The Problem of Black Tipped Tails

One of the curious quirks of research, and the salvation of many a graduate student, is that it is possible to design a study to ask one question and then be led to a valid answer to a different question. Powell's convictions about the effect of raptor predation on the numbers of weasels in ecological time stimulated him to design a series of simple, elegant experiments with trained red-tailed hawks. They produced a legitimate answer to a long outstanding question in a different field altogether, the effect of raptor predation on the morphology of weasels in evolutionary time. He came up with a simple and convincing explanation for why stoats and longtails have black tips on their tails and least weasels do not.

Whether or not predation by raptors affects populations of weasels, it is a disaster for the individuals caught, and for that reason they avoid open spaces if they can (Figure 11.6). A raptor or an owl can stoop down on a fleeing weasel with terrifying speed, and is unlikely to be deterred by the defensive shriek and "stink-bomb" that might put off a fox or cat.

Only if the talons fail to pierce the thin body of a weasel may the raptor find it has picked up more than it bargained for. Anderson (1966) saw a buzzard swoop, pick up a weasel, and flap away with it, and later he saw it fall to the ground with its captive. When he reached the fallen buzzard, he found the bird lying dead on the ground, with its underparts bloody, and the weasel gripping its breast with meshed teeth. Sometimes, raptors survive such an encounter, although without being able to dislodge the weasel's death grip. Seton (1926) reported an eagle that was found with a bleached weasel skull fixed to its neck.

The white winter fur of the northern weasels is probably itself a defense against attack from the air, even though it does not match the snow exactly. For example, one study area near Robinson Lake, Idaho, supported nine longtails and four stoats during the winter of 1950-1951. The stoats changed to white, but the longtails did not, and two of them were caught by raptors (Musgrove 1951).

Stoats and longtails have another defense against raptors, which is effective all the year round: their black tail tip. Powell pointed out that many small animals, such as fish and butterflies, have spots of contrasting color on their hind ends, which are believed to attract a predator's eye and deflect its strike away from vital body parts such as the head and neck. He asked whether the black tail tip of stoats and longtails might have the same function. The black tip is conspicuous at any time, but especially in winter when it remains black after the rest of the coat has turned white. He decided to test whether the black tail tip is an additional hindrance to raptors hunting white weasels on snow.

Powell had three captive, wild red-tailed hawks, which could be tethered to a running wire laid out between two perches set 30 m apart on the flat, white-painted roof of the old Lion House at Brookfield Zoo outside Chicago. He presented a series of model weasels to each hawk in turn, pulling them across the hawk's flight path attached to a fishing line looped around two pulleys. He estimated each hawk's attack speed, and adjusted the movement of the dummy so that the hawk could only just catch it. The model weasels were made of white artificial fur in two sizes, one 40 cm long, to represent a male longtail, and the

Figure 11.6 A weasel spotted on open ground by a hawk or an owl is nearly defenseless. Its only hope is that it can, by making rapid twists and turns, outma-neuver the predatory bird or, if it is a stoat or longtail, that the bird is confused by the black tip on its tail.

other 17 cm long, representing a male least weasel. Some had a black spot on their backs, some on their tails, and some were plain white. The models were presented in random order to the three hawks until each had seen all six models 12 times (Table 11.6).

The models representing long-tailed weasels with tail spots and least weasels with no tail spots, which were most like the real thing, were missed by the hawks much more often than the other models. Powell's explanation was that the hawks focused their attacks on the black spots. They nearly always caught

Table 11.6 Results of Powell's Experiments on the Deflective Value of the Black Tail Tip to Dummy White Weasels Hunted by Hawks Against a White Background

Number of chances missed

Table 11.6 Results of Powell's Experiments on the Deflective Value of the Black Tail Tip to Dummy White Weasels Hunted by Hawks Against a White Background

Number of chances missed

Model

Hawk #1

Hawk #2

Hawk #3

Total

Longtail, no spot

1

1

O

2

Longtail, tail spot

11

4

9

24

Longtail, body spot

2

O

2

4

Least weasel, no spot

9

7

9

25

Least weasel, tail spot

1

O

1

2

Least weasel, body spot

3

O

O

(From Powell 1982.)

either size of model if the spot was placed on the body. If the spot was placed on the long thin tail of the larger models, the hawks failed to grasp it, and they also sometimes checked their attack at the last moment, as if they had not seen the rest of the model until then.

On the other hand, if the spot was placed on the short tail of the smaller model, they usually caught it because the rest of the body was close enough to be within the talons' reach. Larger models with no spots were still visible, even though they were all white against the white-painted concrete roof. But the hawks took fractionally longer to notice and react to the smaller ones with no spots, so often missed them. Powell concluded that the black tail tip on stoats and longtails is a classic predator-deflection mark, and that least weasels do not have it because their tails are too short to hold the mark far enough away from the body.

One might ask, Powell added, why least and common weasels do not have longer tails so that a black tip would be a benefit instead of a liability. He offered the reasonable explanation that they may be too small to keep a longer tail warm during the long northern winters. Perhaps another reason is that they are less exposed to raptors than are larger weasels, because they spend so much more of their time under snow cover.

Snow does not last all year round, however, and, except in the far north, most weasels are brown for more months of the year than they are white. In Switzerland, winter-whitening stoats live alongside non-winter-whitening common weasels in the same areas (Güttinger & Müller 1988). We would like to see Powell's idea tested with models of brown weasels against various natural backgrounds.

Another important factor to consider is the great range in body sizes of weasels with and without black-tipped tails (see Figures 4.5 to 4.7). Some future test should include models representing the races of small stoats that do have short tails with black tips and the races of large European common weasels that have short tails without black tips. In the meantime, Powell's conclusion on the function of the black tail tip as an advantage to individuals seems secure, and it remains valid whether or not predation by raptors has any effect on weasel population dynamics. In nature, predation can have a profound effect on morphology, by determining which animals survive, without affecting at all the number that survive.

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