| Date | November 2009 | Marks available | 2 | Reference code | 09N.3.HL.TZ0.4 |
| Level | Higher level | Paper | Paper 3 | Time zone | TZ0 |
| Command term | Calculate | Question number | 4 | Adapted from | N/A |
Question
Cooperative breeding in birds occurs when more than two birds of the same species help to rear the young from one nest. For the Australian superb fairy-wren (Malurus cyaneus), mature non-breeding birds help to protect and rear the young, although they are not parents of any of them.
The bar charts below show the effect of the presence of helpers on mean egg volume, mean mass of six-day-old chicks and the probability of survival of the breeding females until the next breeding season.
State the effect of the presence of the helpers on mean chick mass.
Calculate the percentage decrease in mean egg volume found in the presence of helpers as compared to the parents only. Show your working.
With reference to the data, suggest why the activity of the helper affects the probability of survival of the breeding female until the next breeding season.
Cooperative breeding is an altruistic behaviour. Outline the evolution of altruistic behaviour.
Markscheme
(mean) chick mass increases in presence of helpers
with helper, smaller egg volume;
less resources from mother to produce egg/rear young;
more resources toward survival of mother;
(altruistic) behaviour is inherited;
adaptations/behaviours that help the colony survive at expense of individual;
selected for (natural selection);
will be more likely to be passed on;
Examiners report
Most candidates correctly identified an increase in the mean mass of the chicks in the presence of the helpers.
Many candidates correctly stated the percentage decrease in mean egg volume. Some were reluctant to round up to one decimal point. A few used an alternative method for this calculation.
No candidate used all the data. That is, no candidate used the smaller mean egg volume to explain that this may mean that the breeding female would be using less resources as the egg volume was smaller. Many incorrectly wrote of helpers protecting the breeding female from predators- as this did not use the data in any way. Marks were awarded if data was used, and most did suggest that with the helper the female may use less energy and that this may increase survival.
Very few candidates applied the theory of natural selection here. Although many did correctly state the meaning of altruistic behaviour, which was the starting point leading towards the idea of selection pressure.
