With the characterisation of a magnetic compass, the research stemming from Monarch migration has received new life. Revisiting the established discovery that blocking antennae light-detection resulted in complete disorientation, it was now clear that not only the sun compass but also the magnetic orientation was blocked. Therefore, it was concluded that the magnetic compass must be light-dependent. But what role might light have in the detection of magnetic inclination angles? The answer belonged in the relatively new field of ‘spin chemistry’. To fully understand the role of molecules in magnetic detection, the fundamental chemistry of biological molecules comes into play.
All molecules contain chemically bonded atoms. All molecules exhibit a constant rotation and vibration in the space in which they exist. The energy associated with this movement is termed the thermal energy of the molecule. This energy is the value to which all other energies the molecule is subjected to are compared. For example, it is fundamental that the energy contained within the chemical bonds joining the molecule is greater than the thermal energy. If it wasn’t, every molecule would shake itself apart; something that wouldn’t spell great things for the universe as we know it. This fundamental rule also tells us why magnetic fields do not tend to affect chemical reactions. The Earth’s magnetic field is relatively weak and most molecules are not strongly magnetic, so the energy input from the magnetic field is not substantial enough to overcome the thermal energy of the molecules involved in chemical reactions.
However, this is not to say that magnetic fields can never affect a chemical reaction. They are perfectly able to do so, should the sufficient initial energy be provided from another source. An analogy can be found in the dynamics of a rectangular block of granite. Standing on its end, the block will not fall over without a huge input of energy focused on one point. However, if the block was somehow perfectly balanced on one of its points, it would not take nearly as much energy to make it fall (perhaps even a butterfly landing on the left or the right of the point!) The same concept can be true of chemical reactions – small inputs of energy can have large effects if there is an original input of energy from another source that pushes the system almost to the point of no return.
Returning the Monarch’s magnetic compass, it seemed that ultraviolet (UV) light, a high energy part of the electromagnetic spectrum sent out into space by the Sun, provides the initial energy to the molecules involved in magnetic detection. A chemical bond between atoms contains two electrons within it, shared between each atom. UV light has the energy to rip these bonds apart, splitting apart the molecule to which the bond belonged. In doing so, it generates two new molecules, with each one taking one of the electrons contained within the bond with them. In general terms, molecules existing with an extra electron are highly unstable and high reactive. The name given to these molecules is ‘free radicals’. The characteristics of the spare electron on a free radical defines the wider characteristics of the molecule. Most important of these electron characteristics is electron ‘spin’, which can be described as up or down. So, UV light splitting up a molecule creates a pair of radicals, which can either be up or down in terms of their spin. When taken as a pair, two forms can exist, a ‘singlet’ orientation in which the two radicals have the opposite spin (up, down) and a ‘triplet’ orientation in which the two radical have the same spin (up, up or down, down). Radical pairs can rapidly oscillate between these two types and do so in a coherent manner; when one radical changes its spin, the other radical in the pair will work with it and change its spin accordingly.
Crucially, if the radical pairs are far apart, the energy associated with this oscillation is less and only a small input of energy is required to shift from a singlet to a triplet state or vice versa. This is where magnetic fields come in. It is hypothesised that magnetic fields can determine the type of radical pair that is formed. Different inclination angles can result in a different concentration of a certain type of radical pair and this can be detected and interpreted by the Monarch brain to give a latitudinal geolocation cue. To give some legs to this theory, researchers attempted to determine the identity of the molecule that was split up to form radical pairs. It would turn out to be a familiar name that was responsible. The molecule(s) in question were suggested to be the CRY proteins involved in the circadian clocks in both the brain and the antennae. Their remarkable dual-role was suggested after the magnetodetection capabilities of CRY-deficient flies were restored when Monarch CRY production was stimulated.
