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LENScience

What is Magnetic Sense?

This page supports specialist and non-specialist teachers by providing background information about the concepts that underpin the LENScience resources on animal navigation.

  1. » Migration
  2. » Homing
  3. » The Earth's Magnetic Field

Migration

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The remarkable ability to navigate over long distances, associated with either migratory or homing behaviour is common to many animals. Examples are seen in a wide range of taxa including insects, birds, fish, crustacea and mammals. These innate behaviours make use of a range of sensory receptors (Box 1), allowing animals to respond to environmental stimuli and navigate across unfamiliar territories.

 

Migration is defined as the regular and intentional mass movement of animals from a breeding area to another area where they do not breed. Migratory movements are regulated by internal clocks in response to environmental cues. The kūaka or bar-tailed godwit migrates 11,000 km annually from breeding grounds in the tundra of Alaska to the rich feeding grounds of the New Zealand tidal mud flats. As with many migratory journeys, adults leave the tundra in advance of the juveniles, providing clear evidence that this behaviour is innate and not learned.

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Homing

Homing is the ability of an animal to return to its nesting site after travelling beyond this site, usually to find food. While this is often across territories which are familiar, this is not always the case. The toroa or albatross exhibit homing behaviours, flying across vast areas of the southern oceans to feed and returning to their breeding grounds in New Zealand and the Sub‐Antarctic Islands. Bees similarly can return from distances of up to 10 kilometres from their hives. 

 

Understanding the mechanisms by which animals navigate across unknown territories has been the focus of interest and challenge for scientists for many years. In both homing and migration the animal must navigate from starting points beyond the reach of sensory information relating to their goal location. This means that the animals must be able to sense location, to determine their starting point, and direction, to determine the path they will take in order to reach their goal location. This ability is clearly demonstrated in the domesticated homing pigeon, columba livia. In addition to being able to return to their nest site after feeding, homing pigeons are also able to return to their nest from distant and unfamiliar sites. Homing pigeons transported to unfamiliar starting sites, (meaning they have not navigated the outward journey), when released, still find their way back to their nest site, even if transported to the release site under general anaesthesia. This means that, as well as having some form of compass to determine direction, the homing pigeon must also be able to determine where it is starting from, demonstrating that the homing pigeon has a sense of location.

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The Earth's Magnetic Field

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The Earth is composed of four layers (Fig 4). The solid inner core is surrounded by a molten outer core of iron. This, in turn, is surrounded by a semi‐liquid mantle on which the solid crust floats. Convection currents within the molten core create a strong magnetic field which can be represented as a bar magnet (Fig 5), but tilted 11 degrees from the spin axis of the Earth. This tilt creates the difference between geographic north and magnetic north. Notice that in Fig 5 the south pole of the magnet representing the Earth’s magnetic field lines up with the Earth’s magnetic north pole. This is because unlike magnetic poles attract. The field lines in Fig 5 represent the intensity and inclination of the magnetic field which increase systematically between the magnetic equator and the magnetic poles. If you compare these to the pattern created by the iron filings around the bar magnet (Fig 6), you will notice the similarity. The arrows indicate the direction of the magnetic force while the distance between the lines represents the strength of the field. The closer the lines are together, the stronger the magnetic field so that the Earth’s magnetic field is strongest at the poles. When you use a compass to determine your direction, the compass needle is actually a magnet which will line up with magnetic north and determine where you are facing in comparison to magnetic north. In the same way, if an animal has a magnetic receptor, it will be able to determine direction.

 

In addition to the strong magnetic field created by the Earth’s core, localised magnetic fields are created from magnetised rocks in the crust. These localised fields are known as anomalies and although much weaker than the main magnetic field of the Earth, create observable variations (Walker et al 2002). Fig 7 shows the effect of these localised fields in the Auckland area. The red diagonal lines represent contours of equal intensity of the magnetic field produced in the Earth’s core. The black contour lines show the variations in the magnetic field caused by magnetic anomalies. This map shows you the magnetic topography of the area that is added to the systematic variation in magnetic intensity that is produced in the Earth’s core. The closer the lines are together, the steeper the gradient in the magnetic topography. Notice the effect of the volcanic island of Rangitoto on these contours. The high iron content in this volcanic rock is influencing the magnetic field, creating a local anomaly.

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What Makes an Effective Environmental Cue for Navigation? 

Animals have been shown to use a variety of environmental cues in navigation. These range from visual stimuli (such as the relative position of sun, stars, moon and location of landmarks) to detection of low frequency sounds (such as those created by waves breaking) and detection of magnetic fields. For an environmental cue to be of use to an animal in navigation, the cue must be consistent, vary systematically in space to provide information about specific points on the Earth’s surface, be stable over time and provide enough accuracy to allow the animal to reach its specific goal destination. Although there is some variation in the Earth’s magnetic field, there is adequate consistency to make this a viable environmental cue for navigation. Importantly, unlike other environmental cues such as the position of sun and stars which are inaccessible when there is cloud cover, the Earth’s magnetic field will give consistent information that can be used to determine position and direction at all times and in all environments. 

 

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How Can Animals Detect Magnetic Fields? 

Organisms from bacteria through to higher vertebrates demonstrate an ability to sense the Earth’s magnetic field. The fact that such a wide range of organisms are magneto‐receptive suggests that the ability to sense the Earth’s magnetic field evolved prior to the radiation of the animal phyla and shares a common origin in early prokaryotic organisms. Whether enabling taxic orientation responses in bacteria or navigation in higher vertebrates, the ability to sense the Earth’s magnetic field, as with other sensory abilities, will have provided a selective advantage for these organisms. Over the course of time, magneto‐reception in animals will have evolved to become more sensitive, eventually specialising to monitor both the direction of the magnetic field and variations in the intensity of the magnetic field (Kirschvink et al., 2001).

 

Experimental evidence has shown that the receptor that allows bacteria and animals to sense the Earth’s magnetic field uses ferromagnetic materials such as magnetite (Fe3O4). In bacteria and eukaryotic algae that are magneto‐taxic, chains of magnetite or greigite (Fe3S4) produce a magnetic moment (a moment is a measure of the strength and direction of magnetism produced by a system) large enough to rotate the cells so that they line up with the Earth’s magnetic field (Schuler et al., 1999). If the bacteria or algae naturally seek the north pole of the magnetic field, this can be reversed by exposing them to pulse‐remagnetisation experiments (Kalmijn, 1978), in the same way that you can reverse the poles on a magnet. This ability to reverse magnetic direction is only found in ferromagnetic materials. Similar experiments on bees and birds have demonstrated the same pole reversal effect, suggesting that they too contain some kind of ferromagnetic sensory receptor (Kirschvink et al., 2001). 

 

To learn more about magnetic sense, see the case study associated with this topic.

 

All images © Michael Walker unless otherwise acknowledged.

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