I am an amateur naturalist trying to learn something about everything living in my garden.
Photo 1, taken back in late Summer, shows a Painted Lady butterfly enjoying a well-earned rest on a leaf in my garden. I say 'well earned' as this butterfly will likely have undergone an amazing 1000 mile migration, to arrive in my garden from North Africa.
The entire British population of Painted Ladies (Ladys?) arrives here in Spring and leaves again in Autumn (strictly not all leave, but those that don't fail to survive the British winter).
To have encountered a Painted Lady in my garden last Summer is perhaps unremarkable when you learn that 2009 was a mass migration year for Painted Ladies to the UK. Millions arrived, with one flutter (the collective noun for butterflies) alone of 18,000 spotted off the South Coast of England.
A fun thing to know (who knows, it may help you win a pub quiz one day!) is that the Painted Lady is the only species of butterfly recorded from Iceland. I got this fact from the admirable UK Butterflies site, which contains numerous facts and photos about the lifestyle and food preferences of the Painted Lady that I'll not reiterate here. Suffice to say that caterpillars of the Painted Lady are dark and hairy and feed on thistles and nettles.
My searches for information on my butterfly were complicated by the fact that some sources seem to use the Latin name Vanessa cardui, others Cynthia cardui, and still others talk about the species cardui in the genus Vanessa and sub-genus Cynthia. I've not had a chance to sort out which is the professionally accepted name. Anyone?
Another topic my searches led me to was the (new for me) subject of 'foraging theory'. This is a huge topic and the reader should be take my (decidedly amateur) understanding and description with a 'health warning'. Briefly however my understanding goes like this: We've all watched bees and butterflies drifting through patches of flowers, or watched little songbirds working their way through the tree tops pecking at tidbits. Perhaps, like me, you've never really noticed any particular pattern or method to the foraging of these animal. At a glance, butterflies for example, seem to drift along haphazardly, landing on any such plant as they encounter and (one might presume) staying there for as long as it takes to drink a flower's nectar dry. In fact, many years of fabulously detailed studies by armies of biologists have shown that the foraging practices of many animals are anything but random. Quite the contrary, foraging animals have evolved highly specific methods and rhythms, carefully fine tuned to allow them to optimally gather food from their environments.
A classic example of decidedly non-random foraging was revealed by the studies by Messrs. Richardson and Verbeek (you can find one of their papers here) on the feeding habits of a population of crows in British Columbia. The crows were foraging on a beach for clams. Now, you might naively assume that a crow would simply gobble up any clam it came across. This fails to take account of the fact however, that a crow has first to open up a clam's shell in order to get at the meat inside. Now, little clams don't take much time and energy to open...but then, they don't yield much meat either. Huge clams yield lots meat...but they require the crow to spend a lot of time and energy to get them open. From this you start to realise that if a crow is to get the maximum food benefit from an hour (say) spent feeding, there will be some optimal clam size the crow should target in order to spend the least time for the most meat. Amazingly, this is what the studies showed: the crows were selecting just those sizes of clam that allowed them to maximise their average energy intake.
Similar studies have been replicated across many animals with the same results: crabs show optimised strategies similar to those of crows when selecting the size of mussels to open and eat; studies on brooding starlings show that parent starlings will continue to hunt for worms in the field until they are have just the number of worms held in their beaks that optimises the bird's efficiency in getting to and from the nest (Carry too few worms and the parents must make too many energy-sapping flights back and forth to feed the chicks. On the other hand, spending too long in the field trying to peck up worms with a beak already stuffed full is slow and cumbersome and results in the parent trying to carry an excessively heavy load back to the nest); Male Yellow dung flies show behaviour that optimises the balance of time and energy spent feeding vs. the time and energy spent in moving between dung heaps looking for females with which to mate.
Actually, although foraging behaviour can be discussed using words as above, in using phrases such as ' the maximum energy acquired per unit time' etc. the numerically minded amongst you may start to realise we are approaching the possibility of a mathematical desciption of foraging ( time-rates-of-change of quantities are the 'bread and butter' of the calculus you may recall from school). A mathematical description of foraging is just what the professional biological community has developed. The classic textbook Foraging Theory by Stephens and Krebs, gives a flavour.
Some of the seminal early work on foraging was by Charnov in 1970's. Charnov developed an important theorem known as the marginal value theorem which dealt with the situation of an animal foraging between patches of food spaced some distance apart (separated patches of flowers in a meadow for example). The question is, how long should a feeder spend in any given patch before moving on? Spend too long in one patch and the available food dwindles away (the animal is reduced to hunting around for the few remaining 'scraps' so to speak). Equally it takes time and energy to fly between patches. Charnov's theory was developed to predict the optimal time an animal should remain in any one patch in order to maximise its average rate of energy intake (or to put it another way, the foraging pattern resulting in the animal getting, on average, the most calories per hour).
All of which preamble, brings me back to the Painted Lady and the papers I came across by F.R. Hainsworth. Hainsworth studied whether Painted Ladies follow the predictions of the marginal value theorem. Specifically he studied how Painted Ladies reacted to being offered sugar-water solutions of varying strengths. Should Painted Ladies be following the predictions of the marginal value theorem then they should preferentially feed on those sugar solutions that would allow them to take up, on average, the most energy per hour. Now, a crow having to wrestle with opening a clam shell or a fly having to divide its precious time between feeding and mating is one thing. But you may wonder what's to stop a butterfly simply opting to feed on the most sugary solution offered every time? The answer is satisfyingly subtle: you must remember that a butterfly is constrained to having to feed through a straw! (The proboscis). If you imagine yourself being hungry but constrained to eat through a straw, you'll realise that whilst a thick gloopy syrup will certainly provide you more total energy than a thin watery one, sucking a jar of treacle through a straw is certainly no quick meal! When you realise this, an answer to the question of what sugar concentration in water will allow you to imbibe the most calories per hour is suddenly not so obvious.
I can't help but digress at this point to briefly address the question: How do you gather data on the feeding preferences of a butterfly? One option is to chase your specimen around a forest with a stopwatch! Preferable of course, is to find some way to get a captive butterfly to eat 'on demand' in the laboratory. This might sound tricky but Hawksworth provides a wonderfully simple solution that any suitably motivated amateur could replicate (naturally, you need a supply of butterflies but there are various companies on the web that will sell you eggs and caterpillars). The trick is to know that butterflies can taste through their feet! Gently hold your butterfly between thumb and finger, lower its feet into a dish of sugar water and, bingo!, it will instinctively unroll its proboscis and begin to feed. Get yourself a stopwatch and you're all ready to test the marginal value theorem!
Anyway to return to our main question: Are the eating habits of Painted Lady's in agreement with the marginal value theorem, with butterflies eating so as to maximise their average rate of energy uptake? Interestingly, the answer seems to be no. Instead, Painted Ladies go for meals that give them the most energy in one sitting (even though it may take longer to eat such a meal). This doesn't mean the marginal value theorem is 'wrong' (as above, it applies well in some situations), it simply means that one or more of the assumptions upon which this theorem is based don't apply to the Painted Lady. One suggestion is that rather than playing the 'long game' of choosing sugar solutions that maximise the calorie intake over a long period of time, newly emerged female butterflies are keen to pack in high calorie meals early, in order that they can quickly take on board enough energy to enable them to lay a large clutch of eggs. Whether this is the full story however appears to require more study...but then of course, you, dear reader, now know how to approach the task of conducting butterfly feeding experiments. I'm entirely confident therefore, that the answers will be with us shortly!