficiency, particularly in folivores. This unique case of decoupling of body and canine size reveals that large canines carry an ecological cost.

Static Scaling and the Evolution of Extreme Canine Size in a Saber-Toothed Cat (Smilodon fatalis)

The canines of saber-toothed cats are a classic example of an extreme morphology, yet important questions pertaining to their evolution remain unanswered. Recent analyses suggest these structures functioned as tools of intrasexual combat where trait size acts as both a weapon of battle and signal of competitive ability. However, classic skeletal reconstructions suggest saber-tooth canines evolved as specialized hunting tools. Either scenario could have led to the evolution of extreme canine size and distinguishing between these hypotheses is therefore difficult. This is made more challenging by the fact that natural observation of saber-toothed cats is impossible, and biologists must rely on measures of static morphology to study the patterns of selection that favored extreme canine size. Here I analyze the static intraspecific scaling relationship between canine size and body size in the saber-toothed cat, Smilodon fatalis, to determine whether or not extreme canine size functioned as a sexually selected signal. I review the literature surrounding the evolution of sexually selected signals and the methods recently established by O’Brien et al. (2018), show how static scaling relationships can be useful, reliable tools for inferring patterns of selection, especially in fossil organisms, and provide evidence that extreme canine size in saber-toothed cats was not the product of selection for effective sexual signals, but instead evolved as either a pure intrasexually selected weapon or a hunting tool.