Urban / rural acorn ant metabolic rate and running speed

Metabolic rates of ectotherms are expected to increase with global trends
of climatic warming. But the potential for rapid, compensatory evolution
of lower metabolic rate in response to rising temperatures is only
starting to be explored. Here, we explored rapid evolution of metabolic
rate and locomotor performance in acorn-dwelling ants (Temnothorax
curvispinosus) in response to urban heat island effects. We reared ant
colonies within a laboratory common garden (25 °C) to generate a lab-born
cohort of workers and tested their acute plastic responses to temperature.
Contrary to expectations, urban ants exhibited a higher metabolic rate
compared with rural ants when tested at 25 °C, suggesting a potentially
maladaptive evolutionary response to urbanization. Urban and rural ants
had similar metabolic rates when tested at 38 °C, as a consequence of a
diminished plastic response of the urban ants. Locomotor performance also
evolved such that the running speed of urban ants was faster than rural
ants under warmer test temperatures (32 and 42 °C) but slower under a
cooler test temperature (22 °C). The resulting specialist-generalist
tradeoff and higher thermal optimum for locomotor performance might
compensate for evolved increases in metabolic rate by allowing workers to
more quickly scout and retrieve resources.