Almost as soon as the first organism developed the ability to change its position, answering the question “Where am I?” became a matter of life and death. Over a billion years of evolution, animals have developed multiple redundant systems for navigating among the landscape’s carrots and sticks, following cues of light, polarization, odor, taste, sound, pressure, electrical charge, magnetic charge, and almost any other parameter that changes over space. Despite the universality of animal navigation, some of the most effective long-range systems remain half-veiled mysteries.
Two groups of researchers have recently delved into historical datasets to illuminate the tools used by two prodigies of long-range navigation, Pacific salmon and homing pigeons.
Pacific salmon choose a route
Recent research revealed a candidate magnetoreceptor that is found distributed (albeit thinly) throughout the key sensory tissues of one salmonid fish, the trout—in the olfactory bulb, inner ear, lateral line, and cornea.
Nathan Putman and David Noakes—part of a team from Oregon State University and the Universities of North Carolina, Washington, and California—analyzed a 56-year set of fisheries data tracking the return of the Fraser River sockeye salmon to their home river. The Fraser River flows out of British Columbia into the Strait of Georgia behind Vancouver Island. The homing fish must detour around the 290-mile-long island, entering either from the north, via the Queen Charlotte Strait, or from the south, via the Juan de Fuca Strait. The researchers call the percentage of fish opting for the northern route the “diversion rate.” It varies widely, from 85 percent in some years down to 2 percent in others; most of the time, naturally, the diversion rate falls somewhere in between.
The fisheries scientists tabulated data on which route the salmon took, year-by-year, and correlated it with two environmental variables: transient drift in the Earth’s magnetic field and water temperature (salmon are cold-water creatures and will generally pick the chillier of two evils). The geomagnetic field fluctuates predictably. The researchers used the established Geomagnetic Reference Field model (GRF-11) to calculate the field strength at the mouth of the Fraser River in the year the salmon left the river, and the field strengths at the river mouth and each strait entrance when they returned two years later.
They found that the salmon generally opted for the entrance whose magnetic field strength was closest to that of their home river estuary—though too-warm water (over about 9-10 C) at the southern entrance trumped magnetic preference, and prompted the fish to take the cooler northern route.
Overall, the group found, field drift accounted for about 16 percent of the fishes’ navigational choices, while water temperature determined 22 percent of the variation, and synergy between the two determined 28 percent of the variation.
Homing pigeons lose their way
The analysis of homing pigeons used data gathered by Cornell researcher from 1968 to 1987 to try to figure out how they navigate by analyzing situations in which their homing sense failed. Despite their reputation for infallibility, homing pigeons sometimes have trouble figuring out which way to fly, and occasionally lose their way altogether. Pigeons from some lofts will become confused more often when released at specific locations—but these zones of befuddlement are different for pigeons based at different locations. Apparently, there is some factor that ties the two locations together—but ties them loosely enough so that the link sometimes breaks.
Cornell biologist William Keeton (1933-1980) studied pigeon homing behavior, and was especially interested in how they acted at three sites where their homing instincts seemed to become unhinged on some days, causing them to yaw consistently off course or even fly off in all directions. (This occurred even though pigeons brought into these same locations from other lofts could find their way without difficulty.) His data was extended after his death and made available to researchers.
U.S. Geological Survey geophysicist Jonathan Hagstrum reviewed the records from the problem release sites. While others focus on the effects of sunlight, terrain (though even birds fitted with frosted lenses can find their way), magnetism, and even odor, Hagstrum focused on sound. In particular, he analyzed the possible contributions of very low-frequency sound waves, 1 hertz to 0.1 Hz. These infrasounds have wavelengths of 340 meters to 3.4 kilometers and have (as whales and elephants are known to exploit) extremely long ranges. For example, while a 1000 Hz tone loses 90 percent of its energy at 7 km, a 1 Hz sound carries for 3000 km before attenuating to the same degree. A 0.1 Hz sound can circumnavigate the earth before losing 90 percent of its energy.. We are constantly bathed in waves like these. Among other sources, frequent tiny earthquakes (microseisms), produce a continuous 0.1 Hz to 0.5 Hz rumble.
These very-low-frequency waves can behave capriciously, though: wind, humidity, and (particularly) temperature gradients can refract them so that they leapfrog over some locations. Using detailed meteorological records and models from the Hamiltonian Acoustic Ray-tracing Program for the Atmosphere (HARPA), Hagstrum calculated the propagation of possible infrasounds from the pigeons’ home loft at Cornell to the problem release sites (Jersey Hill, 120 km west; Castor Hill, 143 km north northeast; and Weedsport, 74 km north).
The results in brief: On the days when the pigeons were disoriented, the problem sites were deep in acoustic shadow. Infrasounds radiating from their home loft jumped inaudibly over the pigeons’ release hilltops. And on the days when they did uncharacteristically head immediately off in the right direction, the atmospheric conditions dropped the home-loft rumble onto their release points.
Images: Oregon State University