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A while back...let me amend that, a couple of years ago I was caught up in an interesting debate about how migrating animals – in this instance birds and eels (how it was narrowed down to just those I don’t recall) – managed to navigate sometimes long distances to mating grounds. The individual I was discussing this with insisted that some invisible, external intelligence guided these creatures each season. I, on the other hand, disagreed and I figured I’d share some of the information from that discussion here. Is this blog filler? If you think so…you’d probably be correct.

The research done by Heyers et al. (2007) shows that, in migratory birds, the proposed pathway in question is the thalamofugal pathway – composed of retinal ganglia expressing cryptochrome and an area in the forebrain called Cluster N. Crytpochrome just denote the receptors which are specialized to detect blue which also play a role in the circadian cycles of some animals. Cluster N is a collection of forebrain areas in migratory birds which play a role in night-vision and as Heyers et al. propose, their internal “compass”. In Anguilla Anguilla, among the physiological changes which take place before migration is the shift in their retina pigments from green-sensitive to blue-sensitive (Wood, P. and Partridge, 1993).

The authors also note that within cluster N, there is “high, sensory driven neuronal activity as indicated by the expression of the Immediate Early Gene ZENK during magnetic orientation”. This is supported previously by independent evidence is several migratory bird species (Mouritsen, Feenders, Liedvogel, Wada, & Jarvis, 2005; Liedvogel, Feenders, Wada, Troje, Jarvis & Mouritsen, 2007).

ZENK expression was utilized as a measure of neuronal activity. The specific genes in question would be the cryptochrome CRY genes which are also involved in circadian cycles as well as the regulation of PRL (prolactin) which is involved in avian reproductive cycles (Yasuo, Watanabe, Tsukada, Takagi, Iigo, Shimada et al., 2004) as well as reproductive cycles in coral (Levy, Appelbaum, Leggat, Gothlif, Hayward, Miller et al., 2007) and involved in time-place learning in mice (Van der Zee, Havekes, Barf, Hut, Nijholt, Jacobs et al., 2008).

Electromagnetic orientation is not restricted to birds, a study of eels showed that there is a definite, seasonal dependent change in orientation in accordance with Earth’s magnetic field (van Ginneken, Muusze, Breteler, Jansma, van den Thillart, 2005; Westerberg & Lagenfelt, 2008). Specifically, Westerberg & Lagenfelt showed that underground electrical power cables (which generate their own EMFs) disrupted the travel of the eels.

In birds, migration has been shown to be a genetically controlled process. For instance, such behavior can be produced or changed to sedentary behavior within several generational breedings by intermixing migratory and sedentary birds (Berthold, 1999). Also Moller (2001) notes that arrive time is dependent upon genetics and such can be linked directly to reproduction, stating:

"…competition for early arrival among males may lead to condition-dependent migration associated with fitness benefits of early arrival achieved by individuals in prime condition."

As for migration in marine species, it does seem to be evolutionarily advantageous, as Roff (1988) notes:

"Migration both influences the evolution of other traits and is contigent upon the evolution of other behavioural and demographic characters. The interaction between such factors is illustrated by considering the relationship between the cost of migration in relation to fecundity and the advantages and disadvantages of schooling, a phenomenon hypothesized to favour the evolution of migration."

The development of migration itself may seem like it may take a heavy toll, however, this is not the case as Alerstam, Hedenstrom and Akesson (2003) show. They also indicate that migration should not be seen as an isolated behavior or mechanism but migration is “an extension of general seasonal adaptations in movement, homing, metabolism etc”. It is also noted that migratory behavior is not a conserved behavior either and is linked to resource exploitation, breeding, disparity between survival and breeding grounds, and so forth. These similarities are seen across taxa with variation (which the article includes eels in their consideration of migration). Alerstam et al. also note that the eel migration is aided by currents although the energetic cost of the travel is fairly low. This was previously shown by Castonguay and McCleave (1987) which showed that Anguilla anguilla stay in the Gulf Stream on their travel to Europe.

The misconception that the eels travel from the same exact spot from their spawning grounds to some exact spot close to Europe and back to the same exact spot is elementary and inaccurate. Here is the variation in their journey:

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The long-distance trek of Anguilla anguilla is quite amazing and it may not be know exactly why their spawning grounds are so far away – however, as research has shown it may have built up distance over a period of time and is directly related to their reproduction. Takaomi, Limbong, Otake & Tsukamoto (2001) conclude that this is this may indeed be the case, stating:

“…ancestral eels most probably underwent diadromous migration from local short-distance movements in complex currents in tropical coastal waters to the long-distant migrations characteristic of present-day temperate eels, which has been well-established as occurring in subtropical gyres in both hemispheres.”

Which is later expounded upon in another article the following year by Tsukamoto, Aoyama & Miller (2002) stating:

“…the large-scale migration of temperate eels probably evolved from local migrations of tropical eels as a result of long-distance dispersal of leptocephali from spawning sites in tropical waters of low latitude to temperate growth habitats at higher latitudes.”

Specifically, Tsukamoto & Aoyama (1998) conclude that the tropical origins of the eels were somewhere around the Western Pacific, close to modern-day Indonesia and their clade originating around 10 million years ago.

