Because of the constant struggle for existence of organisms, very little in nature is random. There is great pressure for organisms to be as well-adapted to their environment as possible, in order to survive and reproduce effectively (Berry, 1977). On the face of it, a resting animal aligning itself with a magnetic field may seem a trivial decision. However, current work is revealing the reasons behind this phenomenon, which is being discovered in new organisms regularly (Begall et al., 2013).
There are two ways in which an animal may interact with magnetic fields; to utilise a magnetic compass for navigation or to exhibit magnetic alignment. The focus of this essay will be on magnetic alignment (MA) in which an animal aligns itself relative to a magnetic field when at rest or performing an activity ‘on the spot’. The key difference between MA and compass use is that MA is not directed by a locational goal (Begall et al., 2013).
Selective attention, antipredator responses and group directional coordination can all be facilitated by MA, as will be shown in this essay.
Selective Attention, Antipredator Behaviour and Group Coordination
Because the stimuli entering the brain from sensory organs compete for processing power, individual senses are heightened when others are dulled. This is known as selective attention (Stein and Stanford, 2008). In magnetoperceptive animals, the perception of a magnetic field acts as an extra sensory stimulus which could decrease the sensitivity of the animal’s other senses. The animals can, when at rest, counter this by always aligning to a certain direction in which they experience the least sensory disturbance (Begall et al., 2013).
In my opinion, this brings about a problem. If magnetoperception is in competition with other senses, then it may lower the fitness of an animal. The fitness benefits of well-developed magnetoperception must therefore outweigh the costs of decreased sensitivity of other senses. The fact that magnetoperception has been proven in such an evolutionarily diverse array of animals (Begall et al., 2013) infers that the sense evolved prior the diversification of these groups, similar to vision (Williams, 2016). It also infers that most, if not all, animal species have the propensity to be magnetoperceptive but that the sense may regress when its fitness falls below a threshold. This would result in species with little or no meaningful magnetoperception, such as our own (Carrubba and Frilot, 2007), analogous to the regression of vision in cave-dwelling organisms (Espinasa and Espinasa, 2008).
On the other hand, MA is often cited to be an antipredator adaptation (e.g. Begall et al., 2008; Červený et al., 2016) and it seems to me that MA could enhance the sensitivity of other senses more important for predator detection, such as sight and hearing, facilitating a faster antipredator response.
However, current literature suggests that directionally coordinated group flight response is the antipredator function. This means that all individuals in a group align at rest so that they can take the optimal escape route at the same time with fewest collisions (Hart et al., 2012). Obleser et al. (2016) also showed that flight responses were more pronounced in groups of European Roe Deer Capreolus capreolus than singletons and that they showed a preference for fleeing along a north-south axis, as opposed to directly away from the perceived threat. However, there seems to me to be no reason why this theory and that of improved predator detection must be mutually exclusive.
‘Nonsense orientation’ in which birds fly in a particular direction, irrespective of any homing behaviour was proposed a long time ago (Matthews, 1961). The theory is similar to that of the directionally coordinated flight response of Roe Deer and other animals as nonsense orientation works to reduce collisions between individuals in a group. It has recently been augmented by the work of Nováková et al. (2017), who have investigated MA and nonsense orientation in the colonial and flock-forming flamingos (Phoenicopteridae). They concluded that MA ranks fairly lowly in a hierarchy of stimuli determining the nonsense orientation of take-off and landing, with wind direction (with its aerodynamic benefits) and azimuth of the sun (with its ease of perception) being more important. However, it is still important when the conditions are right. Nonsense orientation is particularly important for heavy and flock-forming species, such as flamingos, which have long braking periods and hence a directionally fixed ‘landing roll’. The landing roll is the final phase of landing.
In conclusion, I believe that directionally coordinated group flight response is an important facet of MA. It seems to have an important function in at least fish, mammals and birds (Hart et al., 2012; Obleser et al., 2016; Nováková et al., 2017) and helps to explain the long-standing mystery of how animals coordinate their direction of movement in groups while avoiding collisions.
I also believe that the interaction MA has with selective attention as its potential antipredator function may be important and could compliment the flight response theory. Work needs to be done on this subject to confirm this.
The fact that MA has been found in such an evolutionarily diverse array of animals suggests that magnetoperception is a plesiomorphic trait which regresses when it is not advantageous. There is a hypothesis that MA could be a trait left over from migratory ancestors, but this does not conform with the plesiomorphic argument, nor the fact that MA is more prevalent in animals which are resting moving (Begall et al., 2013).
A lot of studies in this area are theoretical, rather than empirical, and there is a subsequent lack of tightly controlled variables. MA is often assumed to be the only possible cause when the authors believe that other factors are random, such as the weather. Additionally, given our lack of knowledge about MA until relatively recently, who is to say that there is not another stimulus detectable by some animals, but not humans? This could potentially interact with MA and influence alignment. Overall though, the evidence for MA seems to be strong.
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