All that remained for the CRY magnetodetection model was for a proof-of-concept molecule to be synthesised and put to the test. This work was carried out by a team at the University of Oxford, led by Christiane Timmel. They produced a synthetic molecule known as CPF and split it into a radical pair by shining a high-energy laser at it. They then measured the lifespan of the singlet/triplet state of the radical pair. They discovered that the lifespan of one particular state was increased by around 100 times when exposed to a magnetic field as opposed to when the system was shielded from all magnetic effects. It seemed that at long last, the final piece had been placed in the remarkable puzzle of Monarch migration.
Image adapted from: Maeda, Henbest et al. 2008
Of course, to put the astonishing accuracy of the Monarch migration down to just a few processes is highly simplistic. There will undoubtedly be other common migratory factors at play. Monarchs are thought to borrow a page of out avian books and use high-speed seasonal wind currents to ease the burden on their paper-thin wings. Flying at high altitude on these currents would likely save energy and reduce the inhibitory effect of cross-winds that would threaten to drag them off course.
Olfactory cues are likely to play an important role. Monarchs have a remarkably impressive sense of smell and it is thought that this may be important in fine-tuning their migration. A great deal of work has centred on just how Monarchs, always on their maiden journey, can find their way to the same fir groves with each migration. It has been suggested that while the sun compasses and magnetic detection may give gross orientation cues, it may be olfactory cues from the fir groves that guide them to their final destination. Monarchs have also been shown to be highly gregarious. It has been suggested that social interactions, possibly mediated by release of pheromones, may serve to fine-tune the migration, serving as a bread-crumb trail to their final destination or to night-time nesting sites.
It seems, however, that when it comes to the Monarch butterfly, this is the story that keeps on giving. In the last couple of years, physicists have picked up the baton and are exploring just how radical pairs could define future research. Radical pairs are known to exhibit coherence as they work in tandem, switching between singlet and triplet states. However, recent work has gone on to show that they may be joined by an even stronger, more mysterious bond. This relationship, known in perhaps the sexiest scientific term of them all as ‘quantum entanglement’, states that the spin characteristics of an electron is affected by what happens to the other electron spin, even at a substantial distance. This fascinating characteristic, first described by Erwin Schrödinger in 1935 and described by Albert Einstein as “spooky action at a distance”, is of great interest to quantum physicists as the close bond between substances lies at the heart of quantum computing, a theoretical process that may allow for phenomenally efficient calculations to be carried out.
Such a system is difficult to produce because quantum entanglement can only be produced using laboratory conditions that are difficult or expensive to maintain, such as near-perfect vacuums and ultra-low temperatures. Quantum-entangled substances also may only exist fleetingly, for around a pico-second (that’s 0.0000000001 seconds!).
However, if the radical pairs produced in Monarch magnetic field detection are indeed quantum-entangled this could revolutionise this field. It may be the case that nature has provided an example of biologically available molecules that exhibit quantum entanglement at normal temperatures and pressures, in as simple an environment as a Monarch’s antennae or a Robin’s eye. Spin chemistry has suggested for decades that radical pairs can maintain their entanglement for a relatively long period of time, as long as a microsecond (that’s still 0.0000001 seconds, but in quantum terms, that is that isn’t bad!). While the story of the science underpinning the remarkable Monarch butterfly migration seems to finally be complete, the tale of the research that has arisen from the past few decades of work seems to be just beginning.
In its most basic form, science of all disciplines can be described as the quest to satisfy insatiable curiosity. It is about never knowing when to stop asking questions, never being satisfied with the work that has come before you. For these people, the Monarch butterfly is the gift that keeps on giving. The unravelling of its unique migration has touched on numerous scientific disciplines; from the initial question of just how these insects knew where they were going on their annual migration, to the discovery of the cellular processes that underpin their accuracy and finally to the work that will arise from this story, including advancements in spin chemistry and quantum entanglement research. This story, like so many others that play out around us on this planet, is a testament to the beautiful complexity of the natural world and the resilience of the scientists that have worked to unravel it.
For the Monarch butterflies, life carries on as normal. In just a few months they will begin their journey once again. They are doomed not to complete it themselves, but thanks to their beautiful in-built circuitry, generations of offspring will, slowly but surely, find their way back home.
The butterfly counts not months but moments, and has time enough.
Rabindranath Tagore
