I thought I’d share some recent and preliminary data from my thrips, which should give a flavour of what I’m doing in the outback. These are quite tentative, and I’m hoping to increase samples next time I go out.
Dunatothrips build silk nests (see our recent paper), and there are usually between 1 and 4 adult females inside a nest (although this can get up to 20). If I experimentally damage the nest, the females repair it. You can see this in this video I took down the microscope:
Notice that one female remains in the background, doing nothing, while the others work hard to add silk to the nest. I saw quite a few females refusing to help in this way. I recorded which of the females chose to engage in what I assume is risky and costly repair behaviour. I removed the females in the order they began adding silk to the nest, and classified them into “repaired” versus “did not repair”.
Then, I dissected them to see whether they were reproductive (i.e. whether they had developing eggs in their ovaries). I found that females that chose to repair the nest were usually reproductive, while those that did not repair were non-reproductive (see below – I’ve applied some scatter to the points to make the figure clearer).
(The bars show the probability of me finding the female has developing ovaries when I dissect her, given what I have observed of her behaviour).
Thus, some females are neither reproducing nor helping to maintain the nest. Sounds a bit like human teenagers to me. Except these are not offspring (the nest hasn’t matured yet, and nests only support one generation of offspring). They are most probably sisters or near relatives of the reproductives (see this paper here for evidence).
Can we say anything else about these do-nothing thrips at this stage? I have also found (very tentatively) that you tend to get nonreproductive females in small nests, while in larger nests, everybody reproduces:
If this indeed turns out to be true, maybe the size of small nests stops some females reproducing. This would make sense, because there is only a limited surface area of leaf to feed on inside a nest. If we look at the wider sample of nests I’ve collected on all my field trips combined, we can see the pattern of offspring production also supports this idea. In large nests, more females produce more total offspring. In small nests, more females do not produce more offspring.
This suggests that in small nests, some females are neither having their own offspring nor helping their nestmates produce more offspring (in both cases, we would have seen increases in total offspring with numbers of females).
Of course, “lazy” nonreproductives may be helping in ways I can’t detect – perhaps by helping the offspring to feed, by defending against an enemy I haven’t encountered yet, or by maintaining nest hygiene. In those small nests, the offspring may still SURVIVE better if there are more adult females in the nest. That’s something I’m going to have to test, maybe by measuring offspring size, or development rate in the field. Alternatively the reproductives might survive better if they don’t have to work as hard when helpers are present, something that would certainly be difficult to test in the field.
The big question is, what determines who gets to reproduce in a small nest? Intuitively, there should be competition over breeding status, as happens in (for example) paper wasps and joint-nesting ants. As we described in our last paper, these thrips appear to be pacifists, with no apparent aggression at all. Also, there’s no evidence that nonreproductive females are smaller than reproductive females from the same nest. Unless they are conducting some kind of chemical warfare, there is no indication of conflict at all.
In other species, lazy workers are sometimes waiting for a chance to reproduce themselves – the more they work now, the less energy they have for later reproduction. In Dunatothrips, this is probably not the case because nests probably only have one generation. However, that is not set in stone, and it may be that females in a small nest end up taking turns to reproduce in what we would call a social queue. That would be really exciting because it would make this system quite close to Polistes paper wasps. These are questions I’m hoping to solve with experiments next time I hit the field. I am currently thinking about how to design these, and constructive ideas are most welcome!