Conflict within families 2

Question 1

Parent-offspring conflict model
Whenever parents have to invest in more than one offspring, the parent-offspring conflict model suggests that parents and offspring will disagree over the optimal investment of parental care (Trivers 1974 American Zoologist 14: 249-264).

Answer 1

The benefit and cost of parental behaviour towards
an offspring as a function of the amount of fitness
the parent invests in the act (PI).

B = benefit to parent and offspring
C = cost to parent
C/2 = cost to offspring

At P, parent’s inclusive fitness (B - C) is maximised.

At O, offspring’s inclusive fitness (B - C/2) is maximised.

Question 2

Sibling Rivalry
The most extreme form of sibling rivalry is siblicide, i.e. killing of one offspring by another. This may precede cannibalism (e.g. spadefoot toad, sand tiger shark), but this is not usually the case in mammals and birds.
e.g. hyenas (Frank et al. 1991. Science 252: 702-704). Siblicide is the norm in same sex twins, probably as a means of reducing competition for high social status.
e.g. black eagles, blue-footed boobies, great white egrets. Siblicide occurs routinely and may be facilitated by parents. There are several characteristics of this behaviour
- resource competition (e.g. white-winged chough; Boland et al. 1997 Journal of Animal Ecology 66:683-691)
- monopolisable resources
- weaponry (although not essential)
- spatial confinement
- competitive disparities (usually under parental control)

Answer 2

There are essentially two likely explanations for why parents allow/promote this behaviour:
1. Insurance - if siblicide always occurs, the potential victim may be ‘insurance’ against death/infirmity of other offspring
2. Parental optimism - in species where siblicide only occurs under poor conditions parents lay an optimistic clutch that sometimes pays off, but not always. Parents should optimize brood asynchrony to maximize productivity (Mock & Ploger 1987. Animal Behaviour 35: 150-160).
These are extreme forms of sibling rivalry, where death usually results, but effects may be more subtle, e.g. in pre-industrial Finns (Rickard et al. 2007 Proceedings of the Royal Society B 274: 2981-2988).