Fleas

When live-trapped weasels are anesthetized, the fleas in their fur, which are also anesthetized, fall out and can be collected by hand. Several field workers undertaking live-trapping studies for quite other reasons have extended their field routine to collect fleas during the short time that they have the unconscious weasel in their hands. When weasels are collected freshly dead, they often have some fleas still on them, and these are also worth collecting as a routine part of the dissection procedure.

Neither method can be claimed to search the fur thoroughly enough to find all the fleas that might be present, and, of course, lice and mites are harder still to see. Worse, dead weasels collected in kill traps quickly lose many of the fleas they had, since fleas are not confined to the host's body and leave a carcass as it cools (King 1981b). These problems aside, the systematic collection of fleas in various places has produced some intriguing insights into the natural history of weasels.

The larvae of fleas are free-living scavengers of organic material, and they find the best conditions of temperature and food supplies in the nests of small mammals. Fleas have evolved for millennia alongside their hosts, and have developed close relationships with mammals that have substantial nests that are occupied or frequently revisited over a long enough time for the fleas to complete their nonparasitic larval stages. Hence, mammals with small home ranges and permanent dens, such as badgers, moles, shrews, and rabbits, have specific fleas. Those that move around a lot and have only temporary dens or none at all, such as foxes, weasels, and hares, have no flea species of their own. They do carry fleas, but only those normally found on other animals.

Weasels were long assumed to pick up fleas from the bodies of their prey and, perhaps less often, from casual encounters in the grass. But these are probably not the usual ways that weasels acquire their fleas. If they were, the list of the normal hosts of the fleas identified on weasels should closely match the normal diet of weasels, but it does not. The list in Table 11.7 includes fleas specific to hosts that weasels seldom or never eat, such as moles, and some of these in substantial numbers; conversely, there are few fleas specific to hosts frequently eaten, such as rabbits and birds.

The explanation for this apparent contradiction appears to be that weasels normally pick up their fleas from the burrows and nests of their prey (King 1976). These are the places where adult fleas lie in wait, ready to jump onto the first warm furry creature that passes by or stays for a snooze. Weasels feel the cold badly, and since they do not make their own dens, they depend on finding warm nests to sleep in made by other animals. They would be very likely to pick up the fleas specific to hosts that make substantial nests of the right size, even though these hosts are seldom eaten.

Judging by the records of fleas listed in Table 11.7, common weasels must often borrow the nests of moles (see Figure 8.4), and stoats the nests of rats and squirrels, because the fleas of moles, rats, and squirrels are found on common weasels and stoats much more often than their hosts are eaten by them. One flea, Rhadinopsyllapentacantha, normally found only in the nests of voles rather than on their bodies, turns up on common weasels remarkably often. Stoats radio tracked in Ireland and in New Zealand liked to sleep in rats' nests and in holes in the ground where rats and rabbits might shelter (Sleeman 1990; Murphy & Dowding 1995).

On the other hand, the nests of many birds offer little protection from the elements. Weasels eat birds often, but seldom rest in birds' nests, and bird fleas rarely turn up on weasels. Irish stoats do eat shrews, yet do not carry shrew fleas. Presumably, a shrew's nest is too tight a fit even for a small Irish stoat.

There are, of course, other considerations. For example, some fleas are specific to one particular host and drop off any other quickly. This may explain why so few rabbit fleas are found on weasels. Less fussy fleas are likely to stay on the "wrong" host for a while, especially if they are hungry and actively searching for a meal. This could be a stronger reason for hopping onto a weasel visiting a nest that has been deserted for days than merely being disturbed by a weasel eating the host they are already on.

Table 11.7 Fleas Found on Common Weasels and Stoats1

Flea species

Normal host

On common

weasels

On stoats

England

Scotland

Switzerland

New Zealand

Britain

Ireland

Megabothris walkeri

Voles

82

Ctenophthalmus nobilis

Voles, mice

48

67

28

Hystrichopsylla talpae

Rodents, insectivores

25

18

3

Malareus p. mustelae

Voles

26

11

C. bisoctodentatus heselhausi

Moles

17

20

3

Palaeopsylla m. minor

Moles

18

17

2

Rhadinopsylla pentacantha

Nests of voles

9

16

1

2

Dasypsylla g. gallinulae

Birds

5

1

3

Megabothris turbidus

Voles

4

1

Peromyscopsylla spectabilis

Voles

1

Nosopsyllus fasciatus

Rats

662

17

Palaeopsylla s. soricis

Shrews

1

Megabothris rectangulatus

Voles

14

1

Ctenophthalmus agyrtes impavidus

25

C. b. bisoctodentatus

Moles

47

C. s. solutus

1

Monopsyllus s. sciurorum

Squirrels

6

Peromyscopsylla bidentata

1

Leptopsylla segnis

House mice

8

Ceratophyllus gallinulae

Birds

1

Parapsyllus nestoris

Birds

1

Orchopeas howardi

Squirrels

2

Spilopsyllus cuniculus

Rabbits

1

Number of examinations

338

NR

380

1501

NR

122

Reference

(King

(Mardon

(Debrot &

(King &

(King 1976;

(Sleeman

1976)

& Moors

Mermod

Moody

Mardon &

1989a)

1977)

1982)

1982)

Moors 1977)

1. Columns give total number of fleas found; number of animals inspected, often the same ones on successive days, summed at end.

1. Columns give total number of fleas found; number of animals inspected, often the same ones on successive days, summed at end.

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