Question

2. Write the methods (include materials) that you would use to determine the Standard Metabolic rate of the silver desert ant? (10 points)

Answer 1

The Silver desert ants were maintained
in a laboratory, on a normal photoperiod at a temperature of 20-24
^oC. Water and food consisting of rolled oats and wheat germ
were supplied ad lib. Experiments were carried out within 3
weeks of capture.
Constant-volume 02 and CO2 respirometry was carried out
during daylight hours on individual ants as previously described
(16). An automated version of the same technique
was used to measure the rates of oxygen consumption (Vo2)
and CO2 production (VCo2) of groups of ants. Ten to 30 ants
were put in a respirometer chamber (volume 15 ml) that was
part of a system through which a stream of outside air,
scrubbed of H20 and CO2, flowed continuously at a rate of
50 ml-min -1. The input and output ports of the chamber were
connected to solenoid-controlled changeover valves. When
closed, the solenoid valves shut off both ports, allowing the
animals in the respirometer chamber to alter the concentrations
of 02 and CO2 in it. When the changeover valves closed
off the chamber, they opened up a bypass circuit, causing the
airstream to flow through it. When the solenoid valves were
open, the air stream passed through the respirometer chamber
rather than through the bypass. Thus, the airstream was used
alternately to establish a reference baseline and to measure respiratory gas exchange. The concentrations of 02 and CO2
in the airstream were monitored continuously with an S-3A
02 analyzer from Applied Electrochemistry (Ametek Thermox,
Pittsburgh) and an AR-50 CO2 analyzer from Anarad
(Santa Barbara, CA). After the respirometer chamber had
been isolated for a controlled interval, the solenoid
changeover valves were energized, and the air inside the
respirometer chamber was drawn through an H20 scrubber
and then through the gas analysis instruments. After the
respirometer chamber had been completely flushed with
reference air, the two solenoid valves were de-energized,
again sealing off the respirometer chamber, and the cycle of
measurement was repeated. The entire operation was computer
controlled, including the timing and energizing of the
solenoids and the regulation of the temperature of the
respirometer chamber to within 0.5OC at any level between 10
and 45TC.
The computer also continuously recorded 02 and CO2
concentrations in the airstream. It calculated the volumes of
02 consumed and CO2 produced by integrating the concentration
profiles, using the reference airstream as a baseline.
It also determined rates of 02 consumption and CO2 production
by using the calculated gas volumes and enclosure times,
correcting Vo2 for the presence of CO2, and converting
the values to STPD (standard temperature and pressure, dry).
The effects of temperature at 5YC increments from 10 to
450C on Vo2 and Vco2 were determined on groups of workers
in humidified air. Group size at 450C was 10; at the other
temperatures, it was 30. Successive readings of Vo2 and Vco2
were made until the ants were almost completely inactive,
and the readings had stabilized at a minimal level (ca. 3 hr).
Thereafter, repeated determinations were made, at 30-min
intervals, for 6-8 hr. The means of four or five of the minimal
readings were used for analysis. The relative humidity in the
respirometer chamber was maintained near saturation [the
situation experienced by ants in their underground nests]
by a water-saturated wad of cotton wool. To determine the
losses in body mass during the experiments the ants were
weighed individually to 0.1 mg prior to respirometry; after
respirometry, the ants were reweighed in groups. Changes in
body mass were calculated by subtracting the final mass of
the group from the summed masses of the individual ants at
the start of the experiment.
The effects at 24°C of low and high humidities on Vco2
were measured with a changeover valve system like that
described above. However, during alternate 40-min periods,
the 50-ml-min 1 stream of dried CO2-free air was humidified
under computer control by bubbling it through a 30-cm
the column of water containing 2% KOH. The humidity change
had no effect on CO2 concentration readings when there were
no ants in the respirometer chamber. The relative humidity of
the air entering the chamber was monitored with a Brady
Array humidity sensor (Thunder Scientific PC-2101C; Albuquerque,
NM). In most of the experiments, the activity of the
ants was monitored with a photoelectric system as described
elsewhere.
Regression equations were calculated by the method of
least squares and tested for significance with analysis of
variance. Differences between means were tested with Student's
t-test. Means are accompanied by standard deviations
and sample sizes.