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First published online January 8, 2007
Journal of Experimental Biology 210, 181-186 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02629
Commentary |
Going wild: what a global small-animal tracking system could do for experimental biologists
1 Department of Ecology and Evolutionary Biology, Princeton University,
Princeton, NJ 08544, USA
2 Mammal Lab, New York State Museum, CEC 3140, Albany, NY 12230,
USA
3 Department of Mechanical and Aerospace Engineering, Princeton University,
Princeton, NJ 08544, USA
4 Copenhagen Bird Ringing Centre, Zoological Museum, University of
Copenhagen, DK-2100 Denmark
5 Goddard Space Flight Center, NASA, Greenbelt, MD 20771, USA
6 Department of Electrical and Computer Engineering and Department of
Astronomy, University of Illinois at Urbana-Champaign, Urbana, IL 61801,
USA
* Author for correspondence (e-mail: wikelski{at}princeton.edu)
Accepted 26 October 2006
 |
Summary |
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300 g) currently can be followed globally
because of power and size constraints on the tracking devices. And yet the
vast majority of animals is small. Tracking small animals is important because
they are often part of evolutionary and ecological experiments, they provide
important ecosystem services and they are of conservation concern or pose harm
to human health. Here, we propose a small-animal satellite tracking system
that would enable the global monitoring of animals down to the size of the
smallest birds, mammals (bats), marine life and eventually large insects. To
create the scientific framework necessary for such a global project, we formed
the ICARUS initiative
(www.IcarusInitiative.org),
the International Cooperation for Animal Research Using Space. ICARUS also
highlights how small-animal tracking could address some of the `Grand
Challenges in Environmental Sciences' identified by the US National Academy of
Sciences, such as the spread of infectious diseases or the relationship
between biological diversity and ecosystem functioning. Small-animal tracking
would allow the quantitative assessment of dispersal and migration in natural
populations and thus help solve enigmas regarding population dynamics,
extinctions and invasions. Experimental biologists may find a global
small-animal tracking system helpful in testing, validating and expanding
laboratory-derived discoveries in wild, natural populations. We suggest that
the relatively modest investment into a global small-animal tracking system
will pay off by providing unprecedented insights into both basic and applied
nature. Tracking small animals over large spatial and temporal scales could prove to be one of the most powerful techniques of the early 21st century, offering potential solutions to a wide range of biological and societal questions that date back two millennia to the Greek philosopher Aristotle's enigma about songbird migration. Several of the more recent Grand Challenges in Environmental Sciences, such as the regulation and functional consequences of biological diversity or the surveillance of the population ecology of zoonotic hosts, pathogens or vectors, could also be addressed by a global small-animal tracking system.
Our discussion is intended to contribute to an emerging groundswell of scientific support to make such a new technological system happen.
Key words: small animal, ICARUS initiative, migration pattern, migratory bird orientation, satellite, field experiments, tracking technology, telemetry, songbird, bat, insect
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Applications of a global small animal tracking system |
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TOPSummaryApplications of a global...The power of combined...A global challenge for...Beyond Aristotle's migration...Why current technology is...A global solution to...ICARUS can make a...References |
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;
Dallimer and Jones, 2002).
Queleas are the world's most abundant birds and have a breeding population in
excess of 1.5 billion. Single colonies can contain up to 30 million birds and
these large individual colonies can destroy up to 5% of grain crops in the
Sahel zone of Africa (Bruggers and Elliott,
1989). Queleas can migrate long distances, sometimes more than
2000 km, to seek appropriate breeding areas or to avoid food shortage
(Ward, 1971). Beyond the
obvious applied benefit of planning for human health emergencies such as
widespread famines in the present and projected paths of these `feathered
locusts' (Manikowski, 1988;
Malthus, 1995;
Mullie et al., 1999),
researchers could also test whether our mechanistic, lab-derived understanding
of migratory drivers truly reflects migratory decisions of individuals that
join the swarming herds in fields and deserts
(Marshall and Disney, 1956;
Desdisney et al., 1959;
Wolfson and Winchester, 1959;
Jones and Ward, 1976).
