Divergent Circadian Foraging Strategies in Response to Diurnal Predation Versus Persistent Rain in Asian Weaver Ant, Oecophylla smaragdina, Suggest Possible Energetic Trade-offs

Abstract

The study of chronobiology of foraging behavior in social insects offers valuable models for the investigation of circadian rhythms. We scored hourly nest entries and exits of Oecophylla smaragdina (Asian weaver ant) workers in 9 active non-polydomous nests on days with and without rain and with and without a primarily diurnal predator present. After determining that Oecophylla display a high nest fidelity, we focused exclusively on analyzing nest entry counts: we found a significant decrease in overall entry counts of individual ants on rainy days compared with non-rainy days (p < 0.0001). They usually maintain a typical diurnal pattern of foraging activity; however, that regularity was often distorted during rainy periods but appeared to quickly revert to typical patterns following rain. This lack of compensatory foraging activity following a period of rain supports the hypothesis that these ants have enough food reserves to withstand a pure masking-induced suppression of foraging activity. Predation through bird anting, too, decreased foraging activity but appeared to cause a reversal in foraging activity timing from diurnal to nocturnal foraging. Daily periodicity of foraging was significantly disrupted in most nests during rain; however, daily foraging periodicity was disrupted in only one nest due to presence of predators. Thus, rain and predation both exert significant impacts on the overall foraging activity of Asian weaver ants, but while persistent pressure from rain seemed to primarily cause masking (diminution) of circadian foraging activity, predation restricted to the daytime resulted in phase-inversion to nocturnal foraging activity, with little diminution. This is consistent with different energetic strategies being used in response to different pressures by this species.

Keywords: Asian weaver ants; activity rhythm; fidelity foraging plasticity; masking; temporal niche plasticity.

Figure 1.

Hourly entry counts, hourly exit counts, and daily entry-exit differences for nests 1 through 6. (a-i) Time-series of hourly entry and exit counts for nests 1 through 9. Days with rain are highlighted in blue, and days with predation are highlighted in purple. (j-r) Differences between total daily entries and exits for each nest over all available days are shown; positive differences are highlighted in green and negative differences in red. The median entry/exit ratio across all days in a nest is denoted ‘Ratio’; the total sum of daily entry-exit differences across all days for a nest is denoted ‘Area’.

Figure 2.

AUC comparison of rain days to non-rain days by nest followed by a comparison of seasonal differences in AUC. (a-f) Time-series of 24-hour segments of entry counts into nests 1 through 6 on days without rain (grey) and days with rain (where applicable, light blue). Opaque black and blue lines indicate the median 24-hour time-series of entries for days without rain and days with rain, respectively. (g) Median 24-hour time-series of entry counts across individual days in nests 1 through 6 on days without rain (black) and days with rain (blue). The median absolute deviation of the median time-series is shown in grey and light blue, respectively. The p-value for the AUC of days with rain compared with days without rain is indicated on each subplot, where applicable. (h) Comparison of AUC for days without rain from the dry season (light orange) to the rainy season (light purple). The opaque orange line indicates the median 24-hour time-series of the individual days for nests subject to the dry season (nest group 1). The opaque purple line indicates the median 24-hour time-series of the individual days for nests subject to the rainy season (nest group 2). (i) Comparison of AUC for days with rain from the dry season to the rainy season with the same colour scheme as (h). The p-value for the comparison of AUCs from the dry season compared with AUCs from the rainy season are indicated on the subplots. *p < 0.01, **p < 0.002, ***p < 0.0002, ****p < 0.00002 using the two-sided Mann-Whitney U test.

Figure 3.

Periodicity of foraging activity and similarity of rain days to non-rain days. (a-f) The CWT was applied to all available entry data for nests 1 through 6, respectively. The maximum normalised magnitude for each column in the magnitude scalogram is shown in yellow. Time periods with rain are boxed and highlighted in blue. (g) Euclidean distance between a 24-hour, non-rain foraging activity template for each nest and each day in that same nest. Euclidean distance before Day 0 was compared to Euclidean distance on and after Day 0 for each nest using the two-sided Mann-Whitney U test, the p-values of which are indicated in the subplot.

Figure 4.

Overall foraging activity and periodicity of foraging activity for nests under predation and comparison to other nests. (a) Entry counts of nests under predation (nest group 3: nests 7 through 9) and all other nests (1 through 6). (b-d) The CWT was applied to all available entry data for nests 7, 8, and 9, respectively. The maximum normalised magnitude for each column in the magnitude scalogram is shown in yellow. Time periods with predators present are boxed and highlighted in purple. (e) 24-hour time-series segments of entry counts for nest group 1 (nests 1 through 3) on days without rain during the dry season (grey). The opaque black line indicates the median 24-hour time-series of the individual days for nest group 1. Median AUC is provided. (f and g) Show data in the same manner as (e) but for nest group 2 (nests 4 through 6) during the rainy season and nest group 3 under predation, respectively. ****p < 0.0002 using the one-sided Mann-Whitney U test to determine whether night-time foraging activity exceeds daytime foraging activity.