10.5061/DRYAD.K0P2NGF4B
Baudier, Kaitlin M.
0000-0001-8450-3788
Arizona State University
O'Donnell, Sean
0000-0002-6338-7148
Drexel University
Data from: Rain shadow effects predict population differences in thermal
tolerance of leaf-cutting ant workers (Atta cephalotes)
Dryad
dataset
2019
Organization for Tropical Studies
Tyson Research Fellowship
Drexel University
https://ror.org/04bdffz58
William L. McLean III Fellowship
2019-11-18T00:00:00Z
2019-11-18T00:00:00Z
en
4870956 bytes
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CC0 1.0 Universal (CC0 1.0) Public Domain Dedication
Tests of hypotheses for the evolution of thermal physiology often rely on
mean temperatures, but mounting evidence suggests geographic variation in
temperature extremes is also an important predictor of species’ thermal
tolerances. Although the tropics are less thermally variable than higher
latitude regions, rain shadows on the leeward sides of mountains can
experience greater diel and seasonal variation in temperature than
windward sites. Rain shadows provide opportunities to test predictions
about the relationships of extreme temperatures with thermal physiology
while controlling for latitude. We tested the hypothesis that populations
of leaf-cutting ants (Atta cephalotes) in leeward, montane, and windward
sites in Costa Rica would differ in upper thermal tolerances (CTmax) of
workers. As predicted from rain shadow effects via extreme high
temperatures, the leeward rain-shadow site yielded the highest mean CTmax
(rain shadow site 42.1±0.3 °C, Montane site 38.2±0.5 °C, windward site
38.2±0.3 °C). This suggests that high-temperature extremes in tropical
rain shadow forests can select for higher thermal tolerances. CTmax
increased with worker body size within sites, but CTmax increased with
body size more gradually at the two lowland sites, as predicted if local
high temperatures selected more strongly on the most thermally vulnerable
society members (small workers). This suggests that warmer lowland
climates selected for colonies with less variation in heat tolerance than
cooler high elevation climates.
Ant collections We studied thermal tolerance of the leaf cutting ant Atta
cephalotes across its elevational range extremes in Costa Rica, sampling
high elevations near the continental divide and lowlands on both the
Pacific rain shadow (leeward) and Caribbean less variable (windward)
slopes of the Tilarán mountain range. All field work was performed during
the rainy seasons of 2014 and 2016 in premontane wet forests of Monteverde
(N10.31°, W84.31° 1296-1311 m asl), lowland rainforests of La Selva
Biological Station (N10.42°, W84.02° 50-58 m asl), and seasonal dry
forests of Santa Sosa National Park (N10.85°, W85.64° 297-300 m asl).
Although we searched the continental divide near Monteverde up to over
1800 m above sea level (masl), A. cephalotes was only common at and below
circa 1300 masl, suggesting that our sampling was representative of the
elevational extremes for this species in the region. Within each site,
active and mature nests of Atta cephalotes at least 500 m linear distance
apart from one another were selected. Twenty worker ants across the range
of body sizes we encountered were collected from the tops of the nest
mounds and transported (within 2 hours of collection) to the lab to use as
subjects in thermal tolerance assays. Three colonies were sampled in Santa
Rosa and La Selva, and two colonies were sampled in Monteverde. Voucher
specimens from each colony were deposited in the entomology collection of
the Academy of Natural Sciences of Drexel University, Philadelphia, PA,
U.S.A. Ambient temperature measurements Daily maximum, minimum, and mean
temperatures were calculated from temperatures recorded round-the-clock in
each site using iButton thermochron data loggers (Maxim Integrated, San
Jose, CA). Temperature sampling was concurrent with CTmax sampling of the
ants (rainy seasons of 2014 and 2016). Each temperature logger was placed
in a plastic housing and beneath a plastic sheet (to shield the probe from
direct insolation) and inserted beneath the leaf litter on the forest
floor. Loggers recorded surface soil temperature once every 5 minutes for
three continuous days. Loggers were arranged in transects of three, spaced
5 m apart from one another and 3 m into the forest alongside dirt trails
at each site. A total of 8 such transects were deployed in Santa Rosa, 9
in Monteverde, and 2 in La Selva. Both raw ibutton temperature data files
and summaries of these temperature data showing daily temperature range,
means and standard devition have been uploaded to this digital
repository. We also obtained bioclim extrapolations of local temperatures
at each site. These extrapolated climate data are reported in the
master file alonside CTmax and head width. Thermal tolerance assays We
ran standard dynamic heat tolerance assays using digital dry heat blocks
(USA Scientific). Ants were placed in 1.5 ml microcentrifuge tubes
stoppered with cotton at the top to prevent access to thermal refuge in
the tubes during the assay. Tubes were placed in the dry heat block and
started at 30ºC, increasing in temperature at a rate of 1ºC every 10
minutes. Every 10 minutes we checked for loss of mobility response to
light tapping on the tube. For each ant lack of motor response to tapping
was interpreted as having surpassed the critical thermal maximum (CTmax).
For each colony, five ants across the range of caste sizes were kept in
tubes but not put into heat blocks for the duration of the assays, to
ensure that factors other than increasing temperature did not account for
loss of mobility. None of the control ants in this study lost mobility,
suggesting that the cause of loss of mobility was due to temperatures
experienced within the heat blocks rather than handling and confinement.
Body size After performing CTmax assays, we collected all ant workers and
measured body sizes from head capsules. Each ant was photographed at 15X
to 40X magnification (depending on size of ant) using a scope-mounted
digital camera. ImageJ software was used to measure the width of heads at
eye-height from photographs. A micrometer accurate to 0.1 mm was used to
convert head width measurements from pixels to millimeters. Head width was
used as a proxy for body size across all analyses. Both CTmax and head
width data are reported for each assayed ant in these uploaded files.
Most headers have been named intuitively, with abbreviations such as
"Avg." meaning "average" and "SD" indicating
"standard deviation". All temperatures and CTmax values are
reported in degrees celcius. Below is a list of less intuitive headers
within these files and their exact meaning. Downloaded bioclim data
based on nest coordinates: BC_MAT - mean annual temperature (BIO1)
BC_MDRngT - Mean Diurnal Range (Mean of monthly (max temp - min temp))
(BIO2) BC_TARng - temperature annual range (BIO7) BC_Aprecip - Annual
precipitation (BIO12) BC_PrecipSnlty - Precipitation
seasonality (Coefficient of Variation) (BIO15) BC_MaxTWM - Maximum
temperature of warmest month (BIO5) BC_MinTCM - Minimum temperature of
coldest month (BIO6) Ibutton-based data in master CTmax file:
ibutton_Min - average minimum daily temperature for this site ibutton_Max
- average maximum daily temperature for this site ibutton_Avg - average
daily temperature for this site Warming tolerances (discussed only in
supplementary material): Night_Warming_Tolerance - individual ant CTmax
minus average daily minimum temperature (ibutton data)
Day_Warming_Tolerance - individual ant CTmax minus average daily maximum
temperature (ibutton data) Mean_Warming_Tolerance - individual ant CTmax
minus average mean daily temperature (ibutton data)