"Mystery of Mysteries" in a Tiny Fish
Animal migration is an active area of research. The editors of Current Biology lay out the requirements:
'True' navigation is the ability of an animal to travel to a relatively precise target at a considerable distance without the need for familiar landmarks. To do this, the navigator must normally have a 'map' to show where it is relative to its goal. Having inferred the direction to the target from this map, the organism then needs a compass to steer itself along the appropriate vector. (Emphasis added.)
The hatchlings of Pacific salmon spend a couple of years in their freshwater streams, many miles above cascades and obstacles that will later challenge their journey to the ocean. The "small fry" with their yolk sacs grow to where they can feed on their own. As they get their fins and grow to "fingerling" size (so called because they are about the length of a human finger), their memories become imprinted with the specific mix of proteins and other chemicals in the water unique to their birthplace. Growing into juvenile "parr," then adult "smolt," they prepare for the adventure of a lifetime: traveling out to the ocean. Years later, having grown into large fish, they will once again head home. Finding their native stream, they swim against the current sometimes for miles, leaping over powerful cascades and dodging hungry bears. Exhausted as they reach the same spot where they were born, they spawn, then die.
The migration of salmon is well known in general, but new particulars continue to come to light. One of the big questions is how the fish navigates. How does a young salmon find the feeding grounds, never having been there before? It can't rely on circulating ocean currents that make passive transport unreliable. Then, how does it re-discover its native stream again after years in the open sea? Thousands of miles from its starting point, out of reach of the aroma of home, unable to see very far through murky water, how does it find its bearing? Its parents are long gone. It can't see the stars. Ocean currents don't head that way. The situation would seem hopeless to a human underwater in a submarine with no instruments, yet the salmon gets home on schedule.
New work by a team working at the Oregon Hatchery Research Center sheds more light on this phenomenon. Published in Current Biology, the paper by Oregon State's Nathan Putman et al. describes how salmon hatch with a map already prepared, inherited from their parents.
Here we experimentally demonstrate that juvenile Chinook salmon (Oncorhynchus tshawytscha) respond to magnetic fields like those at the latitudinal extremes of their ocean range by orienting in directions that would, in each case, lead toward their marine feeding grounds. We further show that fish use the combination of magnetic intensity and inclination angle to assess their geographic location. The "magnetic map" of salmon appears to be inherited, as the fish had no prior migratory experience. These results, paired with findings in sea turtles, imply that magnetic maps are phylogenetically widespread and likely explain the extraordinary navigational abilities evident in many long-distance underwater migrants.
Equipped with a compass that measures angle and intensity, the fish follows the map home. This map must have incredibly high resolution! It fits within the tiny brain inside the tiny head of a salmon fry emerging from its little egg, but won't be used to find home until as a large, mature fish it needs it years later.
A tiny fish brain must be able to read its compass in one direction going to the feeding grounds, and in the opposite direction returning home. But having a compass alone is not enough; a navigator needs a map. Since magnetic field inclination and intensity are not parallel, they provide a "bicoordinate grid," the researchers explain. A grid doesn't need to be orthogonal (arranged at right angles) to be useful. Measurements of specific inclination and intensity at given locations provides waypoints for marking the way out and the way back. In effect, the waypoint says, "You are here; orient in this direction." In the case of sea turtles and salmon, the waypoint map appears to be inherited. The editors call it "an innate look-up table which requires no prior experience or calibration."
To test the hypothesis, the researchers experimented with artificial magnetic fields on Oregon salmon in a hatchery ("And the field was not even strong enough to deflect a compass needle," Putman said). As predicted by the inherited-map hypothesis, young salmon in the "parr" stage, less than a year old -- with no prior navigation experience -- preferentially oriented themselves according to magnetic field inclinations and intensities corresponding to waypoints on their future migration route. Without the applied field, the orientations were random. For another test, the scientists mixed up the intensities and inclinations from different waypoints. Again, the orientations of the salmon were random. The data support the theory that the fish inherit a map with the pre-calibrated waypoints that they can follow out to sea and back. (To clarify, there are not specific waypoints, but continuous variations along the route's curve, like the "instantaneous slope" we learn about in calculus, providing usable readings at any point along a gradient.)
Amazing as this is, it's just a rough cut. There's even more involved in navigation:
This does not imply, however, that environmental factors are unimportant; it is possible, for example, that the responses we observed are influenced by early experience with the local magnetic field. Even so, without the opportunity to learn gradients in magnetic intensity and inclination angle, the response to the change in magnetic field parameters must be inherited. It is particularly noteworthy that the fish tested were parr, the stream-dwelling juvenile stage. Thus, it appears that the fish possess orientation responses necessary for successful ocean navigation prior to even migrating toward the sea. Given that these salmon make their oceanic migration only once, to locations where they have never been, and without the benefit of following experienced migrants, a navigation system based on inherited instructions is likely to be highly adaptive and possibly necessary. Furthermore, the magnetic orientation instructions that juveniles inherit to magnetic fields corresponding to broad oceanic regions (Figure 3) may serve as the building blocks for the subsequent and more sophisticated navigation that is hypothesized to allow these animals to return with precision to their natal site for reproduction.
