Geschlechterunterschiede im Gesicht als Anpassung an Kampf?

Eine Studie wirft die These auf, dass Unterschiede zwischen den Geschlechtern auf eine verstärkte Anpassung der Männer an Kämpfe beruhen könnte:

When humans fight hand-to-hand the face is usually the primary target and the bones that suffer the highest rates of fracture are the parts of the skull that exhibit the greatest increase in robusticity during the evolution of basal hominins. These bones are also the most sexually dimorphic parts of the skull in both australopiths and humans. In this review, we suggest that many of the facial features that characterize early hominins evolved to protect the face from injury during fighting with fists. Specifically, the trend towards a more orthognathic face; the bunodont form and expansion of the postcanine teeth; the increased robusticity of the orbit; the increased robusticity of the masticatory system, including the mandibular corpus and condyle, zygoma, and anterior pillars of the maxilla; and the enlarged jaw adductor musculature are traits that may represent protective buttressing of the face. If the protective buttressing hypothesis is correct, the primary differences in the face of robust versus gracile australopiths may be more a function of differences in mating system than differences in diet as is generally assumed. In this scenario, the evolution of reduced facial robusticity in Homo is associated with the evolution of reduced strength of the upper body and, therefore, with reduced striking power. The protective buttressing hypothesis provides a functional explanation for the puzzling observation that although humans do not fight by biting our species exhibits pronounced sexual dimorphism in the strength and power of the jaw and neck musculature. The protective buttressing hypothesis is also consistent with observations that modern humans can accurately assess a male’s strength and fighting ability from facial shape and voice quality.

Quelle: Protective buttressing of the hominin face

Protective Buttressing ist sozusagen das Einziehen besonderer Stützpfeiler in der Architektur, also ein Verstärken von besonders belasteten Bereichen. Die Studie führt einige interessante Unterschiede zwischen Männern und Frauen auf:

Although humans are generally viewed as exhibiting low to moderate levels of sexual dimorphism (McHenry, 1994; Plavcan, 2001, 2012; Reno et al., 2003), the relatively low body mass dimorphism of humans is largely a consequence of human females having substantial fat stores (Pond & Mattacks, 1987). When fat-free masses are compared, men are 41% more massive (Mayhew & Salm, 1990; Lassek & Gaulin, 2009) and have 48–65% more muscle mass than women (Illner et al., 2000; Abe, Kearns & Fukunaga, 2003; Kim et al., 2004; Shen et al., 2004). As in gorillas (Zihlman & McFarland, 2000) and australopiths (McHenry, 1986, 1991, 1996), the upper body of humans exhibits more sexual dimorphism in size and strength than do the legs (Abe et al., 2003; Raadsheer et al., 2004; Lassek & Gaulin, 2009; Price et al., 2012). Among young adults, the muscles of the arm are 69–109% stronger in males than in females, whereas strength dimorphism of leg muscles range from only 23 to 66% (Bohannon, 1997). The most sexual dimorphic part of the human body, in terms of muscular strength, may be the neck. Maximum moments produced by the muscles of the neck are 100–150% greater in men than in women (Vasavada, Li & Delp, 2001). Pronounced sexual dimorphism in cervical muscles is surprising given that humans do not fight by biting with the jaws. However, as discussed above, energy absorption by the muscles of the neck can protect against concussion when the head is struck. Thus, humans do show very high levels of sexual dimorphism in the parts of the postcranial musculoskeletal system that appear to be most important in fighting (Lassek & Gaulin, 2009; Puts, 2010; Carrier, 2011; Sell, Hone & Pound, 2012; Morgan & Carrier, 2013).

Because sexual dimorphism is often greatest in those characters that enhance a male’s capacity to dominate other males (Clutton-Brock & Harvey, 1977; Parker, 1983; Andersson, 1994), the observation that the face is the primary target when males fight leads to the expectation of sexual dimorphism in buttressing of the human face (Puts, 2010). The protective buttressing hypothesis predicts that the most dimorphic parts of the hominin skull will be those that are most frequently injured during fighting, namely the mandible, nasal region, zygomatic arch, orbit and maxilla.

Also erheblich mehr Muskeln bei Männern, aber auch an Stellen, an denen sie eigentlich nur zum Auffangen von Energie Sinn machen.

