(This exercise is based on Angilletta, M. J. Jr., R. S. Wilson, A. C. Niehaus, M. W. Sears, C. A. Navas, and P. L. Ribeiro. 2007. Urban physiology: City ants possess high heat tolerance. PLoS ONE 2: e258.)
(Note: The reference above links directly to the article on the journal’s website. In order to access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)
Cities, particularly large ones, are often hotter than their surrounding areas. In the largest cities, this difference can be as much as 12°C, the difference between the temperature of a warm room and human body temperature. Several factors, including the properties of surface materials (e.g., concrete and asphalt) as well as the lack of evapotranspiration and the high concentration of people, all contribute to what is known as the “urban heat island” effect. Does this additional heat in urban areas impose an added burden on the organisms that dwell in cities? If so, do these organisms evolve adaptations in response to the higher temperatures?
Michael Angilletta and his colleagues were interested in these questions as they pertain to leaf-cutter ants (Atta sexdens) within and outside the city of São Paulo, Brazil. At just under 11 million people (as of 2007), São Paulo is the fifth largest city in the world in population and larger than any city in the United States.
Angilletta’s group first wanted to find out what surface temperatures these ants actually experience. To do this, they recorded the temperatures on the surface of active trails in full sunlight from March 19–22, 2006. Their results are shown in Figure 1.
Figure 1
Question 1
What maximum temperature was reached each day, and at approximately what time of day did the temperature peak?
Question 2
The authors note that the temperature would likely be higher still in the summer. During which months of the year would São Paulo experience summer?
Question 3
Angilletta and his colleagues assessed heat tolerance as the time it took to lose mobility at a stressful temperature (in this case, 42°C). This type of assay is sometimes known as knock-down resistance. Do these ants experience such temperatures in nature?
Question 4
Is this a reasonable test of thermal tolerance?
Figure 2
Question 5
Angilletta and his colleagues examined knock-down resistance from four colonies of ants collected from the city (urban colonies) and five colonies of ants collected from nearby surrounding regions (rural colonies). In Figure 2, black lines show the results from urban colonies; gray lines show the results from rural colonies. In the least resistant colony, how long did it take for 40% of ants to lose knock-down resistance, and what type of colony was the least resistant colony?
Question 6
In the most resistant colony, how long did it take for 40% of its ants to lose knock-down resistance, and what type of colony was the most resistant colony?
Question 7
What overall trend was observed between the urban and rural colonies with regard to heat tolerance?
Question 8
Can we say for sure that the difference between the urban versus rural colonies in heat tolerance is due to the urban ants being better adapted to the heat? If not, what is an alternative hypothesis?
Question 9
How could one test whether the difference between the urban and rural ants is actually one of adaptation?
Figure 3
Question 10
Trade-offs in performance under different circumstances often occur in nature. Animals and plants that are superior in one task can be inferior at others. Angilletta’s group was also interested in whether the more heat-tolerant urban colonies experienced a trade-off in having lowered cold-tolerance. They assessed the tolerance to cold of the same urban and rural ant colonies by chilling the ants to 0°C, (a temperature that immobilizes all ants), and then observing how long it took the ants to recover mobility once they were warmed. In Figure 3, black lines represent urban colonies; gray lines represent rural colonies. What differences, if any, in cold tolerance are observed between the urban and rural colonies? Can you explain these results in light of trade-offs as discussed above?
