Question 22N.2.HL.TZ0.1
| Date | November 2022 | Marks available | [Maximum mark: 16] | Reference code | 22N.2.HL.TZ0.1 |
| Level | HL | Paper | 2 | Time zone | TZ0 |
| Command term | Calculate, Compare and contrast, Outline, State, Suggest | Question number | 1 | Adapted from | N/A |
Common shrews (Sorex araneus) are small mammals found in Northern Europe. Their diet includes insects, slugs, spiders, worms and amphibians. They do not hibernate in winter because their bodies are too small to store sufficient fat reserves.
[Source: [Shrew], n.d. [image online] Available at: https://www.pxfuel.com/en/free-photo-jslkw [Accessed 29 October 2021].]
To study brain size in shrews, researchers anesthetize them, X-ray their skulls and measure the height of the braincase (BCH) where the brain is located. The graph shows the relationship between BCH and the brain mass of individual adult shrews.
[Source: adapted from Lá zaro, J., Hertel, M., LaPoint, S., Wikelski, M., Stiehler, M. and Dechmann, D.K.N., 2018.
Journal of Experimental Biology 221. http://doi.org/10.1242/jeb.166595.]
The researchers found that the BCH of any individual adult shrew could vary seasonally. They collected shrews at different times of the year. The BCH of each shrew was compared with its body mass. The results are displayed in the chart.
[Source: adapted from Schaeffer, P.J., O’Mara, M.T., Breiholz, J., Keicher, L., Lázaro, J., Muturi, M.,
Dechmann, D.K.N., 2020. R. Soc. Open Sci. 7. http://dx.doi.org/10.1098/rsos.191989.]
Shrews were observed in different seasons and the time they spent on a particular activity was recorded and expressed as a percentage of the total observation time. The circles in the kite shapes represent the mean value of time for each activity.
[Source: adapted from Schaeffer, P.J., O’Mara, M.T., Breiholz, J., Keicher, L., Lázaro, J., Muturi, M.,
Dechmann, D.K.N., 2020. R. Soc. Open Sci. 7. http://dx.doi.org/10.1098/rsos.191989.]
The researchers were interested in the seasonal differences in searching for food. They set up a square arena with sides of 110 cm and four entrances (A, B, C and D). Containers were placed in the arena, some with food and others with no food. The diagram shows a top-down view of the arena.
[Source: adapted from Lázaro, J., Hertel, M., LaPoint, S., Wikelski, M., Stiehler, M. and Dechmann, D.K.N., 2018.
Journal of Experimental Biology 221. http://doi.org/10.1242/jeb.166595.]
Each shrew was starved of food for two hours before its cage was opened at one of the entrances to the arena. The length of the path taken by the shrew to obtain food was measured. This was standardized by dividing the path length by the straight-line distance from the entrance to the containers with food. Each shrew was used for 10 trials.
The graph shows the standardized mean path length taken by all the shrews at different seasons of the year. The letters show where the cages were placed for each trial.
[Source: adapted from Lázaro, J., Hertel, M., LaPoint, S., Wikelski, M., Stiehler, M. and Dechmann, D.K.N., 2018.
Journal of Experimental Biology 221. http://doi.org/10.1242/jeb.166595.]
State the relationship between BCH and brain mass of shrews.
[1]
positive correlation/the greater the BCH the greater the brain mass;
No mark for ‘positive relationship’ or for directly proportional
Most candidate described the relationship shown on the graph as a positive correlation. Some referred to it incorrectly as directly proportional, but that would be revealed by data points along a straight line on the graph. The term 'positive relationship' is vague and should be discouraged.
Outline how the shrew labelled P differs from the normal relationship between BCH and brain mass.
[1]
- high BCH but brain mass is low/lower than expected/lower than others with similar BCH;
- (fairly) low brain mass but BCH is high/higher than expected/higher than others with similar brain mass;
Almost all candidates understood what made P an outlier, but not all could express lucidly how it deviated from the trend.
Suggest a reason that researchers use BCH rather than brain mass to indicate brain size.
[1]
easier to measure/doesn’t require dissection/non-invasive / shrew not harmed/killed/more ethical;
The mark can be awarded for one of these reasons even if it is not the first reason given in the answer.
Many candidates came up with unlikely reasons for using BCH as a measure, rather than that it was easier to measure and less harmful to the shrew. A common misconception was that brain mass was an inaccurate measure of brain size because brain density might be variable. It was surprising that so many candidates thought the height of the brain case would give a better measure of brain size, especially as the scatter graph showed that BCH is not directly proportional to brain mass.
