Urban colonies are more resistant to a trace metal than their forest counterparts in the ant Temnothorax nylanderi

Abstract

Cities can be used as open-sky replicates to assess the responses of organisms to environmental changes, such as increased heat or pollution. In contrast with the numerous communitylevel studies, population-level studies are rare, hugely biased towards birds, and rarely based on multiple city replicates. They are also mostly based on solitary species, despite some social insects being ecologically dominant and economically important. Our study aimed at assessing whether colonies of the ant Temnothorax nylanderi from forest and urban populations exhibit distinct responses to pollution.We collected colonies from four pairs of forest and urban habitats and reared them in the laboratory under normal or cadmium-supplemented diets. We then measured the emergence rate and size of newlyproduced workers as well as the mortality rate of adult workers. We also assessed the effect of some social factors such as colony size or size of field workers. In all four forest/city replicates, urban colonies were less negatively affected by cadmium than forest colonies. We did not find any effect of social factors on cadmium resistance, in contrast with what was found in other eusocial insects. We discuss the potential adaptive nature of this differential response.

Disentangling the roles of social and individual effects on cadmium tolerance in the ant Temnothorax nylanderi

“…Temnothorax nylanderi is a tiny ant, nesting in acorns or twigs, widespread in western and central Europe and living in small colonies (a few dozen workers), and common in urban and forest habitats (Foitzik, Strätz, & Heinze, 2003). We collected colonies at two sites where a differential response to cadmium has been documented (Jacquier et al, 2021a). We collected 103 city colonies in an urban park in Paris (Parc des Buttes-Chaumont, 48°52'47.1"N; 2°23'45.6"E) and 109 forest colonies in a forest located 50km away from Paris (Chantilly forest, 49°10'59.8"N; 2°28'43.6"E).…”

“…Control colonies were fed three times a week with a mixture of diluted honey, soya yogurt and dried crickets. Cadmium colonies were fed with the same mixture containing 100μg/g cadmium (Jacquier et al, 2021a). The concentration used in the food corresponds to the LC 50 (the concentration that triggers 50% mortality over a given period of time), a physiologically but not ecologically relevant dose.…”

“…However, in the field, recurrent exposure to pollutants can lead to bioaccumulation and cadmium concentration can reach 102 mg/kg in the ant Formica polyctena, for a soil concentration of 2 mg.kg −1 (Starý & Kubizňáková, 1987). Therefore, the concentration that we used was high but still in the upper range of real field values (Jacquier et al, 2021a). We used 15 "CC", "FC" and "FF" colonies, and 16 colonies "CF" with control food.…”

“…In this study, we used a cross-fostering experiment while rearing colonies under common laboratory conditions in the ant Temnothorax nylanderi in the presence or absence of a stressor (cadmium). A previous study showed that city colonies were less negatively affected by cadmium than forest ones for two traits: the size of produced workers and the survival of larvae (Jacquier et al, 2021a). In the present study, we used the same laboratory rearing setup and we measured the same traits (i.e.…”

“…body size at emergence and survival) as well as the development duration. Moreover, in contrast with Jacquier et al (2021a) who only assessed worker traits, we also measured sexual production and size of sexuals. Size of sexuals is an important trait linked with fitness in ants as larger sexuals have greater mating success and colony founding success in some species (Abell et al, 1999;Wagner & Gordon, 1999;Wiernasz et al, 2001;Wiernasz & Cole, 2003;Fjerdingstad & Keller, 2004).…”