Now, the problem with wanting to show mutation is responsible for a particular behavior is (as you should know) very difficult since behaviors are not governed by the idea of one gene = one trait/state. Genetic roles can be shown and have been in the migration of animals including the eels along with environmental cues to imprinting (Westin, 2003). It is known, however, that no mutation is a requirement for the Hardy-Weinberg equilibrium and therefore makes it an integral part of the evolutionary process. Mutations lead to genetic variants --> natural selection acts upon these variants --> the selected traits grow throughout the population and may become indicative of that organism. It also must be kept in mind that most mutations are effectively neutral to the organism – that is they do not confer any real selective advantage or disadvantage.

There is definitive evidence for a major role for genetics in migratory behavior, along with environmental cues (electromagnetic fields in this case) and learning (imprinting – which can be shown to be disrupted as in the article from Westin).

As for “instinct” this too is the product of variation and selection. Did anyone teach you how to suck on a nipple when you were a baby? No, that fixed action pattern already existed and has even been observed in vivo prenatally – obviously a beneficial trait to have. Another example is the innate drive to procreate or at least engage in the activity thereof. However, these instincts can be modified by experiential learning (conditioning) or found in variation in which a particular genotype may not exhibit the usual innate behavior – in such an instance without some intervention this would be bad for that particular individual.

There is ample evidence for a completely naturalistic explanation for migration behavior without the appeal to some invisible, intelligence, guiding force for which there is no empirical support.

References:

Alerstam, T., Hedenstrom, A. & Akesson, S. (2003). Long-distance migration: evolution and determinants. Oikos, 103, 247-260.

Berthold, P. (1999). A comprehensive theory for the evolution, control and adaptability of avian migration. Ostrich, 70, 1-11.

Castonguay, M.& McCleave, J. (1987). Vertical distributions, diel and ontogenetic vertical migrations and net avoidance of leptocephali of Anguilla and other common species in the Sargasso Sea. Journal of Plankton Research 9, 195-214.

Heyers, D., Manns, M., Luksch, H., Gunturkun, O., & Mouritsen, H. (2007). A Visual Pathway Links Brain Structures Active during Magnetic Compass Orientation in Migratory Birds. PLoS, 2(9), e937.

Levy, O., Appelbaum, L., Leggat, W., Gothlif, Y., Hayward, D., Miller, D. et al. (2007). Light-Responsive Cryptochromes from a Simple Multicellular Animal, the Coral Acropora millepora. Science, 318, 467-470.

Liedvogel, M., Feenders, G., Wada, K., Troje, N., Jarvis, E. & Mouritsen, H. (2007). Lateralized activation of cluster N in the brains of migratory songbirds. European Journal of Neuroscience, 25, 1166-1173.

Moller, A. (2001). Heritability of arrival date in a migratory bird. Proceedings of the Royal Society B, 268, 203-206.

Mouritsen, H., Feenders, G., Liedvogel, M., Wada, K., & Jarvis, E. (2005) Night-vision brain area in migratory songbirds. Proceedings of the National Academy of Sciences, 102, 8339–8344.

Roff, D. (1988). The evolution of migration and some life history parameters in marine fishes. Environmental Biology of Fishes, 22, 133-146.

Takaomi, A., Limbong, D., Otake, T. & Tsukamoto, K. (2001). Recruitment mechanisms of tropical eels Anguilla spp. and implications for the evolution of oceanic migration in the genus Anguilla. Marine Ecology Progress Series, 216, 253-264.

Tsukamoto, K. & Aoyama, J. (1998). Evolution of freshwater eels of the genus Anguilla: a probable scenario. Environmental Biology of Fishes, 52, 139-148.

Tsukamoto, K., Aoyama, J. & Miller, M. (2002). Migration, speciation, and the evolution of diadromy in anguillid eels. Canadian Journal of Fisheries and Aquatic Sciences, 59, 1989-1998.

Van der Zee, E., Havekes, R., Barf, R., Hut, R., Nijholt, I., Jacobs, E. et al. (2008). Circadian time-place learning in mice depends on Cry genes. Current Biology, 18, 844-848.

van Ginneken, V., Muusze, B., Breteler, J., Jansma, D., & van den Thillart, G. (2005). Microelectronic detection of activity level and magnetic orientation of yellow European eel, Anguilla Anguilla L., in a pond. Environmental Biology of Fishes, 72, 313-320.

Westerberg, H & Lagenfelt, I. (2008). Sub-sea power cables and the migration behaviour of the European eel. Fisheries Management & Ecology, 15, 369-375.

Westin, L. (2003). Migration failure in stocked eels Anguilla Anguilla. Marine Ecology Progress Series, 254, 307-311.

Wood, P. and Partridge, J. C. (1993) Opsin substitution induced in retinal rods of the eel (Anguilla anguilla (L.)): a model for G-protein-linked receptors. Proceedings of the Royal Society B. 254, 227-232.

Yasuo, S., Watanabe, M., Tsukada, A., Takagi, T., Iigo, M., Shimada, K. et al. (2004). Photoinducible Phase-Specific Light Induction of Cry1 Gene in the Pars Tuberalis of Japanese Quail. Endocrinology, 145, 1612-1616.