Lazy males made to work

The best known insect societies are those of ants, bees and wasps all of which belong to the order Hymenoptera. Individuals in these species organize themselves into colonies consisting of tens to millions of individuals. Each colony is headed by one or a small numbers of fertile queens while the rest of the individuals serve as sterile or nearly sterile workers. The spectacular ecological success of the social insects, their caste differentiation, division of labour and highly developed communication systems are well known. A less studied but equally intriguing aspect of these hymenopteran societies is that they are feminine monarchies – there are queens but no kings and all workers are females. Males do little more than transferring their sperm to virgin queens while all the work involved in nest building, brood care and, finding and processing food is done by the females.

Why don’t males work, at least during the period that they stay on the nests of their birth? Using the Indian primitively eusocial wasp Ropalidia marginata and the important task of feeding larvae as an example of work, we have recently made a novel attempt to understand the secret behind the well-known laziness of the males. We considered three hypotheses:
males are incapable of feeding larvae,
males never get access to enough food to satisfy themselves and have something left over to offer to the larvae (males do not forage on their own and depend on the females for access to food), and
females are so much more efficient at feeding larvae that they leave no opportunities for the relatively inefficient males to do so.

To test these hypotheses, my graduate student Ms. Ruchira Sen offered experimental colonies excess food. This resulted in a marginal amount of feeding of the larvae by males thus disproving the hypothesis that males are incapable of feeding larvae. Then she removed all the females from some colonies and left the males alone with hungry larvae. This experiment was a non-starter because males cannot forage and find food in the absence of females. Ruchira overcame this problem by mastering the art of tenderly and patiently hand-feeding the males. And she gave them more food than they could themselves consume so that they might feed larvae if they could. Her efforts were rewarded when males under these conditions fed larvae at rates nearly comparable to those of the females. Thus males can feed larvae and will do so if they are given an opportunity. It therefore appears that males do not feed larvae under natural circumstances because they do not have access to enough food and/or because females leave them few opportunities to do so. There are several lines of evidence to suggest that the males were not merely dumping unwanted food but that they were actively seeking out the most appropriate larvae and feeding them “deliberately”. But it must be emphasized that from the point of view of the larvae, males were quite inefficient compared to the females. Apart from the fact that males fed only the oldest larvae and ignored all the young larvae, it turned out that many of the larvae under allmale care died.

In addition to their obvious interest, these studies open up a major evolutionary puzzle: why has natural selection not made the males more efficient and made feeding larvae by males a routine matter? Answering one question raises at least one more – and that’s how it should be.