Following a thorough journal trawl, it is revealed who are the major contributors to magnetoreception research, many of which are specialised in a certain group of vertebrate. Wiltschko and Wiltschko (1999) for example, are key providers of magnetoreceptor information in homing pigeons and European robins, Lohmann and Lohmann (1994) for loggerhead sea turtles and Phillips (1986) for eastern newts. These publishers, among others, are pioneers in their field providing multiple experiment results over the years. As magnetoreception is yet inconclusive in many animals, having a large data pool within any species is invaluable to uncovering their magnetoreception status. If identified to utilise an aspect of the earth’s magnetic field, further research can then be carried out to answer further quandaries as to how it is accomplished and why. It is the author’s choice that is ultimately responsible for what species is selected for study. The importance of authors who specialise in a species is that in time, they undoubtedly become more familiar with the chosen species and advance their own research by trial and error methodology, providing increasingly significant results for the latest questions in this field of research.
Species size and age specific theories
Several ecological and evolutionary outlines or ‘rules’ have been put forward since the first conception of magnetoreception, mainly in relation to body size; these are apparent in the literature. Noticeably distinguished is Bergmann’s rule (Miguel, Miguel and Bradford (2006), Cope’s rule (Rensch 1948; Stanley 1973) and Rensch’s rule (Rensch 1950). The functioning core mechanisms tied to these models and their precise workings are still unknown. A keystone study by Nishimura et al. (2008) focused on the connection between magnetic field exposure and animal body size, as Bergmann’s rule holds that organisms tend to be larger at higher latitudes, where the geomagnetic field is more than doubled in strength in comparison to that of lower latitudes. Using data in the literature, a meta-regression analysis was carried out by Nishimura et al. (2008) to determine the effect of electromagnetic exposure on animal weight in contrast to that of unexposed controls. The results demonstrate that electromagnetic field exposure had a considerably positive relationship with relative weight in males, whilst in females this conclusion did not apply. Thereby, the body weight increase would explain Rensch’s rule. Bergmann’s and Cope’s rules would be explained by the male’s relative weight increase. To further support Cope’s rule, it was concluded that, over consecutive generations, animals would increasingly gain a substantial amount of body size if natural magnetic fields and/or electromagnetic fields become stronger over time (Nishimura et al. 2008).
In many migratory orientation experiments only young birds (e.g. Mouritsen and Larsen 2001; Muheim and Akesson, 2002), or the age is not stated (e.g. Able and Dillon, 1977; Able and Cherry, 1986), are used.
If however certain senses develop at a specific age, this must be accounted for when choosing a species to study for magnetoreception. A study by Munro et al. (1997) who found that a short, high-intensity magnetic pulse, an action intended to alter the magnetisation of magnetite, had no effect on juvenile Zosterops lateralis orientation. They continued to select their seasonally appropriate migratory direction (Munro et al. 1997). In comparison, mature silvereyes from the same population had reacted to the same experimentation with a 90° clockwise deflection from their normal migratory course (Munro et al. 1997). This outcome proposes that magnetite is a component in an orientation model used solely by adult migrants. Also, study results from Munro et al. (2014) back the hypothesis that magnetoreception appears in the second week of life in Gallus gallus domesticus. Due to the difficulty in determining to what extent of development this sense is in, further research is needed. This has huge implications for the results of past and future studies. For example, if chickens are found to develop a fully functioning truly directional magnetic response at two weeks of age, all studies that will test or have tested chickens younger than two weeks of age will contain misleading results.
Unlike most senses, the physical foundation being defined, no mechanism for magnetoreception has been identified with certainty. Thus, recognising the limitations could be valuable to the illumination of this mechanism; which has so eluded scientists in the field of sensory biology for more than four decades.
