Joint Nesting in a Digger Wasp as an Evolutionarily Stable Preadaptation To Social Life

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Abstract
One suggested evolutionary origin of insect sociality is joint nesting by females of the same generation. Long before selection favoured joint nesting itself, it might have favoured some other incidental preadaptation such as the habit of 'entering' abandoned burrows, found in the usually solitary wasp Splaex ichneumoneus. We have comprehensive economic records of individually marked wasps. There is little evidence of consistent individual variation in nesting success. Wasps often abandon the nests they have dug, and other individuals adopt or 'enter' these empty burrows. 'Dig/Enter' is a good candidate for a mixed evolutionarily stable strategy : digging and entering decisions are not characteristic of particular individuals; the probability of entering is not conditional upon whether it is early or late in the season; there is no correlation between an individual's size and her tendency to dig or enter; there is no correlation between an individual's egg-laying success and her tendency to dig or enter; individuals do not choose to dig or enter on the basis of immediate past success; individuals do not dig and enter in runs, nor do they alternate; wasps do not choose to dig or enter on the basis of how long they have been searching. At one study site digging and entering decisions are roughly equally successful, but at another entering decisions are perhaps slightly more successful. Entering wasps seem not to distinguish empty, abandoned burrows from burrows that are still occupied. As a consequence of indiscriminate entering, two females sometimes co-occupy the same burrow. Co-occupation should not be called 'communal' because the wasps usually share the same brood cell, not just the same burrow. One might expect that wasps would gain some benefit from co-occupying, but they do not, for a number of reasons: only one egg is laid in a shared cell, and obviously only one of the two wasps can lay it; two wasps together do not fetch noticeably more food than one alone; two wasps together are no quicker at provisioning a cell than one wasp alone; wasps sometimes duplicate each others' efforts when they co-occupy a nest; co-occupying wasps often have costly fights. About all that can be said for joint nesting is that it may reduce parasitism. The risk of joint nesting is the price wasps pay for the advantages of taking over an already dug and abandoned burrow. A mathematical model assuming 'dig/enter' as a mixed evolutionarily stable strategy has some predictive success. If the parameters changed quantitatively, the Sphex model could come to predict selection in favour of joint nesting as such. The selection pressures would have to be very strong to overcome the demonstrated disadvantages of co-occupying. Variants of the Sphex model may be applicable to other species, and may help our understanding of the evolution of group living. The theory of evolutionarily stable strategies is relevant not just to the maintenance of behaviour but to its evolutionary change. One suggested evolutionary origin of insect sociality is joint nesting by females of the same generation. Long before selection favoured joint nesting itself, it might have favoured some other incidental preadaptation such as the habit of 'entering' abandoned burrows, found in the usually solitary wasp Splaex ichneumoneus. We have comprehensive economic records of individually marked wasps. There is little evidence of consistent individual variation in nesting success. Wasps often abandon the nests they have dug, and other individuals adopt or 'enter' these empty burrows. 'Dig/Enter' is a good candidate for a mixed evolutionarily stable strategy : digging and entering decisions are not characteristic of particular individuals; the probability of entering is not conditional upon whether it is early or late in the season; there is no correlation between an individual's size and her tendency to dig or enter; there is no correlation between an individual's egg-laying success and her tendency to dig or enter; individuals do not choose to dig or enter on the basis of immediate past success; individuals do not dig and enter in runs, nor do they alternate; wasps do not choose to dig or enter on the basis of how long they have been searching. At one study site digging and entering decisions are roughly equally successful, but at another entering decisions are perhaps slightly more successful. Entering wasps seem not to distinguish empty, abandoned burrows from burrows that are still occupied. As a consequence of indiscriminate entering, two females sometimes co-occupy the same burrow. Co-occupation should not be called 'communal' because the wasps usually share the same brood cell, not just the same burrow. One might expect that wasps would gain some benefit from co-occupying, but they do not, for a number of reasons: only one egg is laid in a shared cell, and obviously only one of the two wasps can lay it; two wasps together do not fetch noticeably more food than one alone; two wasps together are no quicker at provisioning a cell than one wasp alone; wasps sometimes duplicate each others' efforts when they co-occupy a nest; co-occupying wasps often have costly fights. About all that can be said for joint nesting is that it may reduce parasitism. The risk of joint nesting is the price wasps pay for the advantages of taking over an already dug and abandoned burrow. A mathematical model assuming 'dig/enter' as a mixed evolutionarily stable strategy has some predictive success. If the parameters changed quantitatively, the Sphex model could come to predict selection in favour of joint nesting as such. The selection pressures would have to be very strong to overcome the demonstrated disadvantages of co-occupying. Variants of the Sphex model may be applicable to other species, and may help our understanding of the evolution of group living. The theory of evolutionarily stable strategies is relevant not just to the maintenance of behaviour but...