Saturday, March 20, 2010

Lady's Smock Cardamine pratensis

I am an amateur naturalist trying to learn something about everything living in my garden.

Photo 1 shows an attractive flower I found growing wild in a neglected corner of my garden early last summer.

A little time spent with my trusty copy of The Wildflower Key (Francis Rose) and I'm fairly confident my plant is Lady's Smock. The leaves of my plant are also rather characteristic (photo 2) with the lower leaves tending towards round with the largest leaf being the one furthest from the stem, whilst the upper leaves are rather long and narrow.

My copy of The Englishman's Flora (Geoffrey Grigson) lists around thirty alternative common names for Lady's Smock from the pretty Cuckoo Bread, Cuckoo flower, Lucy Locket and Milking Maids to the less flattering Pig's Eyes and Bog Spink!

The scientific name for my flower is Cardamine pratensis. Pratensis is from the Latin 'of meadows'. From some web searching I understand Cardamine owes its heritage to the anicent Greek physician Dioscorides (ca. 50AD) who used the name for some cress-like plants, karda being Greek for 'heart', and damao to 'tame or overpower'. (I've read that Lady's Smock is an edible, spicy, salad leaf, though I can't vouch for the truth of this.)

Lady's Smock is a favoured food of the caterpillars of the Orange Tip butterfly.

C. pratensis is a perennial, native to the British Isles and is highly variable. The paper here gives a flavour of the lengths the professionals have gone to in attempting to characterise it. My copy of New Flora of the British Isles (Stace) sums things up rather bluntly however as "impossible to subdivide usefully; identity of [named] variants [...] is very dubious". My attempt to get some basic amateur understanding of the issue led me into a study of polyploidy. I'm certainly no expert on genetics but briefly my understanding is this:

As most people know, at times the DNA inside living cells gets packaged into structural units, the chromosomes. A microscope reveals we have 46 chromosomes. Closer investigation reveals that these 46 are in fact present as two largely similar sets of 23 i.e. 23 chromosomes from our mother and a similar 23 from our father. (The phrase 'largely similar' skips over a wealth of detail, such as the fact that a certain chromosome from our mother might carry a different characteristic, say eye colour, to that from our father etc. - but never mind that here) . For humans we say "2n=46". Other mammals have other chromosome counts, for example I read variously on the web that the kangaroo has a mere 2n=12 whilst the European hedgehog boasts 2n=88 (in general there is no link between chromosome number and the size or complexity of a creature).

In the case of many plants, a lesser number of insects, amphibians, fish and a very few mammals (one being an Argentinian Plains Rat - see the parade of polyploids here!) however, things are more complicated. Some varieties of strawberry for example contain not, as we humans, two largely similar sets of 23 chromosomes but no fewer than eight sets of seven chromosomes. This gets written as "8x=56". This condition of having more than two sets of chromosomes is termed polyploidy.

The fact that more than 30% of plants go in for polyploidy would seem to indicate it must have some important benefits. There are many theories as to what these benfits might be. It's been suggested for example, that polyploidy may promote greater variation in a species and thereby help it evolve ('radiate') more easily into new environments, or that polyploidy may provide protection against 'in-breeding' issues that might otherwise arise for plants that self-fertilise. It seems also that some polyploids are healthier and more vigorous than non-polyploids. In all however, there seems to be no universally accepted theory of why so many plants are polyploids. Another of nature's mysteries!

Anyway, all of this preamble basically allows me to comment that my garden weed occurs in a bewildering variety of genetic 'forms' from the 'diploid' 2n(=2x)=16 through types with chromosomes numbers of 14, 15, 16, 19, 20, 21 – 28, 30, 32, 37, 42, 44, 45 and 48.

After the science, who better to have the last word than the Bard. From the song 'Spring' at the end of Love's Labour's Lost:
When daisies pied and violets blue,
And lady-smocks all silver white,

And cuckoo-buds of yellow hue
Do paint the meadows with delight,

Sunday, March 7, 2010

Painted Lady Butterfly Vanessa (Cynthia) cardui

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!

Saturday, March 6, 2010

Helophilus pendulus hoverflies

I am an amateur naturalist trying to learn something about everything living in my garden.

Photo 1, taken back in September, shows two mating hoverflies resting on a leaf of my garden Buddleja bush. The characteristic brown stripes on the thorax quickly let me identify the species as Helophilus pendulus in my copy Hoverflies (Francis Gilbert, Richmond Publishing), an excellent monograph I've talked about before.

I assume the pair in photo 1 is male and female. Apparently it's normally possible to tell the two sexes apart in hoverflies by the eyes: The eyes of males touch at the top of the head, whereas those of females don't. Unfortunately this isn't true of Helophilus species. Why, generally, hoverfly sexes should differ in this regard, and specifically why they don't in Helophilus species, I can't imagine. Can anyone comment?

As I've mentioned previously, it's possible to distinguish hoverflies ('Syrphinae') from other flies by the veinature of the wings. In photo 2 I've zoomed in and enhanced the shot with my camera's software. Hoverfly wings have a 'false vein' (running approximately vertical in photo 2) and a section of vein at the edge of the wing that other flies lack.

