Evidence for evolution in urban systems is increasing, but a key outstanding question in the field of urban ecology and eco-evolutionary dynamics is whether the ability of individuals to adapt to environmental conditions (plasticity) also evolves and how this process can happen. In a new paper, Sarah Diamond and her collaborators looked to examine the evolution of thermal plasticity in a system using acorn ants (Temnothorax curvispinosus) which occupy both urban and natural (forested) habitats. The Diamond’s lab work has been featured here before, including research looking at how higher nighttime temperatures in cities may impact caterpillars and work in this same system using acorn ants. In their previous work, Diamond’s lab has shown that both local adaptation and plasticity have a role to play in how acorn ants respond to the urban thermal environment.
Acorn ants, not surprisingly, have colonies which construct nests inside acorns and other biological structures. These colonies are small, and ants forage relatively close to the nest. In cities, which are known to have higher temperatures than surrounding natural areas due to the Urban Heat Island Effect, ants could be exposed to higher temperatures in the microhabitat they utilize. Diamond tested for this possibility using iButtons to measure the thermal characteristics of nests and the foraging areas surrounding them. She found that nests at city sites heated up faster in the morning than nests at natural, forested sites. Additionally, when she examined the temperatures of foraging areas around the nest, she found greater differences between them in urban areas. This higher variability means that ants foraging in cities are likely to move between areas with different temperatures more often, and might need to adjust more quickly to these temperature swings (which could be quite large for a little ant!).
Diamond et al. brought back many colonies from city and natural sites to the lab and waited for them to entirely replace their workers, so that all ants besides the queens grew up under controlled lab conditions. Colonies grew in one of two different temperature regimes, one mimicking urban conditions and one representing cooler, forested conditions. Workers from each colony were taken and tested for their critical thermal limits, both the high and low temperatures at which the movements of individual ants became uncoordinated. When conducting these trials, the research team also varied the speeds with which the temperatures to which ants were exposed changed, a variable termed “ramp-rate”. They hypothesized that, because urban ants may move between areas with greater differences in temperatures, they may be able to adjust to quickly changing temperatures (via plasticity) more effectively.
The research team found just that result. At the higher ramp-rate of 1 degree/minute, ants from urban colonies showed adaptive plasticity in their upper thermal limits (CTmax) that allowed them to tolerate higher body temperatures than ants from forested sites. This plasticity could allow foraging ants to survive better or forage more effectively in the city. Because Diamond et al. observed this effect in ants raised entirely in the lab, it seems possible that these differences in plasticity may have evolved as local adaptations. Conversely, the researchers did not find differences in plasticity for lower thermal limits (CTmin). They theorize that this may be because they also observed no differences in cooling rates of urban and rural nests (Fig. 1 above), so this type of plasticity may not be advantageous.
This work adds another piece to Diamond’s (and our!) understanding of how these ants may be adapting in response to the novel thermal environments present in cities. It suggests that future research should account for the possibility that plasticity itself may be evolving in organisms utilizing urban environments. This type of adaptation may occur when species are faced with conditions that vary strongly over fine local scales or the microhabitats that they use.
For more details, check out the full manuscript in Conservation Physiology:
Sarah E. Diamond, Lacy D. Chick, Abe Perez, Stephanie A. Strickler, and Crystal Zhao
DOI: 10.1093/conphys/coy030