Thermoregulatory brood transport in the fire ant, Solenopsis invictaClint A. Penick* and Walter R. Tschinkel, Department of Biological Science, Florida State University, Tallahassee, FL 32306

*cap1579@fsu.edu

Figure 3. Temperature profile of S. invicta mound (south side) spanning 48 hours at four different depths (gray areas indicate nighttime hours). The first day was sunny, while the second was cloudy. Shallow depths heat and cool faster and to a greater degree. At around 40cm, nest temperatures show little fluctuation and resemble ground temperature outside the nest.

Figure 2. Temperature profile of S. invicta mound spanning 48 hours (gray areas indicate nighttime hours). Note difference between the temperatures of the north and south sides. The temperature on the south side rises earlier and to a higher degree than the north side, which is almost the same temperature as the air.

Figure 4. Reversed heating experiment. Average brood totals from north and south side samples from experimental (reversed heating pattern) and control colonies. The response was to aggregate all of the brood on the side that was most directly heated. This shows heating controls brood orientation and that there is no behavioral tendency to choose one side or the other based on previous experience.

Figure 5. General shading experiment. Average brood totals in samples from the north and south sides of experimental (generally shaded) and control (open to direct sunlight). While all of the brood was oriented on the south side of the control group (where the sunlight was more direct), the brood was distributed close to evenly between north and south sides of the control colony. This shows that when heating is nondirectional, brood orientation is not correlated with a particular side of the mound.

Figure 6. Heating during night experiment. Average brood totals from samples taken during the night from heated and control colonies. During the night, colonies that were heated brought brood up into the mound, but control colonies did not.

2) We conducted three field experiments to determine how temperature affects brood placement. In the first experiment, the heating pattern was reversed in the nest by shading the south side with cardboard and reflecting sunlight on the north side with a mirror. Brood samples were taken from the surface of both the north and south sides of the mound when surface temperatures approached optimal, and brood was counted in each sample (Fig. 3). The second experiment showed the effect of general shading on brood placement. The mound was shaded on every side with cardboard, and samples were taken from the north and south sides (Fig. 4). In the third experiment colonies were heated at night (when temperatures are normally below optimal) with charcoal. Brood was counted from a single sample taken from the surface of the mound (Fig. 5). The controls in all cases were unmanipulated colonies under natural field conditions.

1) In the field, iButton temperature loggers by Dallas Semiconductor were placed in fire ant nests at varying depths and orientations. We examined heating patterns of the north and south sides of the mound (Fig. 1), and how temperatures varied at four different depths within the mound (Fig. 2). Figure 1 illustrates the aggregation of brood in the mound at midmorning.

Introduction: Fire ant mounds are conspicuous ant constructions in the eastern United States. However, only a minority of ant species build true mounds (i.e., above ground mounds filled with a network of livable galleries). The primary use of these mounds is thermoregulation, which is especially important to brood rearing. Previous observations have shown that fire ants move their brood into the mound on the side (usually the south side) that receives the most direct sunlight

following sunrise. As temperatures rise above optimal (~32º C) they move their brood lower in the nest to cooler regions. Here We focused on two aspects of fire ant thermoregulation: 1) the general patterns of heating and cooling in the nest, and 2) what factors govern where and when fire ants place their brood in the mound. Specifically, We manipulated field conditions to separate a response to temperature from behavioral habit or circadian rhythms.

Results/Discussion: The mound structure provides a heating advantage particularly at regions just below the surface on the side most directly heated by the sun. Temperature was found to be the main determinant for brood placement within fire ant nests, and a behavioral habit or circadian rhythm was not supported . Thermoregulation of fire ant brood is strongly involved in the production of sexual brood and contributes a great deal to the fecundity of a colony and colony fitness. Fire ants are investing a lot of energy into temperature tracking and brood placement at all times of the day.

Soil moisture affects brood placement as well but was not included in this study, and the mechanism that controls temperature tracking and recruitment within the nest is also unknown. Understanding the basic principles that control thermoregulation within a social insect colony can help elucidate properties that lead to the evolution of these behaviors and organization within insect societies.

Brood

Figure 1: Cross section through south side of mound showing brood aggregated just under mound surface. This picture was taken during midmorning (Photo taken by Walter Tschinkel).

Acknowledgements: I would like to thank Kevin Haight, Brian Inouye, Jon Seal, and Josh King for all of their help with this project.

North Side

South Side

North Side

South Side

n = 8

n = 8

North Side

South Side

North Side

South Side

n = 8

n = 8

n = 6

n = 6