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.

Methods
Acorn ant running speed and metabolic rate meta-data

Running speed (AcornAntRunningSpeedDryad.csv)

Colony – a unique, alphanumeric identifier for each colony

Individual – a generic numeric identifier for each individual tested within each colony; the identifier is the same for a given individual across the multiple test temperatures

TestTemperature – a numeric vector corresponding with each of the temperatures at which running speed was tested, including 22, 32 and 42 °C

SourcePopulation – a character vector for whether the ants originated from an urban site or a rural site

Site – a character vector indicating the site name from which the ants originated

RunningSpeedmmPerSecond – a numeric vector indicating the running speed in units of mm per second for an individual at a given distance interval and at a given test temperature

DistanceInterval – a numeric vector indicating the distance interval over which the running speed was recorded; distance intervals are 20 mm each, over a span from 0 to 100 mm; possible distance interval values include 20 (indicating the distance run from 0 to 20 mm), 40 (indicating the distance run from 20 to 40 mm), 60 (indicating the distance run from 40 to 60 mm), 80 (indicating the distance run from 60 to 80 mm), 100 (indicating the distance run from 80 to 100 mm)

NumberWorkers – the number of workers present in a given colony at the time of running speed testing

NumberBrood – the number of brood present in a given colony at the time of running speed testing

NumberQueens – the number of queens present in a given colony at the time of running speed testing

Activity within the metabolic chamber (AcornAntRespirometryActivityDryad.csv)

Colony – a unique, alphanumeric identifier for each colony

TestTemperature – a numeric vector corresponding with each of the temperatures at which activity in the respirometry equipment was tested, including 25 and 38 °C

SourcePopulation – a character vector for whether the ants originated from an urban site or a rural site

Site – a character vector indicating the site name from which the ants originated

NumberWorkers – the number of workers present in a given colony at the time of activity testing

NumberBrood – the number of brood present in a given colony at the time of activity testing

NumberQueens – the number of queens present in a given colony at the time of activity testing

ColonyMassInGrams – the total biomass of the colony in grams

NumberActiveWorkers – the number of workers actively moving around the respirometry chamber during a given time interval

TimeInterval – the number of seconds elapsed since the beginning of activity monitoring

Metabolic rate (AcornAntMetabolicRateDryad.csv)

Colony – a unique, alphanumeric identifier for each colony

SourcePopulation – a character vector for whether the ants originated from an urban site or a rural site

DecimalLongitude – a numeric vector indicating the decimal degrees of longitude for the colony collection site

DecimalLatitude – a numeric vector indicating the decimal degrees of latitude for the colony collection site

Site – a character vector indicating the site name from which the ants originated

ColonyMassInGrams – the total biomass of the colony in grams

TestTemperature – a numeric vector corresponding with each of the temperatures at which metabolic rate was tested, including 25 and 38 °C

CO2 – carbon dioxide concentration in ppm

NumberWorkers – the number of workers present in a given colony at the time of metabolic rate testing

NumberBrood – the number of brood present in a given colony at the time of metabolic rate testing

NumberQueens – the number of queens present in a given colony at the time of metabolic rate testing