Why are women smaller than men? When anthropology meets evolutionary biology

Article (PDF Available)inNature Precedings · January 2008with 6,906 Reads
DOI: 10.1038/npre.2008.1832.1.
Abstract
There are large variations of size among humans but in all populations, men are larger on average than women. For most biologists this fact can be easily explained by the same processes that explain the size dimorphism in large mammals in general and in apes in particular. Due to fights between males for the possession of females, sexual selection has favoured bigger males. Indeed, this factor certainly explains why males are selected for being large but lets aside the question of selection on the female side. Actually, it has been shown that larger females are also favoured by natural selection. This is particularly relevant for women because their probability of dying when giving birth is then reduced. In this paper, the common view that size dimorphism in humans results from the fact that the advantage of being big is stronger for men than for women is challenged by another hypothesis, namely that the difference results from a difference of cost rather than from a difference of benefits. The cost of being big would be higher in women simply because, under gender hierarchical regimes found in all cultures, men are allocated the best food. The interaction between evolutionary forces and cultural practices could then lead to this disadaptive situation.
1
Why are women smaller than men? When anthropology
meets evolutionary biology
Priscille Touraille1 & Pierre-Henri Gouyon2
1Musée de l'Homme Unité Eco-anthropologie (UMR 5145 CNRS-MNHN-P7), Muséum
National d’Histoire Naturelle de Paris, 17 place du Trocadéro, F-75116 Paris.
2 OSEB (UMR 5202 CNRS-MNHN) Muséum National d’Histoire Naturelle de Paris, 12
rue Buffon F-75005 Paris.
There are large variations of size among humans but in all populations, men are
larger on average than women. For most biologists this fact can be easily explained
by the same processes that explain the size dimorphism in large mammals in
general and in apes in particular. Due to fights between males for the possession of
females, sexual selection has favoured bigger males1,2,3. Indeed, this factor certainly
explains why males are selected for being large but lets aside the question of
selection on the female side. Actually, it has been shown that larger females are
also favoured by natural selection. This is particularly relevant for women because
their probability of dying when giving birth is then reduced4,5,6. In this paper, the
common view that size dimorphism in humans results from the fact that the
advantage of being big is stronger for men than for women is challenged by
another hypothesis, namely that the difference results from a difference of cost
rather than from a difference of benefits. The cost of being big would be higher in
women simply because, under gender hierarchical regimes found in all cultures,
men are allocated the best food. The interaction between evolutionary forces and
cultural practices could then lead to this disadaptive situation.
Being large is undoubtedly advantageous for men but also for women. Concerning
men, the advantage is usually supposed to involve survival and/or reproductive output
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(through competition between men, whatever the form it takes)1,2,3. Concerning women,
an important component is the fact that the probability of death of both the mother and
the child decrease with the size of the mother4,5,6. The fact that there seems to exist a
stable body size while there is heritable variation for this trait indicates that the
advantage stated above is compensated for by a cost. This cost is probably at least partly
due to the fact that increasing the body size involves a higher resource acquisition but
can also involve different kinds of scaling effects like the surface/volume ratio which
decreases as the size increases. The expected individual size should be determined by
the point where the cost and benefit curves cross. This point is stable if the benefit is
lower than the cost when the size is below this point and reciprocally (Fig. 1.).
The existence of a dimorphism indicates that at least one of these two curves
(benefit or cost) is different according to the sex of the individual, or at least has been
different in the past. Indeed, such a dimorphism is found in all human populations7. One
could imagine that this dimorphism is simply a remnant of an old one, which existed at
the time of our common ancestor with say Gorillas8,9. However, this hypothesis can be
rejected because (i) the degree of dimorphism is variable among populations and
families10,11 and (ii) this variation is heritable12. Given these facts, the present situation
cannot be simply the perpetuation of the preceding one. Selection should be able to
shape efficiently the size dimorphism in humans6,13,14.
