BIOL3046 Lecture Notes - Lecture 7: Honey Bee
5 Jul 2018
School
Department
Course
Professor
Animal Behaviour BIOL3046 460381099
Lecture 7: Sex ratio theory.
• Even though one male is sufficient to inseminate multiple females, we still find unequal
sex ratios most of the time. Optimal sex ratio is when there are sufficient males to
ensure the successful reproduction of all females.
Numerical sex ratio: Number of males and females.
Investment sex ratio: Investment of males and females; how much energy it
requires.
Example: Honey bee colonies only produce 1 queen per reproductive season,
but produce hundreds of males. Numerical sex ratio is biased towards
males. Investment sex ratio is biased towards females, as they are
essential in queen establishing a colony.
Causes of unequal sex ratio:
Local mate competition: When brothers compete among themselves for mating.
Example: Fig wasps have complete control over their sex ratio
depending on whether they put sperm on the eggs. Normally
produce 1 male to mate with all the females, however if
another wasp lays eggs in the same fig, the number of males is
increased so that they can mate with her females as well as
the wasps females.
Local resource competition: When offspring of one sex complete directly with their
parents. Example: Son and father fight for territory; When
food is scarce, bell miners will produce females because they
will find their own food so they don’t use up the limited
resources.
Local enhancement: When offspring of one sex stay at home to help their parents.
Example: When food is abundant, sex ratio is biased towards
males because they will stay and defend the territory to help
protect the abundant resources.
Unequal reproductive success: When a parent possesses a heritable trait that
increases the reproductive success of one sex. Example: In
deer, the social rank of the mother leads to the rank of the
son. Thus, mothers of high rank will have greater reproductive
