Stoats and Weasels-species packing and variation

         A stoat, courtesy of the Mammal Society.               A weasel, also courtesy of the Mammal Society

  Mustela erminea = Stoat                   Mustela nivalis = Weasel

Ref : Natural History of Weasels and Stoats. King, C.M., 1989

	Weasels are missing from the Isle of Man, the Shetland Isles and Ireland.
	Both Weasels and Stoats show considerable sexual differences. Therefore there are effectively 4 different species : the male and female Stoat and the male and female Weasel.
	The shorter days of autumn cause Weasels and Stoats to lose its summer coat and grow a white winter coat. In northern populations the winter fur is white , whilst further south the colour change may be incomplete or not at all.  Also, more females go white than males. The tip of the Stoat's tail always stays black, giving the black spots on ermine (white winter Stoat) robes.

Another stoat, courtesy of the Mammal Society

How do these species and sexes fit together ecologically?...

Diet

Both Stoats and Weasels are strictly carnivorous species which depend heavily on live prey-they rarely scavenge. They eat largely small rodents such as voles and mice. Lagomorphs (hares and rabbits) and birds are an important secondary prey.

Stoats are larger and have a somewhat more varied diet because they can tackle larger animals due to their larger size. However, rodents still form most of their diet.

Is there any competition between Stoats and Weasels? We shall answer this by looking at the diets of these species in the same places :

Devon...

	small female Weasels eat mostly SMALL RODENTS (especially Field Voles)
	male Weasels eat 50% RODENTS, with MORE LAGOMORPHS than the females
	female Stoats and male Stoats eat LESS RODENTS but MORE RABBITS and BIRDS than Weasels

Wytham Woods Oxford...

	in some years, Weasels have been shown to predate tit nests
	there is a reciprocal relationship between between vole availability and bird predation-when there' are not many voles around, more blue tits are predated
	Ref : Dunn 1977, Journal of Animal Ecology;46;633

Sussex...

	Stoats and Weasels have an impact on game birds
	if they can get Microtus (Field Voles) then that is their preference-when the number of Microtus goes up, fewer game birds are predated and vice versa
	female Weasels tend to take far more Microtus than male Weasels

Conclusion...

There is some avoidance of competition by different diets between the sexes and species : smaller Weasels take smaller prey than the Stoats, but there is an extensive overlap, especially between the sexes.

Habitat...

There is a complete geographical overlap between the species even at the level of territories, but Weasels do try to keep out of the way of Stoats as they are smaller.

Worldwide variation...

	Stoats are always larger on average than Weasels when both are present
	in southern Europe and north Africa, there are no Stoats, but the Weasels here are larger-their skull length is the same as that of the Stoats in Great Britain
	in Ireland there are Stoats but no Weasels, and Stoat size declines from south-north Ireland; Stoats in S.Ireland are as big as in England, whereas N.Ireland Stoats are smaller-intermediate size between British Stoats and Weasels

In general...

Stoats are larger in warmer climates, as are Weasels in Europe and Asia. However, in the USA, Stoats are larger in colder climates.

Reproduction...

Reproduction is related to how the Stoats and Weasels react to the environment and prey availability.

Female Weasel...

	is sexually active from February to September : has a long sexual period
	can carry blastocysts from March to July
	earliest birth is April, latest is August
	lactation occurs from March to October
	they usually have 2 litters per year

Male Weasel...

	produces sperm virtually throughout the year (February to October/November) which matches very well with the female
	Weasels have long reproductive seasons over the summer

Female Stoat...

	anoestrous all year apart from around 3 weeks in April, but the fertilized egg then undergoes delayed implantation until the following spring, doesn't develop until implantation
	takes one month from implantation for the young to develop
	lactation is short
	as soon as the female gives birth, she goes into oestrous again and mates with the male in the territory-the next offspring will have a different father
	young females reach puberty at about 2-3 weeks old, they are then mated by the same male as their mother
	young females are blind and toothless and are not physically mature at this stage to carry a child, but as implantation is delayed, they do not have to do so until the following spring
	there is only one litter per year and 6-10 infants per litter

Tail spots...

Tail spots are used for predator deflection. Powell did work on this using three hawks which he had trained to catch models of weasels and stoats.

Ref : Powell, 1982. American Nature;119:126

Models of Stoats (which are called Long-tailed Weasels in the USA) were made with either NO SPOT, A TAIL SPOT or A BODY SPOT. The same was carried out for the Weasels. The hawks were given a total of 36 chances to catch each of the dummies, and the following results were obtained :

Results...

	Stoat : a Stoat's tail spot is further from the body so the hawk is more likely to miss as it goes for the tail
	Weasel : the tail spot is closer to the body so the hawk is more likely to catch it

Explanations need to be made for...

	sexual dimorphism
	climate differences
	different reproductive systems

1) Bergmann's rule...

The rule states that warm blooded animals get larger where the climate is colder. In Europe and Asia, Weasels are larger in WARMER climates. Moreover, this is nullified by extreme sexual dimorphism. If for example the male size is better because it is larger, in cool climates then the female would be too small.

2) Character displacement...

Weasels are much larger in S.Europe where larger Stoats are missing.

Stoats are smaller in size in Ireland where Weasels are missing.

North America : There is a third species Mustela frenata. Where M.frenata and M.erminea overlap, M.frenata is the same size as the British M.erminea. However, the American M.erminea is the same size as the British M.nivalis (American M.nivalis is smaller that the British M.nivalis).

3) Prey size...

The average Stoat prey is certainly larger than the average weasel prey. However, it isn't clear that males and females differ in this way, or that there is a substantial difference between the female Stoat and male Weasel.

Mustela are good at hunting down small rodents-as they are long and thin they can follow them down their burrows and under the snow. This is especially true for smaller females in all species.

If the animals are hidden beneath the snow, they are less vulnerable to winter weather and other predators. Also under the snow there isn't much competition from other predators such as owls, and the Mustela can take over the rodents' nests.

4) Sex...

Weasels are small with long breeding seasons. They mature quickly and there is no delayed implantation. Because of this Weasels can react to increasing vole numbers more efficiently than Stoats.

Stoats have the advantage that even if food is short and few males are around, due to delayed implantation they are still assured of breeding next season, however they have the disadvantage that they can't respond quickly to the food supply improving.

5) Food exploitation...

As Rabbits died from myxomatosis in the 1950s, Stoats almost died out due to the loss of their main food supply. However Weasels increased because there was more grass and more voles due to the loss of Rabbits. When the Rabbits and Stoats recovered, the Weasels decreased : different food sources are exploited.

In New Zealand there are no voles and few mice but there are various larger prey. Because of this the Stoat is common and the Weasel is very rare.

On the Isle of Terschelling (Holland) there was a plague of Water Voles, so the Weasel and Stoat were introduced to get rid of them. However, the Water Vole was too large for the Weasel to kill, so the Weasel became extinct whereas the Stoat flourished.