11.2 Applications and skills

11.2.4 Diagrams and issues related to sexual reproduction

For this topic, there are a number of diagrams that you should be able to annotate for your exam. Test yourself using the printable downloads. You may check your answers by clicking on the images. 

Skill: Annotate a diagram of mature sperm and egg cells to indicate functions

Figure 11.2.4a – A mature spermatozoonFigure 11.2.4a – A mature spermatozoon

Figure 11.2.4b – A mature ovumFigure 11.2.4b – A mature ovum

Skill: Annotate diagrams of a seminiferous tubule and ovary to show the stages of gametogenesis

Figure 11.2.4c – Spermatogenesis in the seminiferous tubuleFigure 11.2.4c – Spermatogenesis in the seminiferous tubule

Figure 11.2.4d – Oogenesis in the ovaryFigure 11.2.4d – Oogenesis in the ovary

Application: Correlation between animal size and development at birth

  • It is logical to predict that the gestation period of mammals should be proportional to the animal’s body size, as it takes more time to grow and develop a larger fetus.
  • However, the relationship between gestation length and body size is not linear. Assuming that all the proportions of a fetus are doubled, then the volume (and mass) of the larger fetus would be eight times greater than the smaller, so we could predict that a large fetus should take x3 times longer to develop than a small fetus.
  • A problem with this prediction is that not all offspring are born at the same stage of development. Altricial species are born without the ability to walk, to thermoregulate, or to see. Precocial neonates are able to walk and are relatively independent.
  • Altricial neonates tend to be born in litters. Gestation periods are shorter in these species because the development of each individual fetus is limited by the space available in the uterus.
  • Precocial neonates are normally born from single pregnancies. The animal is larger and gestation periods are longer.
  • Figure 11.2.4e shows the average gestation period as a logarithmic function of adult body size for 429 mammal species. Analyse the graph and answer the questions.

Figure 11.2.4e – Gestation periods as a function of adult body mass in mammalsFigure 11.2.4e – Gestation periods as a function of adult body mass in mammals

  1. Outline why a logarithmic scale is used.
  2. Humans have not been included in this data set. The average body mass of a human adult is 6500g, and the gestation length is 286 days. Locate the area on the graph where a point representing humans should be.
  3. Describe the trend between gestation length and animal size for both atricial species (hollow squares) and precocial species (solid squares).
  4. Determine whether humans neonates are atricial or precocial.
  5. Suggest reasons why humans don’t fit the pattern.

Nature of Science: Assessing risks of estrogen pollution

  • Ethinyl estradiol is a synthetic form of estrogen that has been used in female contraceptive pills for many decades. It is excreted in urine and currently there are few treatment facilities capable of removing estrogen from sewage water.
  • At high concentrations, both natural and synthetic estrogens are pollutants that are known to have feminising effects on fish and wildlife. For example, in the UK, male fish found downstream of points of sewage discharge have been found to have smaller gonads than other males.
  • Other estrogen-mimicking compounds are present in food additives and industrial pesticides, but causal relationships are difficult to establish.
  • The most logical way to reduce estrogen pollution would be to properly treat wastewater. However, it remains to be seen how this issue will be resolved in the future.

Figure 11.2.4f – Baby elephantFigure 11.2.4f – Baby elephant
The gestation period of an African elephant, genus Loxodonta, is about 670 days.

Nature of Science

Assessing risks and benefits of research: Estrogen and progesterone are released into the environment as a result of use of the female contraceptive pill. The risks to human male fertility of environmental contamination by steroid hormones have not been not adequately addressed.

Figure 11.2.4.g – TwinsFigure 11.2.4.g – Twins
When twins are born, their combined mass is higher than a single baby, but the gestation length is slightly shorter.

Figure 11.2.4h – Tree kangarooFigure 11.2.4h – Tree kangaroo
Gestation in the tree kangaroo, Dendrolagus, is only 44 days. All marsupials give birth to atricial neonates.