The classic explanation favoured by biologists is that dimorphism results from a
difference between males and females in terms of benefit of being large. Fig. 2. shows
such a case. Indeed, an increased size is beneficial in terms of fitness both to males and
females. But the increase of fitness being higher for males than for females, a similar
cost for both sexes leads to an “equilibrium” situation where males are bigger than
females. This explanation certainly holds for a lot of big mammals, including apes
because individual fights remain an important factor in the determination of social status
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and access to females concerning the males15,16,17. This explanation has thus been
critically discussed for different classes and orders of animals18,19 and particularly for
the primates order20, 21,22,23,24.
In humans, it is likely that this very factor has long been less important than in
other species. Indeed, numerous authors have stated that other environmental factors are
involved, nutrition being one of them14,25. Anthropological data about the non
egalitarian access to resources that characterizes human societies could explain the same
fact. This suggests that we should start thinking in terms of differences in costs rather
than in terms of benefits. It has been shown that in almost all known populations
worldwide, restrictions are very often targeted at the critical period of a woman’s
reproductive lifetime26, and women as a group are supposed to get less food in terms of
quantity and of quality than men do25,27. This reality is expected to be the direct
consequence of an institutionalized inequality in favour of men that the social sciences
have named the gender order28. Even if the benefit of being big is as great for women as
it is for men, and even if there is not any advantage in being big for men anymore, this
difference of resource allocation would generate a difference in cost which can explain
the size dimorphism (see Fig. 3.). This hypothesis holds for humans and perhaps for
some primates where females have no priority for access to resources29, contrary to
what happens for some monomorphic primates30.
It is thus suggested that, in humans, the differential cost associated with growth in
males and females is different, thanks to the unequal sharing of resource between males
and females. This would generate a dimorphism even in the absence of differences of
benefits associated with increased size between the sexes. In order to test this hypothesis
against the “standard” model, one would need to determine the curves of cost and
benefits for males and females in different conditions. A precise knowledge of these
curves would certainly be difficult to reach but it is probably possible to find orders of
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magnitude of variation. Note that the two hypotheses are not exclusive and if they are
both true, the effect will be even greater. It is interesting to notice that in our hypothesis,
the fact that females are smaller than males is far from being optimal. Given that the
reproduction is physiologically realized by women, there would have been incredibly
less child and mother damage in human history if females have had access to a richer
diet: the cost of growing more would be decreased and they would reach a size which
allows them to give birth with a decreased probability of accident and depletion.
Furthermore, from a demographic point of view, this situation would be more
favourable for the population. There has always been in the literature a strong tendency
to interpret cultural affairs in terms of biological success. Such biological arguments
have long been used to justify unfair social rules. The present case could constitute a
counter-example where a cultural trait is counter-adaptive because of its evolutionary
implications.
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P. W. Sexual dimorphisms and breeding systems in pinnipeds, ungulates,
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4. Martorell, R., Delgado, H. L., Valverde, V., & Klein, R. E. Maternal stature,
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Acknowledgements: the authors are grateful to Françoise Héritier who supervised the PhD thesis of
Priscille Touraille (from which the main idea of this paper is coming from) and attracted Pierre-Henri
Gouyon to this subject.
Correspondence should be addressed to touraille@mnhn.fr or gouyon@mnhn.fr
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Fig. 1. The evolutionary stable size of an individual is determined by the point at
which the cost becomes higher than the benefit.
Fig. 2. The classical interpretation of size dimorphism in mammals involves a
difference of benefit according to the sex of the individual. Although both sexes
benefit from a larger size, the advantage is higher for males than for females.
Fig. 3. The proposed hypothesis is that, due to unequal sharing of resources
between males and females, the cost of a large body size is higher for females;
this leads to a size dimorphism even if the benefits provided by a larger size are
equal for males and females or if the benefit for males is lower than for females.
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Fig. 1.
Fig2.
Fig 3.
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