The Weasel
Mustela nivalis
Written by Jenny Morley (Lincoln University)
July 2005
Edited by Michael Hudson, Lincoln University, September 2006
Taxonomy
- Scientific name: Mustela nivalis (Erxleben 1777)
- Synonyms:
- Popular Common names: Weasel
- Other common names: Least weasel, common weasel
- Kingdom: Animalia
- Phylum: Chordata
- Class: Mammalia
- Order: Carnivora
- Family: Mustelidae (the mustelids)
- New Zealand Status: naturalised
- Taxonomy sources: King (1989); Flannery (1995).
Identification
Species Description
Like all of the mustelid family the least weasel has the typical long, slender body, long neck, short legs and sharp-pointed face (Shaun Oglivie, personal communication). The legs and body are brown with white belly margins (personal observation). The tail is uniformly brown and the feet have five fingers, each with sharp claws (Sheffield & King, 1994). According to Fannery (1995) the weasel is the smallest of the world’s carnivore species. In New Zealand males are 220 mm, and the females 180 mm long on average (from head to the end of the body) (King, 1989).
The least weasel has a large range (7–15 hectares), no fixed abode, does not form pair bonds (i.e. the males and females live and hunt separately except during breeding) and the female shifts her litter frequently. These are some of the characteristics which make the trapping and killing of weasels very difficult.
The least weasels small size, and specialised hunting strategies make it a very successful predator (King, 1989). Interestingly the abundance of the least weasel is determined by the abundance of food (King et al., 2002)
Similar species
The least weasel is often confused with the other two introduced mustelids in New Zealand; the ferret and the stoat. Where all three species occur sympatrically the stoat is always the largest, followed by the ferret. Furthermore, stoats and weasels are the most similar but can be distinguished by different markings. On the least weasel the tail is uniformly brown, unlike the stoat the tail has no black tip (King 1989).
The following diagram (figure two) may be of assistance in identifying New Zealand’s introduced mustelids.
Geographic Distribution
Full natural and naturalised range
The least weasel is distributed naturally throughout Eurasia. Introductions (accidental and planned) now see the weasels range extending right around the northern hemisphere, north of about 40ºN (Gittleman, 1989).
New Zealand range
In New Zealand the least weasel is naturalised throughout most of the country. Exceptions are Stewart Island, and south of Cape Foulwind on the west coast of the South Island (Shaun Oglivie, personal communication). In addition eradication programmes have seen the removal of the weasel (and other pests) from offshore islands and mainland island sanctuaries (for example: Warrenheip – a 16-hectare sanctuary in the south Waikato; Karori – a 252-hectare wildlife sanctuary close to Wellington).
Canterbury range
Although found throughout Canterbury the least weasel is not common (Shaun Oglivie, personal communication). Areas of farmland, woodland and scrub are all suitable habitats and are found in abundance in the Canterbury region (personal observation). The least weasel would not be found in coastal areas, or in the highest altitudes of the Southern Alps.
Natural History in Canterbury
Habitats
Weasels are able to occupy a wide range of temperate habitats; primarily open forest and farmland. In addition they are able to live in tundra, taiga (coniferous or boreal forest) forest or rainforest habitats (Sheffield & King, 1994). Usually avoiding very dense forest or very open areas, the preferred habitat of the weasel results in them commonly being found where rodents, especially mice (their most common prey), are abundant (King, 1989). This makes the hedges, farmland and woods of Canterbury ideal habitat for these small carnivores.
Figure 3: The natural and naturalised range of Mustela nivalis (Source: www.invasive-mammals.org)
Preferred habitats
Least weasels (unlike the stoat) can breed twice a year; this is dependant on the availability of food (King, 1989). Breeding in spring and again in late summer, weasels can produce between one and seven offspring (McDonald, 1998). Females born in the spring are sexually mature at three months. If food availability is high these females can breed in their first year. However, late summer/autumn born females will be less well developed and unable to breed until the following year. The gestation period is high at 37 days; but at only 18 days the time to weaning is low (King, 1989).
Longevity
Interestingly, all mammals live for about the same amount of physiological time; that is, for about 800-1200 million heartbeats and 200–250 million breaths (King, 1989). That means the least weasel has a physiological life span of 3–6 years. However, in the wild it is uncommon for a weasel to live longer than one year (King, 1989).
Dispersal
The least weasel is highly mobile and active both day and night. It has a large territory, this means it has been able to disperse widely throughout New Zealand.
Diet
Mice, rats, voles, rabbits, insects, invertebrates, birds (including poultry), bird nestlings and eggs, are all suitable prey for the least weasel (Sheffield & King, 1994). The weasel has been described as a fierce and aggressive predator (Blair, 2002). Albeit the smallest of all carnivores, the least weasel is able to attack and kill prey much larger than itself.
Sheffield & King (1994) describe the least weasel as having a ‘keen sense of smell, hearing, touch and sight’. To communicate with each other and to locate prey the least weasel relies heavily on its sense of smell. As very adept tree climbers, even tree nesting birds cannot escape predation by the least weasel.
Predators
Owls and hawks have all been known to prey on the least weasel (Sheffield & King, 1994).