In other words, the salmon can override the automatic pilot depending on circumstances. The system is flexible enough to allow sub-populations of salmon to segregate into separate feeding grounds. "Thus, differential orientation to regional magnetic fields is a possible mechanism by which stocks segregate into broad oceanic areas." The magnetic map probably takes the salmon to within a few kilometers of its native stream, where olfactory cues can begin providing the necessary provision to find home. And that's not all; Putman added, "They likely have a whole suite of navigational aids that help them get where they are going, perhaps including orientation to the sun, sense of smell and others."
What About Other Migratory Animals?
It's interesting to compare salmon to other, unrelated migratory animals. Consider the cross-continent migration of Monarch butterflies, shown in Metamorphosis: The Beauty and Design of Butterflies. We marvel at the cross-planet migration of the arctic terns described in Flight: The Genius of Birds. By some measures, though, what salmon achieve is even more impressive:
The challenges of long-distance oceanic navigation are considerable, especially when compared to terrestrial navigation. For instance, migratory songbirds making their first migration to a distant, unknown site inherit a simple program in which they maintain a fixed compass course for a set duration of time that leads them, approximately, to their goal. Although birds can be deflected by winds while migrating, they often mitigate drift by maintaining visual contact with the ground and by landing when conditions are adverse. Oceanic migrants, however, are continuously susceptible to the influence of currents. They lack stationary visual references against which current drift can be gauged and cannot "land" when conditions are unfavorable. Thus, the clock-and-compass mechanism used by many birds during their first migration is unlikely to be viable for migrants in the open ocean and indeed is inadequate to explain the distribution of juvenile Chinook salmon during the early stages of their ocean migration.
The researchers suspect that the migration system used by salmon and sea turtles will be found in many other marine animals, such as eels, tuna, sharks, penguins, seals and whales. The editors add newts and spiny lobsters to the list. That encompasses taxa as diverse as mammals, birds, bony fish, cartilaginous fish, amphibians and crustaceans -- animals that could not possibly have gained their navigational abilities by common descent.
We suggest that the use of a large-scale magnetic map might support the successful life history of many marine migrants, allowing them to make efficient use of the spatiotemporal variability in ocean productivity and facilitating ontogenetic shifts in habitat utilization to exploit the environments that are best suited for different life stages. Given that such navigational systems have now been reported in two phylogenetically distant taxa (sea turtles and salmon), it appears likely that similar navigational systems also exist in other marine species with similar life-history patterns.
Applying "vera causa" reasoning, we might note that every time (in our uniform experience) we have observed the development of mapmaking and navigational systems, we learned that intelligence was the cause. Since most of the scientific community rules out such reasoning, however, they are left with the blind, unguided cause of Darwinian selection to account for this "efficient...navigational system" in salmon. That requires a heavy dose of presumption:
Presumably the same strong selective pressure that has led to the convergent evolution of an inherited magnetic map in salmon and sea turtles permits its maintenance in the face of geomagnetic drift (Figure 3). Moreover, positional information inherent in Earth's magnetic field appears likely to provide an important source of navigational information for diverse animals that migrate, home, or wander over a wide range of spatial scales. Further investigations into the behavioral, ecological, and evolutionary implications of this ability are likely to be fruitful.
Information alone, though, is useless without a system to utilize it. It appears more likely that the "evolutionary implications" of further investigations into animal navigation will be fruitless -- or, like a hollow rind that looks colorful on the outside but empty inside.
The researchers say little about evolution except in very broad terms. They certainly did not employ it in their experiments. The editors of Current Biology also attributed all the amazing capabilities of animal migration to Darwinian selection -- even though they had to admit the notion seems fantastic:
That animals might utilize magnetic cues, to which we are entirely blind, measure gradients to a better accuracy than portable human technology could (at least until recently), and employ a non-orthogonal set of coordinates to place themselves accurately even hundreds of kilometers away, seems fantastic.... But, as usual, a shortfall in human imagination does not seem to have limited the potential of natural selection to fashion solutions to life-or-death challenges.
That one word -- "imagination" -- tells you all you need to know about Darwinian theorizing. Science should restrict its explanatory toolkit to realistic causes that are adequate to explain the effects. In the case of animal migration, the one cause we know to be adequate is intelligent design. It's long past time to allow design thinking (rather than "impossible leaps of imagination") to tackle the questions that the editors find mysterious:
The map sense remains animal behaviour's mystery of mysteries. No other set of questions takes us so far from human experience and analogy. The phenomena continue to require almost impossible leaps of imagination to formulate hypotheses, much less to devise practicable controlled tests. Even when successful, we are generally treated to isolated episodes in the navigational life of one kind of animal or another. The study by Putman et al., brings us much closer to an integrated picture from birth to death of a single species -- its receptor systems, juvenile migration, and adult homing. It also reminds us that the best questions are now ready to be attacked.