1. Sex & behaviour: sexual investment
CfE Advanced Higher Biology
Unit 2: Organisms and Evolution

2. SQA mandatory key areas
Comparison of investment in sperm and egg production – number and energy store; greater investment by females. Problems and solutions of sex for sessile organisms.
Parental investment, optimal reproduction and reproductive strategies in terms of the number and quality of current offspring versus potential future offspring. Classification of parental investment into discrete r-selected and K-selected organisms does not reflect continuous range of life history strategies.

3. Key concepts
Parental investment is costly but increases the probability of production and survival of young. Simplistic various reproductive strategies have evolved ranging from polygamy to monogamy.

4. Darwin’s Puzzle: Why are males and females different?
Darwin, C The Descent of Man and Selection in Relation to Sex. 1st ed., Murray, London.

5. Parental investment and sexual selection
Trivers 1972

6. Assumption
Assumption: every organism has adaptations that function to facilitate reproduction
Members of a population/species live in the same environment, so why do some animals have different adaptations than others?
Morphs: age, sex, others
SEX: male and female adaptations are different
WHY?

7. Parental investment
“Any investment by the parent in an individual offspring that increases the offspring’s chance of surviving (and hence reproductive success) at the cost of the parent’s ability to invest in other offspring” (Trivers 1972)

8. Sperm vs. egg
In sexually-reproducing species, the relative size of gametes define who is male and who is female.

9. Sexual dimorphism
Amongst vertebrates, the clearest dimorphism is between gamete (sex cell) size. This single physical difference explains why behavioural sex differences exist.
Females gametes: large, nutrient-filled, expensive to produce, limited in number, and produced infrequently. If fertilised this will lead to high costs to the female.
Male gametes: small, have no nutrients, cheap to produce, constantly made throughout life.
Reproductive Capability: females are thus classed as the ‘slow sex’ and males the ‘fast sex’.

10. Nurturant females
In most animals, and almost all mammals, females provide far more parental investment than just the egg
Internal fertilization protects, but at a cost
Cod vs. gorillas
Humans (mammals):
Prolonged internal gestation (pregnancy)
Placentation
Lactation

11. Female reproductive strategy
Females have much to lose if they mate with the wrong male, they are thus selective about who they mate with.
They look for certain criteria:
Physical Features: size and strength which confer dominance and so preferential access to resources.
Behavioural Features: may indicate willingness to invest or good parenting skills.
Females will compete with other females for the right to choose the most desirable (alpha) males.
They gain little from multiple matings and seek quality not quantity.
Almost every reproductively capable female will be able to find a mate of some sort.

12. Male reproductive strategy
Males are far less choosy as they as they little to lose and everything to gain if they can have as many mating opportunities as possible.
Males are not tied to rearing offspring and so seek quantity.
While they would prefer a superior female, they are less choosy.
If presented with a sexual opportunity they will take it.
Males compete vigorously with other males for access to fertile females.
Male reproductive success is however very variable, a small number of males will achieve many matings, while many males may never mate.

13. Competitive males
Males are fighting with each other to mate with as many females as possible
More females = more offspring (sharp contrast to females)

14. Sexual selection and parental investment theory
For members of the sex that invests more in offspring, reproductive success is limited by the amount of resources an individual can secure for itself and its offspring.
For members of the sex that invests less in offspring, reproductive success is limited by the number of mates one can acquire.

15. Bateman’s Gradient
Bateman (1948) observed that the number of offspring fathered by male fruit flies increased in proportion to the number of females with which the male had mated.
Female reproductive success did not increase as her number of partners increased.
This is 'Bateman's gradient' - the steeper the gradient the stronger is sexual selection.
males
No. of offspring
females
No. of mates
From Anderson & Iwasa (1996) p 54.

