was the advent of radio-tracking technology that allowed researchers to begin addressing many of these detailed questions for such a small, cryptic species. By the late 1970s, technology had advanced to the point that small transmitters could be attached with cloth collars to animals. Biologists had already identified the size, design, and weight of a radio collar that the close relative of the black-footed ferret, the Siberian polecat, could tolerate. They knew that ferrets would present a unique issue compared to other animals. Their small weight limited how large a transmitter they could tolerate without the ferret’s ability to move and hunt being hurt. Biologists knew that without a large battery, the transmitter would have limited range and lifetime. They knew that because a ferret spends more than 90 percent of its life below ground, getting a signal would be difficult at best, and it was more likely that they would have to scan continually, often in the middle of the night, for the chance to detect that a ferret had come above ground.
In addition to the technology encased within the plastic-coated transmitter capsule, for a tubular animal the design and tightening of the collar itself was critical. A collar needed to be loose enough to allow breathing and growth of the animal, tight and rugged enough to stay on, and made of cotton that would degrade so the collar would fall off over time as the battery died. This made the stress of placing a collar on a captured and sedated ferret that much more intense. Not only did you have one of the rarest animals in the world in your hands, you had to mount something around its neck that if done improperly could kill it. If it was too loose, you would risk the chance of the ferret slipping the collar, forcing a second stressful capture if you ever even saw the animal again.
For wildlife biologists, tracking an animal is typically a solitary affair conducted by a biologist with a hand-held antenna and receiver tucked under the arm, headphones sending out a hissing sound. The biologist listens for a faint beep as the antenna is panned around, and hikes from waypoint to waypoint, eventually honing in on the location of the animal and taking note of the location. Yet, because ferrets came above ground so infrequently, just detecting them required teamwork. Rather than sending a dozen biologists out onto the prairie throughout the night, three camper trailers, modified so that large directional antennas poked through their roofs, were placed on the highest points surrounding core prairie dog colonies in a triangular design. This required near-constant tracking by a team of biologists who spent hours hunched over within the trailers, rotating the antennas and scanning ferret transmitter frequencies, listening through headsets for faint electronic beeps in the hiss of static signaling a ferret had come above ground. Once a ferret emerged and was detected by one of the biologists, the biologist would relay the approximate location of the ferret to biologists in the other two trailers. When two or more trailers locked on to a ferret signal, they could accurately triangulate the location of the ferret. Over the span of weeks and months, researchers began to define the territories of individual animals, their activity patterns, and their behavioral interactions with other marked ferrets.
As a break from the trailers, researchers would swap their sedentary nightly duties and hit the sagebrush flats to find ferrets by spotlighting, searching for isolated animals on the peripheral colonies that were not already under radio-telemetry study. The researchers would move around on the prairie and capture the remaining few ferrets, recording their measurements and marking them with “passive integrated transponder” implants (PIT tags for short) injected just below the skin. These small, pill-shaped microchips are passive in the sense that they do not rely on a battery to transmit a signal like radio collars do, but instead require an outside scanner to pass within a few inches to relay a reading. With no bulky batteries to worry about, PIT tags last forever and provide a way to permanently identify individual ferrets. By finding and scanning the PIT tags of ferrets over the entire area, biologists were able to get a count of marked individuals. They could learn a minimum population size with which to assess the status of the population and follow the population through time to assess its viability.
With these tools, the small army of researchers learned that it was unlikely to see two or more ferrets together. The solitary ferrets seen above ground in January and February were likely males in search of mates, roaming their territory that overlapped one or more smaller territories of females. Seeing one male trying to maintain access and breeding rights to multiple females suggested to researchers that ferrets were similar to other members of the weasel family in having a polygamous mating strategy. Ecological theory tells us that in polygamous societies, a male has to produce as many offspring as possible to maximize his genetic lineage. This requires that males keep exclusive access to as many females as possible. It is a matter of quantity, but also quality, because females typically select the best habitat in which to raise their litters. Thus, by association, males that protect prime habitat also are likely to have access to the most females and to females that are most likely to successfully rear their young.
Guarding of mates by male ferrets reached a fever pitch in the mating season from January through mid-March. Then, after a forty-five-day gestation period, kits were born in a natal burrow. Contrary to many other carnivores, such as wolverines and otters that remain in a single nest chamber for the whole litter-rearing period, female ferrets would move kits to new burrows at regular intervals. The mother would carry them with her teeth by the scruff of the neck, one at a time, when their eyes were still shut and they were not yet mobile.
Meeteetse researchers found that females did all the caring for the young, protecting them from birth in spring through to independence in late summer. A mother ferret at first nursed her kits, staying below ground for long periods of time. She killed prairie dogs only occasionally for her own food, until June, when she started moving the whole family to a burrow containing a freshly killed prairie dog. By late July and early August, kits were old enough to have motor control and come above ground for periods of time during the night. They would be cautious at first, with only the boldest of a litter of three or more doing more than sticking its head above ground, and never straying more than a few feet from the safety of the natal burrow entrance.
By mid-August, kits came above ground almost every night. On 93 percent of nights to be exact, typically between 1:00 and 4:00 A.M. The kits stood by their natal burrows as the mother hunted for food, until the silence was broken by one of the litter starting a tussle by charging at another, back arched, mouth agape, tail frizzed. Spotlighters found the kits to be curious, peering up from burrow entrances when the spotlighters approached. The kits noticed that any foreign object placed by their burrow deserved inspection. The small flags the researchers used to mark burrows often induced a spontaneous fit of play. Young kits would lunge off the ground, reaching the flapping flag at the top of the two-foot wire flags and landing in a puff of dust. They seemed amazed that they reached so high after a childhood spent mostly below ground or within two inches of the flat prairie surface. Collecting themselves after landing, as the dust cleared they sometimes chased their own tails, perhaps out of curiosity about their rapidly growing bodies.
By late August, kits grew to be as large as their mother or even larger, and more independent. They dispersed from their mother’s territory by the end of September and the prairie dancers had gone away. They were solitary for the next few months as behaviors shifted from rambunctious play to the pressures of adulthood and survival. By November, the former kits and older adult ferrets were active above ground only for, at most, one to two hours per night. With so little to observe, researchers switched their monitoring from spotlighting to occasional snow tracking when fresh layers of snow fell, searching for slight pawprints leading between burrows in skiffs of thin snow blown like sand into small waves by the bitter prairie winds. Biologists found that during the harsh winter months, when temperatures dropped below freezing and when white-tailed prairie dogs slept below ground for weeks on end in a state of torpor, ferrets similarly slowed their above ground activity. During peak winter periods, ferrets would spend up to six nights and days below ground without moving to another burrow. But the clearest pattern to the researchers was also the one attribute that has been known the longest about this species since its first discovery and description: that black-footed ferrets almost never leave prairie dog colonies; as the prairie dog goes, so goes the ferret.
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