In The Encyclopedia of Land Invertebrate Behaviour, the authors, R&K Preston-Mafham, describe male H. pendulus hoverflies as searching near flowers for females but often pouncing on other species of fly by mistake.

My copy of The Colour Guide to Hoverfly Larvae (G. Rotheray, Dipterists Digest 9) explains that the larvae of H. Pendulus thrive in farmyard drains and wet manure. Nice! My web searches also turned up a paper by one E. Stanley in the Veterinary Record 1845, 1(4) that describes finding an H. Pendulus maggot infesting the spinal marrow of a horse. By appearance they are a dark brown maggot with a tail as long as the body that acts as a breathing tube.

An obvious feature of hoverflies is the resemblance of many species to wasps and bees. Indeed, as explained in the extensive and very readable paper The Evolution of Imperfect Mimicry in Hoverflies by Francis Gilbert, at least a quarter of European hoverflies are mimics. Clearly, the mimicry is simply a matter of hoverflies trying to fool birds into thinking they are a wasp that will sting them....right? Well no actually, or at least it can be said the situation is far more subtle. As the paper above highlights the subject of animal mimicry is a complex one, much studied by biologists, and a topic where numerous questions and controversies persist.

Firstly, one should not assume that the 'warning colours' of hoverflies are, in all cases, a response to the threat of bird predation. There are plenty of other animals such as dragon flies, wasps and spiders will also eat hoverflies. Spiders have been shown to have an ability to recognise the threat posed by wasps in their web, and to treat them with greater caution than they do with other insects. At the same time, some experienced birds such as Flycatchers have been shown to be rather skilled at 'seeing through' the disguise of hoverflies - readily distinguishing them from wasps.

Next there is the issue of whether what we, with our human eyes, see as a resemblance to a wasp, is the same as what a predator perceives with eyes that might have very different characteristics to our own (different colour sensitivity etc.) Experiments studying the willingness of pigeons to peck at images of various hoverflies and wasps have shown that pigeons do broadly rank resemblances of hoverflies to wasps in the same way that we do. But there are exceptions with pigeons regarding Syrphus ribesii as the most wasp-like hoverfly of all, a view not shared by humans.

Next, there is the question of whether it is a bee's or wasp's sting that a predator is avoiding and that a hoverfly is "pretending" to posses. In fact, it seems that birds hardly ever suffer wasp stings and actually it is the unpleasant taste of a wasp's internal venom sack that some birds avoid. With Bumblebees (which some hoverflies mimic) the situation is more subtle still. There is some evidence that the sheer effort (in terms of wasted time and energy) involved in removing all the various hairs and largely inedible, chitinous, 'body armour' of a bumblebee is enough to put off some birds. Contrary to proclaiming its sting, the colouration of bumblebees (and their hoverfly mimics) may be a way of saying "I'm not worth the time and calories you'll get from eating me!".

This brings us to the difference between so-called Batesian mimicry and Mullerian mimicry. Batesian mimincry is the type of mimicry most of us imagine at first, whereby a harmless insect species evolves to copy the warning colours of a harmful one. By contrast, Mullerian mimicry involves two or more species that each have defences of their own, but nevertheless carry a common warning colour or form. Lots of bees and wasps all have yellow stripes for example. One can understand how this might arise: Suppose a predator has a bad experience with a harmful species. If that predator comes to associate the bad experience with a warning marking for that one species (yellow stripes say), then there's clearly potential benefit in other poisonous species adopting similar markings. By adopting similar markings however, one wasp species is not really 'copying' another, rather, both species are gaining benefit by evolving in parallel to advertise their venomous natures through similar body markings (multiple species settling on a 'common format' for advertising their individual danger signals if you will) . Now, in saying that hoverflies 'copy' wasps we are making the tacit assumption that hoverflies are harmless Batesian mimics. This is probably mostly the case. But it need not be universal. For example, it has been suggested that some species of hoverfly concentrate unpleasant tasting chemicals in their bodies through eating aphids that have been feeding on noxious plants. Such hoverflies would have their own defences (their noxious taste) and any evolved resemblance to say wasps, might then be an example of Mullerian mimicry.

The list of fascinating questions surrounding mimicry goes on. There is the question of the drawbacks of mimicry. Naively, it might seem there could little detriment for a species in 'wearing the clothes' of another. But suppose the venomous species being copied becomes rare. If a non-venomous hoverfly were to become more common than a venomous bee (say) it might be copying, what then? Predators would rarely (or even never) meet with the bad experience of finding that the prey in their mouth was the truly venomous one. After a time predators might simply stop associating the mimic's bright colours with danger. Indeed, the bright colours of the hoverfly mimic might become a positive liability, gaudily advertising the presence of a tasty snack! In summary, by choosing to copy the colours of another species, a mimic pays the price of shackling its population size that of the target insect being copied. Exactly the nature of the constraint (i.e. the mathematical relationship between the sustainable population of the mimic vs. the copied species) is a rich topic in its own right.

I could go on still further into a discussion of polymorphism in mimics, whereby a single species of say, hoverfly, comes in a number of different (colour) forms called morphs, and the benefits this confers. I've written enough for now however and so will leave this topic for another day, or, if you cannot wait for then, refer you to the paper above by Gilbert.