Although it remains unclear whether a mechanistic knowledge of quelea movement
would enable agricultural managers to prevent major devastations from the
outset (Manikowski, 1988),
tracking individuals might at least allow for a predictive forecast of
pattern, similar to a tornado or hurricane warning system
(Malthus, 1995). |
The power of combined laboratory and field experiments |
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TOPSummaryApplications of a global...The power of combined...A global challenge for...Beyond Aristotle's migration...Why current technology is...A global solution to...ICARUS can make a...References |
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). Invaluable
progress will be made in integrative and experimental biology once we are able
to track the whereabouts of small animals across the globe
(Lawton and May, 1983;
National Academy of Sciences,
2001). For example, lab-based findings demonstrate that navigation
in laboratory conditions is altered or impaired by hippocampal lesions
(Sherry and Vaccarino, 1989;
Strasser and Bingman, 1997)
and that trigeminal nerve section prevents pigeons from sensing the magnetic
field. However, studies on free-flying birds show that in both cases pigeons
are nevertheless able to home (Gagliardo
et al., 1999; Gagliardo et
al., 2006). Furthermore, whatever the magnetic manipulation
imposed on pigeons, only vanishing bearings are affected; the pigeons are
always able to home (Wallraff,
1999). In the field of migratory bird orientation, Perdeck's
classic field experiment testing songbird orientation mechanisms in nature
(Perdeck, 1958) is still cited
as providing unparalleled insight into changes in orientation mechanisms
between young and adult birds. Furthermore, Muheim et al.'s review
(Muheim et al., 2006) suggests
that a field-based test of cue-conflicts during songbird orientation
(Cochran et al., 2004)
provided data that helped resolve long-standing disputes over results of
lab-based orientation studies. Given this clear importance of tracking small
migrating animals, it is remarkable that even after a century of songbird
migration research, one 6-day track of a single Swainson's thrush
(Catharus ustulatus) conducted 32 years ago (in 1973) by Bill Cochran
(Cochran, 1987) remains our
globally best data set describing the individual decisions of an estimated
forty billion songbirds migrating annually among continents. All other data on
individual songbird migration are limited to one or two nights of migratory
flight (Cochran et al., 2004;
Diehl and Larkin, 1998;
Wikelski et al., 2003;
Cochran and Wikelski, 2005;
Bowlin et al., 2004) or banding studies connecting dots across continents
(Thorup and Rahbek, 2004). We
know even less about migration in small mammals such as bats
(Tuttle and Stevenson, 1977;
Cryan et al., 2004). Even so,
this lack of knowledge of individual migration pattern should not conceal the
fact that major progress in small-animal migration continues to be made, with
spectacular insights being published in rapid succession
(Alerstam and Hedenström,
1998; Berthold,
2001).
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A global challenge for experimental biologists: dispersal and long-distance migration |
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TOPSummaryApplications of a global...The power of combined...A global challenge for...Beyond Aristotle's migration...Why current technology is...A global solution to...ICARUS can make a...References |
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), but even
repeatedly locating resident study animals is rarely easy
(Winkler et al., 2004). Many
animal species are cryptic, shy and faster than the field biologists chasing
after them. Nevertheless, there are many situations in nature where this is
not true, and experimental biologists have taken advantage of this. The
bonanza of knowledge that these situations have produced illustrates the
scientific potential of a global tracking system. The Galapagos are one such
example where animal dispersal is limited. The extraordinary success of
Rosemary and Peter Grant's long-term investigation on Darwin's finches
(Grant and Grant, 1996) relies
on the ability to reliably relocate individuals, enabled because they work on
birds living on a small, desolate crater island 1000 km off the Pacific coast
of South America. In most other `open' study systems, the demonstration of
microevolutionary processes would have been much more equivocal.
