Leptothorax acervorum is a small red ant widely distributed in Eurasia, ranging from central Spain and Italy to the northernmost parts of Scandinavia and Siberia. The head and abdomen are dark, thereby giving the ant a two-toned appearance. Individual ants are small, with workers measuring around 3 millimetres (0.12 in) in length and queens being only 10% larger. They nest in small rotting branches, tree stumps, and under bark. Colonies are small compared to those of other ants—they have anywhere from a few dozen to a few hundred workers and one to several queens.
Bergmann’s rule establishes that among endothermic animals of the same species, body size increases with latitude. Studies have tested whether this rule also applies to social insects. L. acervorum workers were counted in a sample of colonies from Erlangen and Karelia. The worker size was significantly larger in the Karelian population, with the average thorax length being 1.15 mm ± 0.07 mm. The average thorax length from the Erlangen population was 1.08 ± 0.05 mm. As evidenced, the workers from Karelia were on average 10% larger than the workers from Erlangen. The results suggest that larger body sizes in L. acervorum from boreal habitats might result from selection for increased fasting endurance. Larger workers had more fat than small workers, and would survive longer in colder environments. Leptothorax acervorum might extend their survival time in areas with long winters and unpredictable climate by storing more reserves. Thus, the body size of workers of this holarctic ant increases with latitude.
Ecology and Behavior
Leptothorax acervorum is a model organism to investigate the social structure of multiple-queen colonies. Leptothorax acervorum is a facultatively polygynous ant, meaning that colonies with one or more than one queen occur, and these colonies acquire extra queens by adoption—thus polygyny is secondary. Electrophoretic allozyme analysis showed that cohabiting queens are close relatives. This reinforces the assumption that the queens in L. acervorum colonies form mother-daughter-sister groups, which arise from adopting newly mated queens into their natal nests.
Newly eclosed queens mate with unrelated males near the natal nest and then return to it, where they are readopted. Other queens disperse to mating aggregations, mate, and then leave the aggregations to establish new colonies elsewhere. Matings near the nest may occur because L. acervorum queens ‘call’ males through the use of pheromones.
An important behavior noticed in L. acervorum was the eating of reproductive eggs by queens. On average, approximately 69% of eggs eaten were intact. Also in observed colonies, the proportions of eggs eaten out of all eggs laid were 25%, 93%, 125% (i.e. more eggs were eaten than laid in that period) and 64%. This oophagy had a major impact on the colony’s output of eggs. The queens appeared to exhibit no discrimination when targeting eggs. It was actually observed that one queen interrupted an egg-eating queen and removed the egg to eat it herself. Feeding rate is positively correlated with fecundity. In the four colonies where intact eggs were eaten, one of the two most fecund queens was among the top two egg eaters.
A L. acervorum queen eats eggs by picking up the egg with her mandibles and manipulating it against her mouthparts with her forefeet. She pierces the egg’s membranous skin and laps the egg’s fluid through the hole. When the contents of the egg are emptied, typically after a few minutes, the queen will then discard the remaining skin by either dropping it to the floor or placing it on the mouthparts of a larva (which then eats the skin).
A possible explanation for this phenomenon is reproductive competition between queens. However, the overall lack of egg defense and overt aggression seem to provide contrary evidence. It is possible that direct confrontation would increase risk of injury for the egg-laying queen, thereby making egg defense too costly.
Trivers and Hare (1976) proposed that the population-level sex-investment ratio equals the relatedness asymmetry, meaning the ratio of relatedness to the sexes dictates which party controls sex allocation. Thus, the prediction is that sex-investment ratios are 1:1 females:males if queens control sex allocation and 3:1 females:males if there is worker control. This is because the queen is equally related to her sons and daughters (r=0.5 in each case), so she should produce equal numbers of male and female reproductive offspring. However, because of haplodiploidy, full sisters are more closely related to one another because half of their genome is always identical, and the other half has a 50% chance of being shared. Their total relatedness is 0.5+(0.5 x 0.5)=0.75. This means sisters would prefer to skew the population sex-investment ratio to 3:1 females:males. A female is related to her brother by only 0.25, because 50% of her genes that come from her father have no chance of being shared with a brother. This results in 0.5 x 0.5=0.25.
It was found that the population sex-investment ratio for "L. acervorum" changed from significantly female biased to significantly male biased with increasing polygyny. In polygynous colonies where multiple queens reproduce, there is a lack of worker aggression towards queens. This is likely a benefit for multiple queens that reproduce in polygynous populations as a result of dilution of relatedness. Workers simply favor the previous reproductive queen because she is their mother, and would thereby rear full sisters. Thus, multiple reproductive queens would decrease this worker regulation because relatedness is much less certain. The relatedness estimate for nest mate workers in polygynous colonies (0.46 ± 0.040) was significantly lower than that for nest mate workers in monogynous colonies (0.55 ± 0.089). However, this relatedness estimate for nest mate workers in monogynous colonies was distinctly lower than the expected 0.75 value for full siblings.
Seasonal fluctuations of queen numbers may explain why relatedness estimates for workers in monogamous colonies are lower than expected. The seasons shape the composition of the colony—young queens are regularly adopted in their natal colonies after mating in late summer. By seeking adoption in established colonies, young queens might avoid long solitary hibernation—winter mortality was found to be lower in polygynous than in monogynous colonies. Some emigrate from the colony after hibernation in the spring. This may be an attempt to found their own colony solitarily or by budding, leaving the natal colony with their own workers and brood to start a new colony. Some monogynous colonies could have recently been polygynous. Thus, colonies of L. acervorum may easily switch from monogamy to polygyny as a result of adopting young queens and budding, or queen emigration.
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