What's in it for the seagull?

A simple enquiry that arose while riding the Many ferry. I was working at a museum, and needed something to amuse me as I travelled home . . . it must have amused somebody else, as it appeared in the third printed collection of Ockham's Razor talks.

One of the basic assumptions of biological science as a science is that we can always identify either a cause, or a reason, or an explanation for anything if we try hard enough. Whatever reasons and explanations we come up with, they must be plausible, they must be consistent with what we already know or believe, and they should allow us to make some sort of prediction.

This is the scientific way to work, but it's a fairly conservative approach, for we're generally trying to insert new facts into an existing theory. At the same time, of course, we're putting together more evidence to support that theory, aren't we?

I mean, every time that you explain some odd item, some unusual observation from natural history by showing how it would be favoured by natural selection, you're making an even stronger case for accepting evolution by natural selection as the best biological schema on the block.

Working like this serves admirably as a test of a theory: suppose we have an observation which can't be fitted into the standard biological viewpoint, which has no plausible explanation. When this happens, we know that we have to go looking for a better theory. So speculation like this serves a useful scientific purpose.

It can be fun to speculate. I heard Richard Leakey talking recently about the eyebrow ridges of our ancestors, the protohumans. Leakey said that there were people who had actually stuck false plastic eyebrow ridges on their heads.

Then they wandered about, these earnest people in plastic eyebrow ridges, trying to work out what the advantage was in having such things. They couldn't find any advantage at all that could be derived from possessing a fine pair of eyebrow ridges. So far as we can tell, eyebrow ridges have no function, at least not so far as our modern life-style is concerned.

No doubt somebody will eventually put forward a theory that the protohuman eyebrow ridge is a convenient adjunct to head-butting or some other violent activity. And we'll accept an explanation like that, of course, because it matches our preconceptions about violent apes.

But it still might be a wrong explanation. The eyebrow ridge might equally well be a useful sexual signalling device. Suppose, just for a moment, that our female ancestors all looked a bit like Groucho Marx, and the primitive men were doubles for Bob Menzies. Or the other way round, if you really must....

Maybe our predecessors used a rapid up-and-down motion of the eyebrows to convey a message that basically said to other protohumans, "How about it?", or maybe it just meant "Kiss me quick!". In either case, eyebrow ridges that stuck out and accentuated the signal would tend to be selected quite naturally.

Still, it's all a bit far-fetched, isn't it? So that idea remains as a partly-baked speculation of no great value, but it's amusing to think about and play around with ideas like that, as well as being occasionally scientifically useful.

This mode of thinking is also a useful cure for the gloom that the sociobiologists bring us. I'd rather a Groucho Marx for my multi-great grandmother than one of those bone-bashing semi-simians that helped ruin a fine piece of Richard Strauss forever. You know, the ones who reduced the "Sunrise of the World" from "Thus Spake Zarathustra" to the totally mundane "Theme from 2001".

One of the ways in which we fail our students these days is that we don't encourage them to speculate in a scientific way about reasons and causes for things that can readily be observed in our environment. Why don't the snails in my garden eat the weeds in my garden? Why do cicada larvae and most ants have no wings?

Maybe this reticence, this desire to avoid open-ended speculation arises because we're discouraged from such flights of fancy in our own training. I know, for example, that I was nearly failed on a university essay assignment once, merely because I treated a dull topic speculatively.

We'd been asked to write about selection pressure, and I responded by discoursing on the results that might ensue if leprechauns were the pollination vectors for shamrocks. It was biologically sound, but one tutor of botany was less than amused.

But while I'll return to that idea some other day, for I never waste a good theme, I really wanted to talk about seagulls today. It's the way that they follow ships and boats that has me interested just at the moment.

As with any other biological phenomenon, there just has to be a rational explanation for it. Gulls seem to follow their target vessels for quite long distances: surely they must be wasting energy when they do so?

At first I wondered if there was some special advantage in following a boat because people sometimes throw food scraps overboard. Maybe the energy that you get from the food is greater than the energy that you use in following the boat.

But this boat-following behaviour is either instinctive, or it's been learned separately by each gull that follows boats. If it's instinctive, then it must have evolved over a number of generations, and this would take a longer time than there have been boats around for gulls to follow.

But if the behaviour is learned, then it would have to be trial and error learning, where separate individuals discover rewards that come from their behaviour. You know, the sort of thing that Skinner used to do with pigeons and white rats. Or if you've never heard of Skinner, maybe the name Pavlov rings a bell?

In this form of learning, a gull just happens to follow a boat by chance, just happens to collect a scrap of meat, this happens a few more times, and the gull realises that it's onto a good thing, and adopts a life-time habit of boat-following.

The trouble is, too many gulls seem to follow boats: if it was only chance that made them follow a boat in the first place, how come so many of them do it?

You could only explain the large number of chance followers by assuming that there was some sort of following instinct, and I've already ruled that out because boats haven't been around the oceans for a long enough time.

The other thing wrong with this idea of conditioned learning is the extinction of this sort of learning takes place if the rewards aren't kept up. With that many gulls following along. The pickings would have to be fairly slim.