The limitations consist of several factors. First, humans appear to lack the capability to detect magnetic fields (Wiltschko and Wiltschko 2012). In comparison, the majority of non-human senses (UV vision and polarisation detection) are somewhat direct additions of known human abilities, magnetoreception being an exception. Consequently, neither medical literature nor innate comprehension can be used as an advisory tool with regards to human senses. Another impediment is that organic matter is fundamentally not physically affected by magnetic fields (Hulot et al. 2012), resulting in magneto-receptors having the potential to being situated anywhere within the organism (feasibly in microscopic intracellular arrangements) which is dissimilar to the majority of other sensory receptors. Another obscuring factor is that the existence of sizeable auxiliary edifices used for fixing and influencing the magnetic field, for example components of lenses and eardrums, are unlikely to be found due to the lack of natural materials which can impact on magnetic fields (Chauhan and Vaish 2012). Finally, the faintness of the collaboration amid the Earth’s magnetic field and the magnetic instances of electrons and atoms, approximately one five-millionth of the thermal energy kT at body temperature, makes it problematic to even propose a reasonable mechanism (Johnsen and Lohmann 2008). To what intensity and degree each possible mechanism detects the magnetic field of the Earth, is another realm of questions entirely.
Once an ability such as this is identified in a vertebrate species, not only is it a breakthrough in scientific terms, but also has the potential for a multitude of commercial applications which can be used for human and/or species benefit.
A study by Burchard et al. (2003) assesses the assumption that electric and magnetic fields could interfere with dairy production in Bos taurus. Exposure to electromagnetic and magnetic fields resulted in an average reduction of 4.97% in milk yield, 13.78% in fat corrected milk yield, and 16.39% in milk fat and an increase of 4.75% in dry matter intake. Similar studies on cows back up these findings (Burchard et al. 1996; Burchard, Nguyen and Block 1998; Burchard, Nguyen and Rodriguez 2006).
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Domestic chickenshave ferrous naturally occurring inorganic substances in thedendritesin the top mandible and have the ability of magnetoreception (Falkenberg et al. 2010; Wiltschko et al. 2007). Since chickens use orientation guidance from the magnetic field of the earth to navigate in moderately sized areas, this highlights the risk thatbeak-trimming(subtraction of a portion of the beak to minimise harmful pecking often executed on egg-laying hens) compromises the capability of hens to navigate in large arrangements, or to move in and out of structures in free-range networks (Freire, Eastwood and Joyce 2011).
Spiny dogfish (Squalus acanthias), are thought of as pest species because of their large quantities in the western Atlantic, adding to their often formidable numbers in industrial fishing equipment. O’Connell et al. (2012) combined electropositive metal and magnetism, as possible elasmobranch deterrents, on a fishing hook – the SMART (Selective Magnetic and Repellent-Treated) hook and evaluated onspiny dogfishin the Gulf of Maine. Results revealed that SMART hooks reducedspiny dogfish catch by 28.2%, but had no perceived effect on thorny skate (Amblyraja radiata), barndoor skate (Dipturus laevis), and teleost catch. It is interesting to note, further investigation identified that SMART hooks created a mean voltage of 1.05eV for a period of 5 days; subsequently the material rapidly dissolved and the voltage dissipated. Despite being effective, the implementation of the SMART hook may not be financially practical at present as the general target catch (e.g. teleosts) did not compensate the price of the hooks.
Since it has become apparent that elasmobranchs can identify tiny electromagnetic fields, <1 nVcm–1 (Winther-Janson et al. 2011), using their ampullae of Lorenzini (Schäfer et al. 2012; Wueringer et al. 2011), reaction effects to electric fields have been analysed in different species with the purpose of producing shark repellents to minimise shark-human interactions. Huveneers et al. (2013) demonstrated the impact of the Shark Shield Freedom7™ electric repellent on white sharks (Carcharodon carcharias). Two experiments were conducted, the first contained 116 trials using static bait and were executed at the Neptune Islands, South Australia (Huveneers et al. 2013). These results suggest that the quantity of bait taken throughout static bait trials was not influenced by the electric field. Nevertheless, the electric field lengthened the time it took the sharks to ingest the bait, decreased the number of contacts per advancement and reduced the quantity of contacts within two metres of the lure. It must also be noted that this effect of the electric field was not consistent in all sharks. Experiment two involved 189 tows using a seal lure and was executed near Seal Island, South Africa (Huveneers et al. 2013). 0 breaches and just 2 surface contacts were detected for the period of the towing when the electric field was initiated, this was compared to the 16 breaches and 27 surface contacts devoid of the electric field. Interestingly, it was concluded that the decrease in activity resulting from the electric field is situation specific and determined by the motivational state of sharks (Huveneers et al. 2013).
