Abstract
The question on how individuals allocate resources into maintenance and reproduction is one of the central questions in life history theory. Yet, resource allocation into maintenance on the organismic level can only be measured indirectly. This is different in a social insect colony, a “superorganism” where workers represent the soma and the queen the germ line of the colony. Here, we investigate whether trade-offs exist between maintenance and reproduction on two levels of biological organization, queens and colonies, by following single-queen colonies of the ant Cardiocondyla obscurior throughout the entire lifespan of the queen. Our results show that maintenance and reproduction are positively correlated on the colony level, and we confirm results of an earlier study that found no trade-off on the individual (queen) level. We attribute this unexpected outcome to the existence of a positive feedback loop where investment into maintenance (workers) increases the rate of resource acquisition under laboratory conditions. Even though food was provided ad libitum, variation in productivity among the colonies suggests that resources can only be utilized and invested into additional maintenance and reproduction by the colony if enough workers are available. The resulting relationship between per-capita and colony productivity in our study fits well with other studies conducted in the field, where decreasing per-capita productivity and the leveling off of colony productivity have been linked to density dependent effects due to competition among colonies. This suggests that the absence of trade-offs in our laboratory study might also be prevalent under natural conditions, leading to a positive association of maintenance, (= growth) and reproduction. In this respect, insect colonies resemble indeterminate growing organisms.
Highlights
A central question in life history theory is how individuals allocate resources into maintenance and reproduction to maximize evolutionary fitness [1, 2]
Queen survival was identical for both populations (CoxPH model: n = 84, events = 71, coef = -0.115, p = 0.681), but Japanese queens had a lower daily egg-laying rate than Brazilian queens (Wilcoxon rank sum test: W = 485, p = 0.0157; mean Japanese colonies (Japan) = 1.811, mean Brazil = 2.472)
We did not find a significant difference in the onset of sexual reproduction between the two populations (CoxPH model: first sexual pupa n = 77, events = 77, coef = -0.432, p = 0.0895), but the onset of queen production was on average 8 days earlier in the Japanese population (CoxPH model: first queen pupa, n = 73, events = 73, coef = -0.596, p = 0.026) while no differences appear in the production of males (CoxPH model: first male pupa, n = 69, events = 69, coef = -0.042, p = 0.872)
Summary
A central question in life history theory is how individuals allocate resources into maintenance and reproduction to maximize evolutionary fitness [1, 2]. As resources are typically finite, individuals must find a balance between investment in reproduction and maintenance. Causes a negative correlation between reproductive effort and lifespan, which is commonly observed throughout the animal kingdom [3]. Eusocial animals are special because their reproductives are unusually long-lived and highly fertile at the same time [4,5]. Workers have considerable shorter lifespan and forego reproduction [4,6] even though both castes develop from the same genome and live in the same environment [7]. Long queen lifespan has been connected to extrinsic mortality rate [8] or in case of the worker to lower costs due to efficiency gains by the division of labor [6,9]
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