- Trivers (1972) conceived parental investment this is investment by parents in individual offspring which increases the offsprings chance of survival and achieiving reproductive success at the expense of parents ability the invest in other children, either living or yet to be born.
- PI includes the provision of resources
- females have initial investment greater than the male because female gametes are more costly to produce than males.
- being pregnant limits opportunities for further reproduction as women cannot become pregnant again once they are making investment of current offspring even more
- males arent as restricted in their ability to reproduce - though in many species males can provide much parental care
- Trivers (1972) believes that there is an optimum number of offspring for each parent. low investing males can afford many offspring and favour a quantity rather than quality approach. high investing females prefer quality rather than quanitity.
- limiting factor for males in maximising reproductive success = number of women they can impregnate while females it is important to choose males with attractive genetic qualities who will provide resources and protection in the long term.
- given rise to male-male intrasexual competition and male-female intersexual competition
- women are always certain of maternity of their children
- men arent certain of paternity
- why men have several parteners to ensure they can be certain of paternity somewhere
- Predicts there are several ways in which male and female PI will differ:
1. Paternal Certainty - with internal fertilisation males are more likely to desert with external fertilisation as they are unsure of paternity
2. Order of gamete release - internal fertilisation gives males the chance to desert and leave child care duties to the female, while external fertilisation females have the opportunity to desert first.
3. Monogamy - where offspring are born at early stage of development where child care is intensive, pair bonds are exclusive and long lasting = increases offsprings survival chances
4. Grandparental Certainty - maternal side = more certain so more care will come from them
Research - Sex Differences
Gross and Shine (1981)
report that with internal feritilisation parental care is carried out by females in 86% of species while with external fertilisation parental care is carried out by males in 70% of species
SUPPORTS the predictions based on paternal certainty
Krebs and Davies (1981)
report that males of some fish species release sperm first in a nest and then the female lays eggs, meaning that the male has the first opportunity to desert but the males carry out the childcare
GOES AGAINST the idea of the order gamete release hypothesis
FOUND - males who demonstrate cues of a positive disposition to parental investment were seen as more attractive by females
SUPPORTS the evolutionary explanation of parental investment
Evaluation - Sex Differences
- can test the evolutionary theory by making predictions based on the theory and seeing whether real life examples support predicitions
- Dawkins and Carlisle (1976) found that 36 out of 46 species where there is simultaneous gamete release and both sexes have equal chance of deserting the males provide more monoparental care.. REFUTES the evolutionary predictions
- Krebs and Davies (1981) report that it isnt always true that external fertilisation leads to increased parental certainty.
- Neonaticide is the killing of newborn babies by their mothers - this isnt explained by the theory