Of aphids and ants: do mutualisms go awry?

Our group’s discussion this week was based on a topic I’ve always found fascinating: the ant aphid mutualism. Basically aphids eat loads of plant material (much to the chagrin of those with green thumbs) to get enough nitrogen in their diet (essential for building and repairing body tissue), but by a quirk of their metabolism the waste product is incredibly high in sugar. Ants cottoned on to this and started to “farm” them for their tasty sugar, such that 1/3 of all aphids species in Europe now cannot live without their ant carers, and another 1/3 are often kept by ants.

Ant (Lasius niger) with its aphid flock (Aphis fabae)

This obviously looks like a pretty “sweet” deal for the ant, they get loads of free sugar just by keeping the aphids around, while the aphids do alright as well, as in exchange for something they weren’t going to use any way they get protection from predators. Nature seems to have got all nice and soft all of a sudden.

Alas, it is not quite that simple. Like our very own agricultural practices, ants work the aphids to produce more of the sugar solution that they would normally, effectively forcing them to expel useful substances as well as the waste. The aphids get pretty sly as well, with some only squirting out low-quality sugar, but reaping the benefits of protection. This seems more like the nature I know and respect, each species out for themselves. What theory then predicts is that ants will punish and even prey on those cheating aphids, trimming their flock to only those who pump out the good stuff, to the maximum benefit of the ants and the detriment of some of the aphids. However, Vantaux et al. in fact found the ants (Lasius niger) didn’t punish their aphids; they visited high and low sugar aphids equally.  Hmmm

Part of what we do is analyse and evaluate what other scientists have done, and in this study we found quite a few things we weren’t overly satisfied with. For starters they wanted to see if ants would protect low-sugar aphids the same as high-sugar aphids, yet they never introduced any kind of threat or predation pressure, so that question in our eyes remains unanswered. They also didn’t starve the ant colonies as much as previous studies, and in the two different choice experiments they did they starved them different amounts and admitted this could’ve affected the results. Now this may appear like nit-picking, but why on earth would you change your method, find it altered the results and then just move on?  Firstly the starvation period should be standardised between experiments. Secondly if there’s a massively important factor like degree of starvation that you are doing differently to previous studies that revealed really important processes, you need to carefully justify why, and how it does not invalidate your results.

We weren’t done with the disparagement there. We all agreed we were disappointed by this result, as punishment by the ants just seemed so likely and plausible. This perhaps reflects one big issue in science publishing; a positive result is much more likely to be written up in a paper than a negative one, even if it is just as valid. We just seem to prefer something saying: “yes there is a difference!” Rather than: “nope, nothing going on here”. I guess in this instance we have to commend these people for publishing their work anyway, despite the underwhelming result.

In doing so there is a lesson Vantaux and colleagues can teach us. They got a null result, from an experiment that look under 210 hours to carry out, and yet they still wrote a really interesting paper out of it and got it published in a good journal.  That takes no little skill, and it is something we as young aspiring scientists have to learn to order to carve a career for ourselves. Otherwise, unlike the aphids in the ant colonies, we won’t belong no place in academia.

Vantaux, A., Parmentier, T., Billen, J., & Wenseleers, T. (2012). Do Lasius niger ants punish low-quality black bean aphid mutualists? Animal Behaviour, 83 (1), 257-262 DOI: 10.1016/j.anbehav.2011.10.035

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