science

Secondary sex characteristic

Secondary sex characteristic is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Secondary sex characteristic rather than just read about it. In short: A secondary sex characteristic (or secondary sexual characteristic) is a physical characteristic of an organism that is related to or derived from its sex, but not directly part of its reproductive system. In humans, these characteristics typically start to appear during puberty—and include enlarged breasts and widened hips of females, facial hair and Adam's apples on males, and pubic hair on both.

Key takeaways

Secondary sex characteristic belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Secondary sex characteristic to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Secondary sex characteristic from memory before moving on to harder problems.

Reference excerpt

A secondary sex characteristic (or secondary sexual characteristic) is a physical characteristic of an organism that is related to or derived from its sex, but not directly part of its reproductive system. In humans, these characteristics typically start to appear during puberty—and include enlarged breasts and widened hips of females, facial hair and Adam's apples on males, and pubic hair on both. In non-human animals, they can start to appear at sexual maturity—and include, for example, the manes of male lions, the bright facial and rump coloration of male mandrills, and horns in many goats and antelopes. Secondary sex characteristics are particularly evident in the sexually dimorphic phenotypic traits that distinguish the sexes of a species. In evolution, secondary sex characteristics are the product of sexual selection for traits that show fitness, giving an organism an advantage over its rivals in courtship and in aggressive interactions. Many characteristics are believed to have been established by a positive feedback loop known as the Fisherian runaway produced by the secondary characteristic in one sex and the desire for that characteristic in the other sex. Male birds and fish of many species have brighter coloration or other external ornaments. Differences in size between sexes are also considered secondary sexual characteristics.

Secondary sex characteristics vs. primary sex characteristics The reproductive organs in male or female mammals that are usually identifiable at birth are described as the primary sex characteristics or sex organs. In the male, these are the penis, testes, and scrotum. In the female, these are the ovaries, fallopian tubes, uterus, cervix, vagina and vulva. The primary sex organs are different from the secondary sex organs because at maturity they are directly related to the production gametes, which are haploid male or female germ cells, and their uniting with one another during sexual reproduction to form a zygote. The secondary sex characteristics differ in that they will not be identifiable at birth, they will develop as the subject becomes sexually mature. In mammals, these characteristics may include increased mammary tissue in females and greater muscle mass in males. Secondary sexual characteristics have an evolutionary purpose: increase the chance of breeding.

Evolutionary roots

In The Descent of Man and Selection in Relation to Sex, Charles Darwin hypothesized that sexual selection, or competition within a species for mates, can explain observed differences between sexes in many species. Ronald Fisher, the English biologist, developed a number of ideas concerning secondary characteristics in his 1930 book The Genetical Theory of Natural Selection, including the concept of Fisherian runaway which postulates that the desire for a characteristic in females combined with that characteristic in males can create a positive feedback loop or runaway where the feature becomes hugely amplified. The 1975 handicap principle extends this idea, stating that a peacock's tail, for instance, displays fitness by being a useless impediment that is very hard to fake. Another of Fisher's ideas is the sexy son hypothesis, whereby females will desire to have sons that possess the characteristic that they find sexually attractive in order to maximize the number of grandchildren they produce. An alternative hypothesis is that some of the genes that enable males to develop impressive ornaments or fighting ability may be correlated with fitness markers such as disease resistance or a more efficient metabolism. This idea is known as the good genes hypothesis.

In non-human animals Examples of secondary sex characteristics in non-human animals include manes of male lions and long feathers of male peafowl, the tusks of male narwhals, enlarged proboscises in male elephant seals and proboscis monkeys, the bright facial and rump coloration of male mandrills, horns in many goats and antelopes, and the swollen upper lip and elongated premaxillary and maxillary teeth of male spikethumb frogs. Male fish develop "nuptial tubercles", mainly on their snouts, in the breeding season. These are an honest signal of health, and may assist females in sexual selection for species that use lek mating, such as the roach Rutilus rutilus. Biologists today distinguish between "male-to-male combat" and "mate choice", usually female choice of male mates. Sexual characteristics due to combat are such things as antlers, horns, and greater size. Characteristics due to mate choice, often referred to as ornaments, include brighter plumage, coloration, and other features that have no immediate purpose for survival or combat. Male jumping spiders have visual patches of UV reflectance, which are ornamentations used to attract females.

In humans

Sexual differentiation begins during gestation, when the gonads are formed. The general structure and shape of the body and face, as well as sex hormone levels, are similar in preadolescent boys and girls. As puberty begins and sex hormone levels rise, differences appear, though some changes are similar in males and females. Male levels of testosterone directly induce the growth of the genitals, and indirectly (via dihydrotestosterone (DHT)) the prostate. Estradiol and other hormones cause breasts to develop in females. However, fetal or neonatal androgens may modulate later breast development by reducing the capacity of breast tissue to respond to later estrogen. Underarm hair and pubic hair are usually considered secondary sex characteristics, but they may also be considered non-secondary sex characteristics because they are features of both sexes following puberty.

Females In females, breasts are a manifestation of higher levels of estrogen; estrogen also widens the pelvis and increases the amount of body fat in hips, thighs, buttocks, and breasts. Estrogen also induces growth of the uterus, proliferation of the endometrium, and menstruation. Female secondary sex characteristics include:

Enlargement of breasts and erection of nipples. Growth of body hair, most prominently underarm and pubic hair. Widening of hips; lower waist to hip ratio than adult males.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Secondary sex characteristic

Start with the simplest possible case. Write down what Secondary sex characteristic claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Secondary sex characteristic before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Secondary sex characteristic ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Secondary sex characteristic

In research: Secondary sex characteristic appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Secondary sex characteristic in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Secondary sex characteristic is common in secondary-school and first-year university syllabi. It links to neighbouring topics Secondary sexual characteristics, Sexual selection, so understanding it makes those chapters shorter.
In everyday life: Look for Secondary sex characteristic outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Secondary sex characteristic in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Secondary sex characteristic means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Secondary sex characteristic out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Secondary sex characteristic in simple terms?

Why does Secondary sex characteristic matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Secondary sex characteristic?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Secondary sex characteristic.