As predicted, the parts of the human facial skeleton that exhibit marked sexual dimorphism are also the parts of the skull that most frequently fracture when people fight (Table 1). First, as described above, the face of humans exhibits much more sexual dimorphism than the neurocranium and fractures of the face as a result of fighting are much more common than fractures of the neurocranium (Shepherd et al., 1990; Boström, 1997). Second, although relative frequency of fracture type varies among the studies included in Table 1, the sites of facial fracture are the same in each study. Of the mandibular fractures in the Bristol study, 25.4% were to the condyle or coronoid, 35.6% were to the ramus, 35.6% to the angle, and 3.4% to the symphysis. Additionally, the nasal region, zygoma and orbit are also sites of frequent injury and exhibit relatively high sexual dimorphism. Thus, a dramatic correspondence exists between the parts of the skull that are most sexually dimorphic and the parts that most frequently fracture during fighting.

Es zeigen sich also gerade dort mehr Unterschiede, wo zur Vermeidung eines Kampfes Verstärkungen notwendig waren.

Given that humans have relatively small canine teeth, exhibit low canine sexual dimorphism (Frayer & Wolpoff, 1985; Wood et al., 1991; Plavcan & van Schaik, 1997) and rarely bite during fighting (Shepherd et al., 1990; Boström, 1997), it is puzzling that humans exhibit significant sexual dimorphism in the strength of their jaw adductor muscles. Five studies that measured maximum bite force in men and women indicate that, on average, men produce 34.3 ± 10.5% (mean ± S.D.) greater forces than women (Klatsky, 1942; Waltimo & Könönen, 1993; Braun et al., 1995; Raadsheer et al., 2004; van der Bilt et al., 2008). This level of sexual dimorphism is only 7–20% below estimated sexual dimorphism in bite force of gorillas and orangutans (Demes & Creel, 1988; Eng et al., 2013); species in which biting is an important male fighting behaviour. The human masseter muscle also exhibits substantial gender differences in the proportion of fast-twitch (type II) muscle fibres. The average cross-sectional area of type II fibres in the masseter muscle averages 66.9% in males and only 8.3% in females (Tuxen, Bakke & Kenrad, 1992; Tuxen, Bakke & Pinholt, 1999). Type II muscle fibres shorten faster and generate force more quickly when stimulated than type I fibres (Close, 1967). Although human sexual dimorphism in adductor muscle strength and fibre type cannot be explained by aggressive biting behaviour during fighting or by mastication, because diets of human males and females are largely similar, the observed human dimorphism is consistent with the hypothesis of protective buttressing of the face. If the jaw adductor muscles do function to protect against mandibular dislocation and fracture, as suggested above, greater muscle strength and shorter force activation times in males would be expected because of their higher incidence of fighting and facial injury.

Des weiteren wurde auch untersucht, ob aus dem Gesicht Rückschlüsse auf die Eigenschaften als Kämpfer hergeleitet werden können:

The facial features that allow observers to assess a male’s strength, fighting ability and propensity to behave aggressively include the ratio of facial width to height (Carré et al., 2009, 2010), face width, chin breadth, eyebrow prominence and nose size (Windhager et al., 2011; Třebický et al., 2013). These metrics represent aspects of the skull that experience high rates of fracture due to interpersonal violence and are features that increased in robusticity coincident with the evolution of hand proportions that allow the formation of a fist (discussed above). Generally, facial masculinity is viewed as an honest signal conveying information about formidability (Sell et al., 2009; Puts et al., 2012b; Třebický et al., 2013). Puts (2010) proposed that, in addition to acting as a signal, the greater facial robusticity of males may have evolved to protect the face from injury when males fight, as we are suggesting here. It is also likely that masculine features of the face convey direct information about the degree to which the face, the primary target during interpersonal violence, is vulnerable to injury. Male contest competition involves both offence and defence and a robust facial skeleton may make an individual more formidable simply because he is less susceptible to serious injury.

Das wäre für Männer durchaus eine wichtige Information.

Es ist aber auch unter dem Gesichtspunkt der Partnerwahl interessant: Viele Aspekte, die innerhalb des intrasexuellen Konkurrenzkampfes einen Vorteil bieten, werden auch in der intersexuellen Selektion wichtig, denn entsprechende genetische Informationen werden dann auch an einen Sohn weitergegeben. Das könnte erklären, warumfür Frauen ein markantes, kräftiges Kinn attraktiv sein kann: Es spricht eben für ein robusteres Gesicht.

George Clooney

George Clooney

Wenn es eine sexuelle Selektion auf diesen Umstand gegeben hätte, dann hätte das diesen Zugang noch zusätzlich verstärkt.