How long versus when: worker mortality synchrony is widespread but decoupled from body size in Australian ants

Abstract

Colony demographic models often treat worker deaths as independent events, but shared environmental exposure may synchronize mortality within species. Using a 15-month field dataset of 18 Australian ant species and 106 cohorts, we quantified intra-specific mortality synchrony with the intra-class correlation coefficient (ICC) and tested body size and temperature as candidate correlates. Synchrony was widespread: credible intervals excluded zero in 9 of 18 species, and the assemblage mean ICC was 0.21 +/- 0.09, indicating that about 21% of daily mortality variance reflected shared within-species exposure. Body size did not buffer synchrony. Worker mass was unrelated to ICC (Spearman rho = +0.10, p = 0.69; WLS beta = +0.05, p = 0.35), decoupling how long workers live from when they die together. Temperature showed a suggestive lagged association: maximum temperature preceded mortality pulses by 1-7 days in 13 of 15 analyzable species, with mean peak r = 0.16, but no species survived Bonferroni correction. A sensitivity analysis using mean correlation across lags 1-7 days instead of the peak removed the assemblage-level signal (mean r = -0.03, p = 0.93), and ICC was unrelated to temperature-coupling strength (Spearman rho = -0.004, p = 0.99). Mortality synchrony is therefore widespread, size-independent, and only partly thermal. Species-specific thermal tolerance, rather than body mass, is a more promising candidate for explaining variation in synchronized mortality.