16. Sexual selection and parental investment theory
What of it?
Selection acted on males differently than it acted on females
Specifically, differences in parenting strategies cause differences in adaptations
Sex that invests more: adaptations to survive and get resources for offspring
Sex that invests less: adaptations to help them get as many mates as possible
It explains why, in many species, males look and behave differently than females

17. Sexual selection and parental investment theory
Explains primary sex differences (uteruses vs. testes)
Explains secondary sex differences
Differences in weaponry (intrasexual selection)
Differences in ornaments (intersexual selection)
When the sexes have different adaptations, they are “sexually dimorphic”

18. r & K Selection
r/K selection theory relates to the selection of combinations of traits in an organism that trade off between quantity and quality of offspring. The focus upon either increased quantity of offspring at the expense of individual parental investment, or reduced quantity of offspring with a corresponding increased parental investment, varies widely, seemingly to promote success in particular environments.

19. How many, and how often? r Selection (aka. Quick-and-many)
K selection (aka. Slower and fewer)
Age of maturation
Young – usually before the next breeding season
Older – usually many seasons after birth
Number of offspring
Many
Few
Frequency of breeding
Usually frequently (many times a season) – high fecundity = many eggs produced per breeding season
Generally once a season. Low fecundity
Size of offspring
Usually small
Generally larger
Mortality rates
High – many offspring do not live to sexual maturity
Low – offspring generally survive
Examples of species
Mice, rabbits, most insects, cane toads, octopus, mass spawning organisms
Humpback whales, elephants, humans, some birds

20. Eggs or liveborn young? Oviparity Viviparity Literally means
Ovum = egg, parus = bearing
Vivus = living, parus = bearing
Description
Eggs released by mother, embryos develop outside mother’s body, nourished by egg yolk
Embryo develops in mother, born as young. Mode of nutrition varies
Benefits
Reduced energy use in care of young
Yolk provides good nutrient source
More likely for offspring to survive to birth
Drawbacks
Eggs may need to be incubated
Less chance of survival to birth due to eg. Eggs desiccating, predators, poor environment
Energy expenditure for female carrying offspring
Examples
Birds, sharks, reptiles, monotremes
Humans, some snake species, most mammals

21. Oviparity Bony fish and frogs Birds and reptiles Known as Shell-
Amniote eggs
Shell
None, or leathery membrane
Usually a hard, calcerious shell
Benefits
Wedge into safe crevices
Better protected from desiccation – do not have to reproduce in water
Dangers
Desiccation
Damage
Cannot be hidden in crevices
Examples
Port Jackson shark, amphibians
Hens, monotremes, crocodiles

22. Viviparity Egg yolk viviparity Placental viviparity
Types of viviparity are recognised by the nutrient source for the developing embryo
Egg yolk viviparity
Placental viviparity
Other source of nutrient
More notes
Cool habitat – kept warmer within body
Largish eggs
Any – nutrient sent via blood stream to embryo
Very small eggs
Feed them unfertilised eggs
Feed them “uterine milk” – secretion from uterus
Examples
Some sharks and snakes. Sea snakes – so that they do not have to return to land to breed
Mammals except monotremes, hammerhead shark
Porbeagle shark (feeds with eggs), Bat rays (feed with “milk”)

23. Parental care or not? No parental care Care of laid eggs Care of young
What is it?
No contact with offspring after eggs are laid
Guarding and/or incubating eggs to hatching
Care of young after hatching/birth
Benefits
Free to mate more
No energy expenditure
Eggs have protection from predators/ harsh conditions
High chance of offspring survival
Drawbacks
High levels of mortality
Energy expenditure
Some mortality after hatching
Very high levels of energy expenditure – may not be able to mate for many years after offspring birth
Examples
Reef fish, frogs, turtles
Seahorse, diamond python, cephalopods (eg. Octopus, squid), spiders
Humans, primates. Mammals (milk), emperor penguins, emus

24. Assessment task
Choose two organisms to compare reproductive strategies (one r and one K selection) including:
Comparison of investment in sperm and egg production – number, size, energy store.
Parental Investment – number of offspring produced and by which method (oviparity or vivaparity), degree of parental care.
Explain each organisms chance of survival in light of this information.