(akl) Why queen bumblebees restrict themselves to a single sexual partner–often for their entire lives–was not clear until now. Because from the point of view of evolutionary biology, promiscuity is rather advantageous as it leads to increased genetic variability. A new study with ETH participation now shows that it is not only the obvious disadvantages–the inevitable loss of time and energy–that induce the queen to live monogamously (1).

Queen impregnated while she slept

The goal of the study, carried out by ETH Professor Paul Schmid-Hempel from the Institute of Ecology & Evolution (2) and Boris Baer (3) from the Department of Population Biology of the Institute of Biology of the University of Copenhagen, was to investigate the direct influence of the male sperm on the female fitness in the bumblebee. Isolated sperm from one, two, three or four different males were implanted into anaesthetized female bumblebees of the species Bombus terrestris. Remaining components and proteins of the male ejaculate had previously been filtered out. After fertilization the queens were artifically hibernated at 4°C. After two weeks the queens were allowed to found colonies in climate chambers at 28°C and, with abundant nutrition. As soon as the colonies comprised 15 or more workers they were transferred to an open field in the vicinity of Zurich. The scientists monitored the most important key data in the life of the queens during the entire experiment: whether they survived hibernation and how long they lived and how many progeny they produced.

Unfaithfulness doesn't pay

Females who had multiple fertilizations, above all from different males, survived hibernation less often than those who had only mated with a single male. In addition, the surviving ones were less healthy. The results of the study show that females who received a "cocktail" of sperm had fewer progeny and shorter life expectations.

Polyandry in bumblebee queens of the species Bombus terrestris has serious consequences for health and fitness.(Picture: P. Schmid-Hempel.) large

The two ecologists were thus able to show for the first time that certain sperm alone can negatively affect the fitness of the female bumblebee. The scientists exclude the explanation that the loss of fitness is due to the quantity of sperm implanted, since they always cared to implant the same total quantity of sperm. Rather, they hypothesize that the observed effect is due to a sperm incompatibility; the sperms of different males could interact amongst themselves and thus affect the fitness of the queen.

Whether this is the only explanation for the observed loss of fitness due to polyandry is not yet clear, as the scientists say. Nevertheless, they are one step nearer to the long-term goal of their investigation–the details of processes that preserve the genetic diversity in natural populations. The afore-mentioned sperm incompatibility effect would indeed call for a multitude of pairing types that would thus foster genetic diversity.