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.

Hard rocks can have long memories

One of the best ways to understand the geological history of our 4500 million year old planet is to study rocks formed under a wide variety of geological conditions. Geologists, equipped with their vast experience and advanced analytical instruments, can identify and interrogate those rocks that best preserve evidence of past geological events. One such instrument is the sensitive high resolution ion microprobe (SHRIMP), a large specialized mass spectrometer that measures the ages of rocks, their precursors and major thermal events by firing a 10,000 volt ion beam at crystals as small as 0.05 mm diameter and measuring the isotopic abundances of the lead, uranium and thorium that are released.

The reconstruction of the continents that existed in the past is an important part of understanding the dynamic evolution of earth. The ancient supercontinent of Gondwana once consisted of what are now the smaller continents of South America, Africa, Madagascar, southern India, Sri Lanka, Antarctica and Australia. Determining the timing of the geological events involving rock formation and modification (deformation, metamorphism etc.) in these continental fragments is vital in piecing together the evolution of the earth's crust during any period of geological time. Most rocks 'forget' their history if exposed to extreme geological conditions, but there are some rare cases where particular rocks derived from the earth's lower crust have preserved, in their distinctive mineralogy, convincing evidence of the very high temperatures that can be present at depth.
The rocks of the central Highland Complex in Sri Lanka, and some parts of Antarctica and southern India, have been subject to some of the highest peak temperatures of crustal metamorphism known, over 1100°C. At such temperatures most rocks would turn into molten magma, but in the November issue of Geology, Sajeev and others report rocks from near Kandy (Sri Lanka) that not only survived the high temperatures, but contain crystals of zircon in which a uranium-lead isotopic record of their provenance and thermal history have survived. Such survival is contrary to all predictions from experimental studies of the rate that lead should be lost from zircon by thermal diffusion.

From a study of the metamorphic minerals and thermodynamic modelling, and SHRIMP uranium-lead isotopic analyses of zircon and monazite (cerium phosphate), the authors have shown that the rocks near Kandy were originally sediments derived from sources ranging in age from 2500 to 830 million years. The sediments were heated to over 1100°C at a depth of about 25 km about 570 million years ago, and then rapidly lifted towards the surface, while still hot, about 550 million years ago. These Sri Lankan rocks were probably trapped and buried in the violent collision between the two halves of the Gondwana supercontinent about 600 million years ago, superheated by basalt magmas rising from the earth's interior, then forced to the near surface again as the tectonic pressures relaxed. The preservation of the isotopic record of these events is remarkable, and still remains to be fully explained.