One of the most amazing behaviors of animals that we can observe is the ability of migratory birds to travel thousands, or sometimes tens of thousands of kilometers, and then return to the point of origin . Their precision in navigation is so high that some species return in the spring to exactly the same nest they abandoned the previous fall, after a journey of months.
Obviously, they locate the nest thanks to the sense of sight; they memorize the aerial view when they leave, and thus recognize it when they return. However, locating such a specific place requires extraordinary navigation skills . A stork can locate its nest in the Sierra de Guadarrama in a few days if it is within a radius of several kilometers at the end of its migration, but it could not find it at all if its journey ends in the Alps.
Registering the address
Many animals, from insects to fish, birds and mammals, have a type of cell in their brains called head-direction cells . These cells are the neural backbone of the sense of direction.
However, the direction cells do not work like a compass , but rather an accelerometer; they do not align according to the magnetic field, but depend on signals of proper movement. When the animal turns its head, these cells tell it which way it is turning and how much. When it returns to the original position, the animal is aware of it.
This sense of direction without magnetic capacity could be behind the navigation capacity of many animals. For example, if an animal with these abilities memorizes where the sun has risen, or in which direction a certain reference point is, it can stay oriented, regardless of the turns it takes.
For a long time it was thought that the orientation capacity of birds was based on observational navigation, supported by this ability to detect changes in direction. In this way, if during the trip, an unexpected wind changed the flight path, the animal would detect it and could correct its course without the need for an internal magnetic compass.
Direction cells prefer north
There is, however, abundant evidence of animals being able to orient themselves, not relative to a memorized landmark, but relative to the Earth’s magnetic field. Like a real compass .
Cells that are sensitive to the Earth’s magnetic field have been found in certain groups of animals capable of orientation—including many migratory birds. This magnetoception , in birds, is intimately linked with the direction cells of the head, in an area of the brain called the vestibular brainstem. That is, the perception of the magnetic field is integrated with the perception of the direction. Your brain works like a compass.
However, it is convenient to remember that these are two different senses, and that animals that have both can take advantage of one or the other as they see fit. For example, it has been observed that during the migration of the Gray- haired Shearwater ( Calonectris leucomelas ), the younger and inexperienced specimens rely more on their magnetic sense, even if this means crossing dangerous mountainous areas; while the more experienced adults take advantage of their ability to detect changes in direction to avoid mountain ranges and take advantage of more efficient and less risky detours. Although this supposes them to deviate from the original course, they are able after correcting their direction to return to their original course.
Other animals have even more complex systems. Some birds, and also sea turtles, are able not only to orient themselves in a north-south direction, but also to establish mental maps that stay aligned with the magnetic field . This allows them to establish, in familiar terrain, new routes that improve the efficiency of the trip, and in this way, invest less energy in migration.
And in addition, other animals, including many ruminants, rodents or bats, are also capable of orienting themselves on a north-south axis, and, to this day, we do not know exactly how they do it. Although some hypotheses point to the existence of pigments called cryptochromes , which would align according to the magnetic field under certain conditions, much remains to be investigated. There are still many experimental studies to be done on orientation mechanisms in animals.
References:
Burda, H. et al. 1990. Magnetic compass orientation in the subterranean rodentCryptomys hottentotus (Bathyergidae). Experientia, 46(5), 528-530. DOI: 10.1007/BF01954256
Kishkinev, D. et al. 2021. Navigation by extrapolation of geomagnetic cues in a migratory songbird. Current Biology, 31(7), 1563-1569.e4. DOI: 10.1016/j.cub.2021.01.051
Pakhomov, A. et al. 2018. Magnetic map navigation in a migratory songbird requires trigeminal input. Scientific Reports, 8(1), 11975. DOI:Â 10.1038/s41598-018-30477-8
Takahashi, S. et al. 2022. Head direction cells in a migratory bird prefer north. Science Advances, 8(5), eabl6848. DOI: 10.1126/sciadv.abl6848
Wu, L.-Q. et al. 2012. Neural Correlates of a Magnetic Sense. Science, 336(6084), 1054-1057. DOI: 10.1126/science.1216567