The only reliable approach for animal tracking with guaranteed global
coverage and constant access is via satellite. Present satellite
technology allows us to track large (>300 g) animals globally and has led
to spectacular insights. Weimerskirch et al. tracked wandering albatrosses
(Diomedea exulans) around the South Pole and through the Indian Ocean
and related their wanderings to feeding rates and food distribution
(Weimerskirch et al., 1993).
Fuller and colleagues found a breeding snowy owl in Alaska one year, then in
Canada and in Siberia in subsequent years
(Fuller et al., 2003),
presumably also breeding at these locations. Block and coworkers tracked
bluefin tuna across the entire Atlantic and into the Mediterranean Sea
(Block et al., 2005). Although
all of these studies are remarkable, many large animals such as petrels or
albatrosses may outlive their researchers
(Clapp and Sibley, 1966) and
thus do not allow for evolutionary trends to be observed. Similarly, for many
experimental studies, large animals are not ideally suited as they are either
endangered and protected, not numerous enough, again too long-lived or simply
too difficult to keep in any numbers to study selection on certain traits.
The most limiting factor of modern satellite tracking methods is the size
of the tag. The smallest commercially available satellite transmitter in 2006
(9.5 g;
www.microwavetelemetry.com)
is still too large for 81% of all bird species [6106 bird species of 7514
species for which body weights are available weigh less than 240 g
(Bennett and Owens, 2002);
following the <5% body weight rule
(Murray and Fuller, 2000)].
Similarly, 66.8% of the world's mammal fauna cannot be tracked over long
distances, i.e. from space, again because of body weight constraints on
transmitter size [Fig. 1; 3763
of 5630 species for which body weights are available
(Smith et al., 2003)].
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Beyond Aristotle's migration enigma: tracking small animals globally |
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TOPSummaryApplications of a global...The power of combined...A global challenge for...Beyond Aristotle's migration...Why current technology is...A global solution to...ICARUS can make a...References |
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;
Hedenström and Alerstam,
1997; Klaassen et al.,
2000). Aristotle wondered about 2000 years ago where songbirds
disappeared to in winter (Peck,
1968). For many of the species that the famous natural philosopher
was concerned about, we are still rather ignorant - even in Europe, where
scientific bird banding started more than 100 years ago
(Berthold, 2001). For most of
the species wintering in tropical areas, our knowledge about wintering homes
and migration routes is limited to less than a handful of recoveries in the
presumed wintering areas and the information contained in the recordings of
observers (Thorup and Rahbek,
2004).
Other questions for biologists that such new satellite tracking methods
could address are natal dispersal (Winkler
et al., 2004), a mechanism critical to our understanding and
modeling of animal demography (Lawton and
May, 1983), metapopulation dynamics
(Robinson et al., 1995),
life-history evolution (Sillett and
Holmes, 2002) and extinction
(Jackson, 1979;
Webster et al., 2002). For
example, it is unknown whether the fragmented landscapes of the North American
Midwest are population sinks for wood thrushes or whether these long-distance
migratory songbirds sustain healthy populations amidst the corn and soybean
fields, despite high nest predation
(Robinson et al., 1995).
Long-term tracking over large spatial scales could be used to discover when
and where free-ranging animals die, helping to improve our understanding of
the ecology of the different life-cycle stages that are otherwise very
difficult to investigate (Rubenstein et
al., 2002). Furthermore, the paths of birds, bats, rodents and
insects carrying diseases could be followed
(Rappole et al., 2000;
Malkinson et al., 2002).
What is needed to address these problems is a system that can track
hundreds of small animals down to the size of a 6 g hummingbird or large
insects (Naef-Daenzer et al.,
2005; Wikelski et al.,
2006) over large, continental distances and over long periods of
time. Individuals should be tracked over at least one entire year to solve
pressing scientific and conservation questions, such as where individuals die
and what stopover sites are most important
(Moore and Simons, 1992;
McNamara et al., 1998;
Moore, 2000;
Sillett and Holmes, 2002).
