Sexual Body Size Dimorphism

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Background Information: 

Sexual body size dimorphism is a difference in size between the two sexes, usually measured as a ratio of the male to female body weight. In most hominoids, the male is larger than the female. Selection pressures for dimorphism include natural and sexual selection, often correlating with the species’ social and mating system. The mechanisms by which dimorphism occurs include accelerated onset of growth, timing of growth spurts, early and late cessation of growth, the overall rate of growth, delayed maturation, and indeterminate male growth (i.e. continued growth throughout the lifespan). Accelerated onset of growth and late cessation of growth can provide advantages in resource competition where food resources are scarce. Delayed maturation can result in larger size over the long term, and the initial lengthening of juvenile status can reduce the danger of mature competition. The delay in maturation is selected if it results in reduced juvenile mortality and elevated fertility over the lifetime. Indeterminate male growth is believed to be selected for by a sexual selection pressure of intense intermale competition. This occurs in other species such as the elephant seal and African elephant in polygynous mating systems where males compete heavily over access to females. Such species also may develop enhanced weaponry, such as large canines, or structures that emphasize the size and shape of a male, such as flanges and laryngeal sacs in orangutans or manes in lions. Such structures contribute to sexual dimorphism but are separate from strict body size dimorphism as determined by weight.Species which are monogamous, such as gibbons (Hylobates), tend to have minimal body size dimorphism as both sexes are equally aggressive in defending their territory and mate against conspecifics, as in Hylobates. Mechanisms also exist that can reduce dimorphism. For example, although male chimps live in societies with multiple adult males and females, lower within-group male aggression can have the advantageous effects of greater affiliation among males, larger troop size to protect against other troops, and increased access to females.

The Human Difference: 

Human sexual body size dimorphism (male/female ratio) is on average 1.15, though depending on the location values range from 1.09-1.28. Estimates of sexual body size dimorphism in the Homo lineage are controversial. Whereas some claim A. afarensis to exhibit marked dimorphism similar to gorillas, others argue for values intermediate to chimpanzees and modern humans. Due to the scarcity of fossil remains, the large area over which they are collected, and assumptions regarding the sex of the fossils, the estimates of dimorphism are highly variable. Apart from overall body size estimates, the canine size of afarensis is smaller than that seen in chimpanzees, with the reduction in size continuing in modern humans. Gorillas, chimpanzees, bonobos and orangutans all exhibit sexual body size dimorphism, but to different extents and for different ontogenetic reasons. Lowland gorillas show the greatest dimorphism, having a male/female bodyweight ratio of 2.37. Orangutans also show large dimorphism (male/female ratio = 2.23), while the ratio for bonobos and chimpanzees is more moderate (1.36 and 1.29 respectively). In gorillas and bonobos the dimorphism is primarily due to bimaturation, or differences in the duration of growth. That is, male gorillas and bonobos continue growing longer than female gorillas and bonobos. Early cessation in female growth seems to drive the pattern of dimorphism and is thought to result from utilization of ubiquitous folivorous resources and thus decreased competition for food resources in those species. In chimpanzees, the dimorphism is primarily due to differential rate of growth, with both sexes growing for roughly equal durations but for higher rates in male chimps. Chimp females have a later cessation of growth than other primates, likely reflecting greater female competition over food resources. In orangutans, indeterminate male growth contributes to the dimorphism. The high degree of orangutan dimorphism is even more striking given that orangutan females have a later cessation of growth than all apes except humans. Pongo (orangutan) females have slower rates and duration than males. Males have indeterminate growth, can continue growing through life span. Gorilla females have shorter duration than gorilla males. Peak growth spurt in females is earlier, but abrupt and early cessation and growth contributes to the dimorphism. Chimpanzees have roughly the same duration of growth (with female cessation only occurring 6 months before males) but the male rate is higher, so leads to dimorphism. Female growth is lower, but prolonged in comparison to males, thus limiting dimorphism.

Mechanisms Responsible for the Difference: 

Sexual body size dimorphism is correlated with intermale competition and mating system. Species with a monogamous mating system tend to show little to no dimorphism while those with high intermale competition, as occurs in in polygynous or promiscuous mating systems, exhibit greater dimorphism. Dimorphic traits are revealed during adulthood and are less discernible before, suggesting pubertal hormones drive dimorphism. A reduction in body size dimorphism in comparison to chimpanzees, along with the small canine size in humans, might reflect a relaxation in intermale competition over access to females during the Homo lineage, indicating a pair-bonding mating system rather than a polygynous one.

Implications for Understanding Modern Humans: 

By comparing the dimorphism of modern humans and the Homo lineage, we can infer the mating system and selection pressures of our ancestors. For instance, the moderate sexual body size dimorphism seen in humans may indicate a conserved yet tempered predisposition for inter-male violence.

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