• Raghavendra Gadagkar

      Articles written in Proceedings – Animal Sciences

    • Observations on the natural history and population ecology of the social waspRopalidia marginata (Lep.) from Peninsular India (Hymenoptera: Vespidae)

      Raghavendra Gadagkar Madhav Gadgil N V Joshi A S Mahabal

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      Ropalidia marginata, the most common Indian social wasp, belongs to a crucial stage of social evolution showing no obvious morphological caste differentiation but a behavioural caste differentiation and a dominance hierarchy that appears to influence division of labour. The nests consist of a single open comb that can sometimes have up to 500 cells and 10 pedicels. Nests are initiated and abandoned all round the year. Initiation is by 1–20 foundresses, 1–4 being the most common number. There is a great deal of variation in brood developmental times both within and between nests. Male progeny disappear from the nest soon after emergence while daughters stay on at the parent nest for a mean period of about a month. Small nests have a single egg layer while large nests have two or more females with well developed ovaries that presumably lay eggs. Most nests are short-lived, small nests being highly susceptible to failure. Large nests are less susceptible to failure but the emergence of multiple egg layers reduces the average relatedness of workers to the brood which presumably is the cause for large scale emigrations from these nests. An interaction of ecological and soical factors therefore appears to determine the growth of a nest.

    • Evolution of insect sociality—A review of some attempts to test modern theories

      Raghavendra Gadagkar

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      An important feature of insect societies is the presence of a sterile worker caste that makes it possible for the fertile queens to produce a large number of offsprings. The mechanism of evolution by natural selection of such sterility and similar, though less extreme, forms of altruism has long been considered as a paradox. In recent years a large body of theoretical ideas has accumulated that purports to explain altruistic behaviour within the framework of the theory of natural selection. With special reference to insect sociality three theories namely kin selection, parental manipulation and mutualism have been suggested. Some attempts have now been made to empirically test the mutually exclusive predictions arising out of these alternative theories. A somewhat different approach to empirically distinguishing between kin selection and parental manipulation is to measure sex-investment ratios. This approach was at one time believed to have provided overwhelming support in favour of the theory of kin selection. It has now been realised that several complicating factors such as local mate competition and multiple mating have to be considered before arriving at appropriate theoretical predictions of the two rival theories. I argue in this paper that rigorous quantitative studies on inter-individual variations in behavioural strategies in primitively cusocial insects constitutes yet another approach that is likely to help in understanding the forces that mould the evolution of insect societies.

    • Kin recognition in social insects and other animals—A review of recent findings and a consideration of their relevance for the theory of kin selection

      Raghavendra Gadagkar

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      Kin selection is a widely invoked mechanism to explain the origin and evolution of social behaviour in animals. Proponents of the theory of kin selection place great emphasis on the correlation between asymmetries in genetic relatedness created by haplodiploidy and the multiple origins of eusociality in the order Hymenoptera. The fact that a female is more closely related genetically to her full sister than to her daughters makes it more profitable for a Hymenopteran female, in terms of inclusive fitness, to raise full sisters rather than daughters or full siblings with a female biased sex ratio rather than offspring. This is sometimes referred to as the haplodiploidy hypothesis. In reality however, genetic relatedness between workers in social insect colonies and the reproductive brood they rear is far below 0·75, the value expected for full sisters, often below 0·5 the value expected between mother and daughter and, not uncommonly, approaching zero. Such values are on account of queen turnover, multiple mating by queens or polygyny. This situation raises doubts regarding the haplodiploidy hypothesis unless workers can discriminate between full and half sisters and preferentially direct their altruism towards their full sisters only. This would still mean an effective coefficient of genetic relatedness of 0·75 between altruist and recipient. For this to be possible however, workers should be able to recognise their full sisters inspite of growing up with and being habituated to an assortment of full sisters, half sisters and perhaps other even less related individuals. Even outside the Hymenoptera, social animals may find themselves growing up together in the company of individuals of varying degrees of relatedness. An ability to tell apart the more and less related individuals under such circumstances should favour kin selection.

      Much effort is now going into assessing the abilities of animals to discriminate between kin and non kin. In every case studied carefully so far, animals appear to be capable of recognising their kin. Ants, wasps, sweat bees, honey bees, frogs, toads, mice, rats, voles, squirrels, monkeys and even humans appear to be able to recognise their kin in one circumstance or another. An ability to recognize true genetic relatedness requires genetically specified recognition labels and these must therefore be present. Recent findings of the role of the histocompatibility system provides some clues to the possible nature of recognition labels. An ability to recognise full sisters for example, inspite of being habituated to full and half sisters requires not merely genetically specified labels but also recognition templates which are based on the characteristics of the individual animals making the recognition and not templates based on all animals one grows up with. Some animals such as honey bees, tadpoles and ground squirrels appear to have such templates but others such as sweat bees and some mice appear not to. It is entirely possible that our inability to devise natural enough assays for recognition prevents us from understanding the full potential of the kin recognition abilities of many animal species. In any case, genetically specified labels and self based templates should greatly facilitate the evolution of social behaviour by kin selection.

    • On reconfirming the evidence for pre-imaginal caste bias in a primitively eusocial wasp

      Raghavendra Gadagkar Seetha Bhagavan Rashmi Malpe C Vinutha

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      Caste is usually thought to be determined entirely in the adult stage in most primitively eusocial wasps and bees. A pre-imaginal caste bias has however been recently discovered in the primitively eusocial waspRopalidia marginata. This study also suggested that reigning queens and possibly other adults may influence the production of new queens and implied at least partial support to the parental manipulation or sub-fertility hypothesis for the evolution of insect sociality. The interest of these results prompted an attempt at their reconfirmation. Complete reconfirmation has now been obtained using data from an independent experiment and two additional methods of data analysis. We therefore conclude that caste is at least partly determined prior to eclosion in the primitively eusocial waspRopalidia marginata which lacks morphological differentiation between egg-layers and non-egg-layers.

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