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Why current technology is not sufficient |
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TOPSummaryApplications of a global...The power of combined...A global challenge for...Beyond Aristotle's migration...Why current technology is...A global solution to...ICARUS can make a...References |
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).
However, the spatial precision of these methods is inherently limited
(Rubenstein and Hobson, 2004)
and the actual routes that animals take during migration would remain unknown.
Satellite tracking systems such as ARGOS
(www.argosinc.com)
have produced spectacular global location data for large animals, for example
albatrosses (Weimerskirch et al.,
1993). ARGOS instruments are housed on board different satellites
from the US National Oceanic and Atmospheric Administration, The Japanese
Space Agency and the European Meteorological Satellite organization. Two
satellites are operational at any time in 850 km orbits. The ARGOS system
collects data from Platform Terminal Transmitters (PTTs) and delivers
telemetry data directly to the users. PTT satellite tags may be shrunk down to
smaller sizes in the future but will be unlikely to surpass a lower size limit
of 5-8 g. GPS (Global Positioning System) tags are now approaching 5-10 g
(Lipp et al., 2004;
www.technosmart.edu),
but these tags often need to be recovered to read out the positions and they
still remain too large for most animals. GPS tags with a communication network
to transmit data are even heavier (30 g;
www.microwavetelemetry.com).
Miniaturizing cell-phone technology holds high promises but also high hurdles.
The hardware and software (including >1 million lines of code) are advanced
and inexpensive to purchase but very customized to commercial applications. It
is unclear to what degree a small market such as experimental biology can
capitalize on this technology. Furthermore, network coverage remains limited
on a global scale and locational accuracy is low
(Stokely, 2005). Radar
technology, either passive or active, also holds many promises but, because of
the immobility of most radar installations, will not resolve long-distance
migration of individuals (passive radar)
(Gauthreaux and Belser, 2003).
Active radar, such as the cross-band transponder technology, faces similar
power and detection problems as the ARGOS satellite tags
(www.earthspan.org). |
A global solution to track small animals |
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TOPSummaryApplications of a global...The power of combined...A global challenge for...Beyond Aristotle's migration...Why current technology is...A global solution to...ICARUS can make a...References |
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). These problems are not unique to animal tracking.
Another scientific discipline has already addressed similar, albeit inverse,
problems. As radio astronomers cannot increase the power emitted by distant
galaxies or control the nature of the radio emissions, they were forced to
build sensitive receivers with sophisticated antennae and to develop elaborate
signal processing algorithms (e.g. Kraus,
1986; Thompson et al.,
1986). We suggest that the best technical solution for
experimental biologists involves a similar solution
(Cochran and Wikelski, 2005;
Cochran and Wikelski, 2005).
Radio astronomers point antennas into space to locate faint radio sources
against background noise tens of thousands of times stronger. Wildlife
biologists could point antennas towards earth from near-earth orbit to locate
small radio transmitters attached to animals. Radio telemetry receivers and
antennas mounted on satellites have global reach and are a natural extension
of the ground and aerial radio techniques that have been the mainstay of
animal tracking for the past 45 years
(Lord et al., 1962). Extending
this space technology to accommodate much smaller animals than can be served
by the ARGOS program will have very great benefits to the study of dispersal
and migration.
The physics of this scenario has been modeled and shown to provide a
workable solution using presently available radio technology.
Satellite-mounted radio receivers could track radio-tags with a radiated power
as low as 1 mW with an accuracy of a few km under favorable conditions. This
power can be achieved by modern tags as small as <1 g, which could be
carried by, for example, migrating birds, bats, rodents, marine life or even
desert insects. In the marine realm, radio transmitters could not be received
in space when animals are below the surface, but a satellite system could work
for surfacing animals or with the help of pop-up archival tags
(Block et al., 2005). The
remote measurement of physiological parameters characterizing the state of
animals could be added to any of these tags
(Cooke et al., 2004;
Bowlin et al., 2005).
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ICARUS can make a small-animal tracking satellite fly |
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Acknowledgments |
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