Biology Reference
In-Depth Information
and K´ sa, 1978; Kieser, 1990; Scheuer and Black, 2004 ). Traits that are under greater genetic
control seem to appear earlier than those affected more by the environment. Interestingly,
sexual dimorphism is present in the fetal pelvis, becomes indistinguishable during child-
hood and then reappears after adolescence. The adult shape of the pelvic inlet has obvious
ramifications for childbirth, which is likely why it is under intrinsic genetic control. 3
Extrinsic Factors in Sexual Dimorphism
Extrinsic factors in sexual dimorphism are those that are introduced typically from outside
the body. Examples of extrinsic factors include nutrition and the biomechanics of activity and
locomotion, both of which are combined into the additional extrinsic factor of body weight
that acts on the skeleton. Nutrition, an extrinsic factor, has been shown to accelerate the
process of maturation in both humans and nonhuman primates. For instance, high fat and
high protein diets can speed up the age of maturation in humans ( Bogin, 1999; Kaplowitz
et al., 2001; Kaplowitz, 2006 ). Rate acceleration in maturation has been demonstrated in
the worldwide decrease in the age of menarche (the age of a girl's first menstruation), which
many researchers attribute to nutrition ( Bogin, 1999; Kaplowitz et al., 2001; Onland-Moret
et al., 2005; Kaplowitz, 2006; Cho et al., 2010 ). Conversely, delayed maturation and growth
stunting in skeletal elements is also due to extrinsic environmental stresses (e.g., malnutri-
tion, disease) and can reduce sexual dimorphism in body size. Females show buffering to
these extrinsic factors of nutritional stress; thus, growth stunting is less severe in females
than males in terms of rates of growth and development ( Stini, 1975, 1982; Stinson, 2012;
Bogin, 1999; Ross et al., 2003 ). The reason for this buffering in females is unknown, but
hypotheses surround the role females must play in pregnancy and childbirth: females
must be better buffered against environmental stressors, even beginning in childhood, for
the species to survive.
Extrinsic factors in skeletal maturation can also be due to the biomechanical influence of
different forces on the skeleton, such as locomotion and gravity. The plasticity of bones
during growth and development enables our skeletal system to be designed specifically
for our size/weight, activities, and behaviors. If it were not, our bones would simply fail
(i.e., fracture). The load-bearing bones of the lower limb seem to show more plasticity during
maturation, especially in the diaphysis, yielding a high correlation with body mass and
activity levels ( Ruff and Hayes, 1988; Moro et al., 1996; Larsen, 1997; Ruff, 2000; Lieberman
et al., 2001 ).
Intrinsic and extrinsic factors begin to blur when we consider the bones of the pelvis and
the femur. There are no inherent differences in the femora of boys and girls, but this begins to
change with the onset of puberty. There are, however, subtle differences in the fetal pelvis
that disappear in children and reappear in adolescence. As mentioned above, the pelvis is
under genetic control for shape, but this pelvic shape may have biomechanical consequences
for the femur, which can be compounded as a result of an individual's behavior. By looking at
population variation in sexual dimorphism of the skeleton, we can start to discern whether
a skeletal trait is the result of intrinsic or extrinsic agents. If consistent patterns of sexual
3 Simultaneously, the pelvis plays a pivotal role (pun intended) in the biomechanics of the lower leg in
locomotion, influenced by extrinsic factors as described below.
Search WWH ::




Custom Search