Biology Reference
In-Depth Information
SEXUAL D
IMORPHISM: INTRINSIC VERSUS EXTRINSIC
FACTORS
Many factors are involved in the development of the adult human skeleton. Bones are
extremely plastic throughout life and are constantly changing in response to extrinsic factors,
such as the biomechanical effects of load bearing and muscle forces acting on bone. Other
extrinsic factors that can leave a record on the skeleton (in addition to biomechanical
responses to forces) include the effects of nutritional status, activity levels and even body
mass. During growth and development, bone is additionally affected by intrinsic or systemic
factors under genetic constraint, such as hormone levels. Our ultimate goal is to tease
apart these confounding variables to develop methods that enable us as skeletal biologists
to distinguish traits that reflect only biological sex.
Sexual dimorphism is the difference between males and females of a species in terms of
body size, body shape, the rate/timing of development, or behavior. Sexual dimorphism
is a combined result of genetic factors (e.g., hormone levels) and the environment (e.g., nutri-
tion and cultural behaviors) (
Stinson, 2012
). For example, human males and females exhibit
dimorphism in body composition (fat versus lean mass), tooth size, and distal femoral
breadth (
Kieser, 1990; Mahfouz et al., 2007b; Stinson, 2012
). In terms of femur length, human
male femora can be anywhere from 3.3% to 10.7% longer than female femora. Compare this
to male gorilla femora, which are 20.9% longer than female gorilla femora (
Frayer and
Wolpoff, 1985
).
In humans, the primary sexual characteristics of the genitalia begin to differentiate and
develop early in utero (
Ulijaszek et al., 1998; Bogin, 1999
). The secondary sex characteristics
begin to develop at puberty and include differences in body size and pelvic morphology,
among other distinctions. These secondary sex characteristics are more affected by the envi-
ronment than are the primary sexual characteristics (
Frayer and Wolpoff, 1985
). An example
of this is the reduction in size dimorphism in humans when there are nutritional deficiencies.
Males are more affected by nutritional deficits, which results in less sexual dimorphism in
size (
Stini, 1975, 1982; Bogin, 1999; Ross et al., 2003
). Moore and Ross discuss female buffering
to environmental stress later in this volume (Chapter 6). The following discussion will elab-
orate on the distinctions between intrinsic and extrinsic factors that play a role in sexual
dimorphism.
Intrinsic Factors in Sexual Dimorphism
Intrinsic factors in sexual dimorphism are factors that arise from within the body system-
ically as mentioned, a prime example being those controlled by the gonadal or pituitary
hormones. Before the age of 12, it is difficult to accurately estimate the sex of a juvenile skel-
eton, due to the fact that most sexually dimorphic skeletal characters do not develop until
puberty, as already discussed. The onset of puberty is accompanied by high sex hormone
levels and initiates the last major growth spurt in humans (
Bogin, 1999; Scheuer and Black,
2004
). It is this adolescent growth spurt that manifests the secondary sexual characteristics
in the skeleton. Some traits of the skeleton, however, appear to be tightly genetically
controlled and are highly dimorphic. These include the size of the secondary dentition, as
well as the shape of the pelvis, both of which appear very early in development (
Fazekas