The next tack that I tried was to wonder whether gulls follow other gulls if the other gulls appear to be feeding. If you throw a scrap of food to one gull, soon you're knee-deep in screaming gulls.

They spend most of their time yelling and screaming at each other to get away, but obviously this is a rational way to behave, from a gull's viewpoint. Certainly the gulls hang around, even when they aren't getting a share of the food: somehow, the sight of other gulls feeding gives them nice warm feelings..

So working on this assumption, and straying further away from fact and further into speculation, maybe Gull Number One has learned by chance to follow boats, on account of the food scraps thrown overboard from the boat.

And maybe, just maybe, there's some special way of flying that gulls use when they are flying towards a feed, with a pattern that shouts "come and get it!" to all the other gulls.

As I said, if you've ever fed gulls in a park, or at a picnic, then you don't need to be told that there's some sort of visual cue that says "Food Available Here".

So if our original gull expects to be fed, and is sending out the visual message that there's food about, then other gulls, seeing Gull Number One apparently with the nose-bag on, or at least apparently within cooee of a feed, join in the chase as well, and with more of them there, any scraps that do fall are picked up by one gull or another. Then those gulls who still miss out altogether are buoyed up by the joyous sight of some of their con-specifics getting a good nosh.

So I concluded that what I was seeing was some sort of learned behaviour, reinforced often enough so that the gulls didn't give up hope. With that in mind, I started watching gulls as they followed ferries on Sydney Harbour. And I realised that something was wrong.

The first problem was that the gulls I was observing were getting no food at all, for there was none falling or being thrown from the boat as it rushed along.

The second problem was that it was night, so that if any food had fallen, no gull would have seen it. And the third problem was that many of the gulls weren't following the ferry at all, they were flying alongside, up near the bows.

I must be a bit thick, because I missed the second and third clues at first, and so I started thinking about smart gulls, flying along, looking out for fish that were startled by the approach of the ferry, so that they came to the surface, just as small fry do when they are chased by a larger fish. There had to be enough food in it, somehow, for it all to be worthwhile.

Anyhow, the penny eventually dropped, and I noticed that the gulls never, or only rarely, moved from their station: they always flew alongside the ferry in the same position. But what really struck me was the way that they glided along, hardly ever flapping their wings.

Bingo! If you recall, I mentioned a few moments ago that gulls would need to balance out the energy equations in some way. They weren't gulls scrounging a free feed: they were gulls who were scrounging a free ride.

As the ferry runs along its allotted path, there is quite a mass of boat, pushing along through the air. There must be quite an impressive air pressure wave running along with the ferry, and the gulls were surfing along on the crest of an air pressure wave.

So by muddling around with the facts, mulling them over, I'd found some answers. If I'm right, and that's still open to argument, then probably somebody else got there forty or fifty years ago.

But I don't mind: I'd come up with a suitable hypothesis, and it didn't take me long to extend it: porpoises often swim along in front of ships: it's a pretty good bet that they were riding along on a pressure wave as well.

But I still hadn't worked out how it might all have started. I still hadn't managed to come up with a reason why the behaviour might be instinctive, or how it might be learned.

Off-hand, I can't think of any large, fast-moving marine animals that might produce air pressure-waves for gulls to follow instinctively. Still, there are plenty of winds and updrafts around cliffs for gulls to learn to use, whether the learning is racial and instinctive, or merely individual.

I rather suspect that a seagull's range, in the absence of food, is quite large. If I'm right, then by flying high up, a hungry gull can look for other gulls feeding, and swoop down to join them.

So maybe the energy equation is right for the gulls after all. The ferry is just a convenient travelling cliff, making a travelling updraft for the gulls to ride on.

But even if I have managed to account for that, I still have to explain the small hawk that was following me the other day as I waded down a shallow estuary. As I picked my way along, it flew from tree to tree above me, watching me intently. If it was a fish-eater, I'd know what was going on, but according to my bird books, it eats insects and small lizards.

And even if I do solve that one, I still don't know why my snails don't eat my onion-weed. Not that I'm worried: if I ever do manage to sort those questions out, I'll just have to start on the problem of why the only birds that eat Bogong moths around my house are the foreign birds, the sparrows, the starlings, and the Indian mynahs.

It's nice to have lots of things that you don't know the answer to. If you look at it in the right way, being ignorant can be quite rewarding.

A director from another Sydney museum dropped me a note after this one went to air in 1989. This highly intelligent and erudite man expressed the opinion that I was fully correct, that seagulls were the ultimate scavengers. Cabinet Ministers, he added, would make very good seagulls -- or maybe it was the other way around. Either way, when I had the opportunity, shortly after, to assess the merits of the minister we had the misfortune to share, I realised how right he was.


This is one of a set of talks which were originally heard on ABC Radio National in Australia. All of the talks are copyright © Peter Macinnis, 2001, but permission will be readily granted on request for educational and most non-profit purposes. Contact Peter Macinnis specifying the talk(s) you want, and the purpose to which they will be put. For the rest of the talks, go to Six Months of Sundays.