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Appendix
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Subjects
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We used sixty colonies of Temnothorax rugatulus. All had at least one queen, with
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worker populations ranging from 128 to 211 and brood populations ranging from 46 to
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302. Colonies were collected in the Pinal Mountains near Globe, Arizona (N 33° 19.00’,
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N 110° 52.56’, W). Each colony was housed in a nest like those described below, but
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with a small entrance (2 mm) and no light filters. Each nest was kept in a plastic box (11
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cm x 11 cm), the walls of which were coated with Fluon to prevent the ants from
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escaping. Each box was provided with a water-filled plastic tube capped with cotton and
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an agar-based diet that was refreshed weekly [1].
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Nest designs
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Each nest was made from a balsa wood slat (2.4 mm thick) sandwiched between glass
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microscope slides (50 x 75 mm), with a circular cavity (38 mm diameter) cut through the
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middle of the slat. A nest entrance was drilled through the center of the roof slide. Interior
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illumination was adjusted by placing transparent neutral density filters (Rosco Cinegel)
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between the two roof slides. This design prevented ants from directly contacting the
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filters, which can sometimes build up an electrostatic charge that the ants find repellent.
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All filters had a 9.5-mm diameter hole to accommodate the nest entrance. This consistent
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hole size ensured that interior illumination was independent of entrance size.
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To test if colonies adjust attribute weights according to their environment, we designed
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two kinds of target nests, E and L. Nest E had a smaller entrance than nest L (2 mm vs.
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5.5 mm), but a brighter interior (525 lux vs. 2 lux). Subjects started each experimental
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trial in a standard home nest with an intermediate entrance size (3.7 mm) and interior
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light level (245 lux).
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For the treatment phase, we designed two additional nests, one for each treatment. Each
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nest was inferior to the home nest in one attribute, but identical to it in the other. In the
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light treatment, the inferior attribute was light level (1400 lux vs. 245 lux in the standard
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nest); in the entrance treatment, the inferior attribute was entrance size (9.5 mm diameter
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vs. 3.7 mm in the standard nest).
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Supplemental experimental procedure
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Before receiving the preference test, each colony was first moved from its original nest
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into the standard home nest. This was done to minimize any effects of current home
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attributes on site preference [2]. The empty home nest was placed against one wall of a
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small experimental arena with Fluon-coated walls (17.8 x 12.7 cm). Then, the colony in
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its original nest was introduced and forced to migrate by removal of the nest’s roof. All
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colonies successfully migrated within 12 hours. The original nest was then taken away
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and target nests E and L were placed against the arena wall opposite from the home nest.
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Finally, the roof of the home nest was removed to induce migration. We assayed nest-site
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preference by recording the site occupied by the colony 12 hours later. Colonies usually
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showed an unambiguous preference, but they sometimes split between sites. If one site
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contained over 90% of colony members, including all queens and brood items, we
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designated that as the colony’s choice. If no site achieved this criterion, we did not record
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a preference. This occurred for only 4 of the 60 colonies.
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During the treatment phase, colonies made a series of four choices. Prior to the first
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choice, each colony was induced to move into the standard home nest, as described
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above. Then, they were made to choose between two sites: one site was identical to the
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standard home nest; the other was clearly inferior to the home nest in one attribute, but
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identical to it in the other. Each choice usually ended with the colony moving to another
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standard nest, hence the ants did not have to be re-housed before making their next
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emigration. In a few cases they chose the alternative (inferior) nest, and had to be forced
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to migrate to the standard nest before the next choice, to ensure that all colonies
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experienced similar contexts during decision-making. (This happened in only 18 of 224
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emigrations: 4 in the first choice, 7 in the second choice, 4 in the third choice, and 3 in
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the fourth choice.) In other respects, each choice followed the same procedure used in the
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initial preference test. Three days elapsed between choices, during which colonies
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remained in the emigration arena in the presence of the rejected inferior site. This was
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done to maximize colony exposure to the site, and any associated effects on their
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weighting of attributes.
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Once the treatment phase was complete, colonies were again presented with the original
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binary choice between sites E and L. Six colonies split either in the pre-treatment or post-
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treatment tests and so were excluded from further analysis, leaving a total of 54 colonies.
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Analysis
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To compare preference shifts between conditions, we used a generalized linear model.
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Post-treatment choice was the response variable, and the treatment condition was a
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predictor variable. We also included pre-treatment choice as a predictor, in case colonies
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showed consistent differences in their preferences. Because the response variable was
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binary, we used a binomial error structure. We tested the null hypothesis that the
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preference between targets E and L after the treatment was independent of the type of
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treatment. The alternative hypothesis was that the two treatment types drive target
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preference in opposite directions, with E more popular following the entrance condition,
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and L more popular following the light condition. Details of the model results are shown
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in Table S1. The statistical package R (v. 2.15.2) was used for all analyses.
Table S1. Results of two generalized linear models fit to the experimental results. Model
1 includes both treatment and pre-treatment choice as predictors of post-treatment choice.
Because pre-treatment choice had no significant effect, the model was re-fitted without it
(Model 2). In both models, post-treatment choice was assumed to be a binomially
distributed random variable.
Model 1: Post-treatment choice ~ Pre-treatment choice and Treatment
Intercept
Pre-treatment choice
Treatment
Coefficient
-0.55
-0.49
1.38
Standard Error
0.47
0.59
0.58
z-value
-1.18
-0.82
2.37
P
0.24
0.41
0.02
Odds Ratio
z-value
-1.85
2.40
P
0.06
0.02
Odds Ratio
0.62
3.98
Model 2: Post-treatment choice ~ Treatment
Intercept
Treatment
Coefficient
-0.75
1.38
Standard Error
0.40
0.58
3.99
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References
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1 Bhatkar, A. & Whitcomb, W. H. 1970 Artificial Diet for Rearing Various Species of
Ants. Florida Entomologist 53, 229–232.
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2 Healey, C. I. M. & Pratt, S. C. 2008 The effect of prior experience on nest site
evaluation by the ant Temnothorax curvispinosus. Animal Behaviour 76, 893–899.
(doi:10.1016/j.anbehav.2008.02.016)
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