State the season when shrew brain mass is greatest.
[1]
Summer;
90% of candidates correctly identified summer as the season when brain mass was greatest.
Compare and contrast the results for winter and spring.
[2]
Compare part of answer = similarity:
- both have low BCH (compared with summer);
Contrast part of answer:
- greater body mass in spring than winter;
OR
overall/mean/average BCH higher in spring than in winter;
OR
more variation in body mass in spring than winter;
For the second alternative in mpb, the answer must not state simply that BCH is higher in spring as there is much overlap. Do not accept quoted figures without the similarity or difference being stated.
This was a 'compare and contrast' question, so similarities and differences were expected in the answer. There were plenty of choices for differences between winter and spring but only one acceptable similarity — that BCH was lower in both winter and spring than in summer. Some answers failed to discriminate between BCH and brain mass — the trends were not the same so these terms were not interchangeable. Some candidates gave a comparison and then repeated it as the converse statement — for example 'sample mass is higher in spring than winter and is lower in winter than spring'. Clearly this is not necessary.
Suggest a reason for the difference in BCH in summer and winter.
[1]
- large brain (indicated by large BCH) requires/uses much energy;
- shrews need/use much energy in winter (other than for the brain);
- much energy used in winter for keeping warm/searching for food;
- food/energy more abundant in summer/less abundant in winter;
- growth between winter and summer (so BCH larger in summer);
Do not accept hibernation/lower metabolic rate/lower body temperature/lower activity levels/less food eaten in winter. Do not accept answers about body mass.
The mark allocation here was rather low, as a proper explanation of the difference between BCH in winter and summer required several linked ideas. Any one of a number of important elements of an explanation was accepted, making it relatively easy to score the mark.
State the activity and season that occupied the greatest mean percentage of observation time.
[1]
resting in spring;
80% of candidates identified the season and activity correctly.
Suggest a reason for the difference in the time observed eating and drinking.
[2]
- more food/energy eaten/required in winter/cold;
- food needed to maintain temperature/stay warm/generate heat;
- more loss of body heat in cold conditions;
- more energy used hunting for food;
- food less available in winter/harder to find enough food;
This was generally well answered, with candidates mostly focussing on food being harder to find in winter and more energy being needed to maintain body heat.
Calculate the percentage of containers that contained food.
[1]
4(%);
Percentage calculations often cause widespread difficulties but most candidates found this one easy. The only common mistake was to divide 4 by a hundred, and then forget to multiply by a hundred to turn the proportion into a percentage.
Outline a reason that the path length was standardized.
[1]
compensates for the different distances between entrances and food/OWTTE;
OR
to enable (fair/valid) comparison/OWTTE;
Do not accept unspecific answers such as ‘to be consistent’.
This was another question that tested candidates' ability to express a relatively simple idea clearly, rather than having to understand a more complicated idea. The key part of the answer was that standardizing path lengths allowed results to be compared to find the effect of the intended independent variable, by cancelling out another variable.
Compare and contrast the results for trials 2 and 9.
[2]
Similarity between 2 and 9:
- winter path length longer (than spring and summer) in both (trials 2 and 9/from entrances B and C);
Contrast between 2 and 9:
- path length longer in trial 2 than 9/from entrance B than entrance C (in all seasons);
OR
error/bar/standard deviation/variation in data greater in trial 2 than 9/from entrance B than entrance C (in all seasons);
As in part (e), a similarity and a difference were required. When analysing data, candidates should try to keep in mind whether aspects are of interest and lead to conclusions significant conclusions or not. Here the two interesting findings were that the path length was highest in winter in both trials and that the shrews' performance between trial 2 and 90 had improved in all seasons, so they found the food with a shorter path length.
With reference to all the data, suggest a reason for the difference in standardized mean path length for summer and winter.
[2]
- in winter shrews have smaller brains/smaller BCH / converse for summer;
- lower/poorer memory/thinking/cognitive skills/learning/intelligence/senses/sense of smell/ability to find food in winter/converse for summer;
The instruction to refer to all the data does not indicate that conclusions from each graph should be restated, but rather than the research findings should be combined to produce a hypothesis. The expected reasoning here was that the lower BCH of shrews in winter showed brain mass was on average lower, so cognitive ability was reduced and shrews were slower to find food.