Bird deterrents have been used for centuries, in a variety of forms (Avery et al. 1996). Increasing need for new methods have become apparent, for use in airports (Blackwell et al. 2013; Schmidt et al. 2013), farmland (Railsback and Johnson 2011) and other areas where birds are not welcome. Two magnetic tools developed by the Sho-Bond Corporation in Japan are currently being promoted as bird deterrents. The first, “Birdmag”, is made up of globular magnets, 1.5 cm in diameter, threaded along a wire at 25-cm intervals. The wire can be connected beside objects where birds are likely to congregate, nest, or roost. The second, “Birdpeller”, combines four 1.5-cm diameter crescent magnets linked to a propeller at 6-cm interludes. The developer claims that these devices produce magnetic fields which confuse birds, resulting in birds circumventing magnetic field locations. As mentioned (see section 1.3), the Earth’s existing magnetic field is exploited as a directional guide throughout migration or homing by a number of bird species (Moore 1975; Southern 1974, 1978; Wiltschko et al. 1981). It also is recognized that irregularities in the Earth’s magnetic field can cause disorientation in birds (Able 1994). Belant et al. (1997) aimed to clarify the effect of artificial magnets, with field strength of up to 118 Gauss, by inserting them into nest boxes known to be used by European starlings (Sturnus vulgaris). This magnetic field was unsuccessful in discouraging starlings from nesting in the test boxes. However, the effectiveness of synthetic magnetic fields to deter birds has not had adequate research to conclude its effects.
Navigational ability is an essential factor of a vertebrate’s spatial ecology and may impact the invasive potential of a species, whether they relocate naturally or are introduced by man. Pittman et al. (2014) supplies evidence that Burmese pythons (Python molurus bivittatus) have navigational map and compass senses (described in section 126.96.36.199); this is not only a pioneering study for pythons but also for reptiles (see section 3.3.2 – Fig. 4. for reptile study quantity). Similar studies which aim to identify a navigational ability in: fish (Dittman and Quinn 1996), mammals (Holland, Borissov and Siemers 2010), amphibians (Rodda and Phillips 1992) and birds (Wiltschko et al. 2010) add to this growing list of potential species.
It must be noted that not all species are invasive or cause detrimental effects to their new environment or local species. By identifying which species have high invasive potential and subsequently which of these can utilise magnetoreception, a better understanding of the navigational capacity of these animals will become apparent. Identifying the navigational capacity is the first step to creating a systematic understanding of species’ spatial dynamics (Putman et al. 2012) and is essential with regards to range expansion by invasive species (Holway and Suarez 1999). Natural resources that are widely spread or seasonally changing may be acquired by species, due to navigational capacity, which may also decrease risk linked with foraging possible treacherous or foreign areas. Navigational aptitude also influences population mechanics by allowing individuals to survive in large concentrations, including when there as sparse resources (Mueller and Fagan 2008; Barton et al. 2012). Although having possibly considerable effects on movement behaviour and resource use, limited research has been conducted on navigation in invasive species.
Magnetoreception research on certain species has varied uses, the few areas reviewed show potential for future study. It appears negative effects on dairy production and chicken movement and welfare is just the tip of the iceberg. Studies propose SMART hooks, electric and magnetic fields are a theoretically functional tool to: improve discrimination of fishing equipment, induce behavioural responses in sharks and deter birds. Thus, further research is necessary for the progressive development of these uses. Additional improved selection of species for testing in this field of study is not only beneficial for agricultural industry and deterrent methods, but also conservation and ecology management (due to the huge implications for predictions of spatial spread and impacts). Travis et al. (2009) states the addition of density-dependent dispersal to high endurance of dispersers could result in fat-tailed dispersal kernels and advancing invasion borders.
This meta-analytical study is unique in that it does not draw on data within other studies per say, but instead combines other factors (publication years, quantity and species used) for analysis. The aims proposed (section 1.4) in this study have not been acknowledged before, resulting in the lack of comparable data. Some publications in the literature show similar aspects such as Feist (1997) who proposed the quantity of publications affected an individual’s standing in the academic workplace, and Laband (1985) who aimed to correlate quantity and quality of faculty publications, graduate student placement and research success. These employ the idea that a ‘quantity’ search of literature could provide correlations and pin-point gaps in certain fields of research. Although this loosely relates to the present study, it highlights that alternative methods (not commonly used) can be used to address relevant and perplexing problems.
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