Competitors
There are no real competitors to the weasel in New Zealand. The only real limiting factor to the success of the least weasel is the availability of food. Able to respond with rapid breeding, weasels can take advantage to favourable conditions (Gittleman, 1989)
How to find the Weasel
As any good ecologist would tell you, if you see a least weasel in the wild the best course of action is to administer a quick and efficient blow with sufficient force to crush the skull. Blair (2002) suggests a hammer as a useful tool for carrying out this grisly task.
If, however, you simply want to observe this animal without taking an active role in its demise, then there are some tips that may help. It could be argued that to see a least weasel in the wild can be a difficult matter. Their small size, speed, mobility and sharp senses all help the least weasel to remain undiscovered. However, pastoral, lightly forested areas or forest margins would be the most likely place to spot one. Weasels are found in greatest abundance where rodents, such as mice, are also abundant. To add to the difficulty, being active both day and night means there is no ideal time to spot a least weasel.
On the other hand it could also be argued that with enough local knowledge it could be possible to find a least weasel. However, if you set a Fenn trap you will soon be able to observe the freshly dead weasel in the trap; perhaps minus some of its limbs.
Abundance and Conservation Status
- New Zealand: occasional
- Canterbury: occasional
In New Zealand the weasel is considered to be scarce and difficult to study (King et al., 2001). Although it was introduced in larger numbers than the stoat, it has not thrived in New Zealand. It has been suggested that this is due to the lack of abundant prey for mustelids, especially voles (King, 1989). The least weasel is also categorised as uncommon in Canterbury; although it is definitely still considered a pest and there is ongoing work to reduce their numbers.
Perhaps the central point is the discussion on the abundance of the least weasel. It has already been discussed that this species has been implicated in the decline of many of New Zealand’s native bird, lizard, and invertebrate populations. An example is the Whittaker’s skink, which was reduced to one lone population after a combination of predation pressure and habitat loss. Many of New Zealand’s endemic species are all threatened by the presence of the least weasel, and this is what leads to its current classification as a pest.
New Zealand has been isolated for a significant length of time. This isolation was spent in the absence of selective pressure from mammalian predators. After the mustelid introduction to New Zealand many of the native species were put seriously at risk (King, 1989).
Significance for people
Modern Uses
The main ‘use’ of the least weasel by humans has been as a bio-control agent for rabbits. However, at least in New Zealand, this was an unwise use that is commonly regarded as a mistake (personal observation).
Summary
The weasel (Mustela nivalis) is one of New Zealand’s four wild terrestrial carnivores (others are the cat (Felis catus) stoat (Mustela erminea) and ferret (Mustela furo)). All of these have been introduced to New Zealand, and therefore do not occur naturally in these ecosystems.
The least weasel, as well as the ferret and stoat, were introduced from Europe in the 1880’s as a biological control agent; an attempt to control rabbit numbers. Rabbits were also introduced to New Zealand but as their population grew to such large numbers they quickly became serious agricultural pests. There followed pressure from farmers to introduce the ‘natural enemy’ of rabbits (the mustelids) in order to reduce numbers. Despite the objections of bird experts the stoat, weasel and ferret were all released onto farmland. Their subsequent natural spread has lead to their distribution across almost all of New Zealand (Gittleman, 1989).
The introduction of all three mustelid species to New Zealand has been described as ‘a spectacular failure’ (DOC, 1999). The results of this introduction have been catastrophic to many of New Zealand’s native species such as the weka, kiwi, skink and many other invertebrate and bird species (Sheffield & King, 1994).
The least weasel is now considered a pest throughout New Zealand and is subject to pest management strategies. On mainland New Zealand the aim of pest control is one of population control rather than eradication (Blair, 2002). There is currently no effective bio-control or toxin available, leaving trapping as the most effective option for reducing least weasel numbers (Blair, 2002).
References
Blair, (2002). Field guide to Mustelid trapping. Yellow eyed penguin trust, New Zealand.
Department of Conservation (1999). What shall we do about Ferrets? A public discussion document. Wellington, New Zealand.
Erxleben, J. C. B. (1777). Systema Regni Animlais. Impesis weygandianis.
Flannery, T. (1995). Mammals of the South West Pacific and Moluccan Islands. Reed books, Australia.
Gittleman, J. L. (1989). Carnivore behaviour, ecology and evolution. Cornell University Press, New York.
King, C. M., Flux, M., Innes, J. G. and Fitzgerald, B. M. (1996). Population biology of small mammals in Pureora forest park 1. Carnivores (Mustela erminea, M-furo, M-nivalis, and Felis catus). New Zealand Journal of Ecology 2:241–251.
King, C. M., Griffiths, K. and Murphy, E. C. (2001). Advances in New Zealand mammalogy 1990-2000: Stoat and weasel. Journal of the Royal Society of New Zealand, 31:165–183.
McDonald, R. A., Webbon, C. and Harris, S. (1998). The diet of stoats (Mustela erminea) and weasels (Mustela nivalis) in Great Britain. Journal of Zoology 252:363–371.
Sheffield, S. R. and King, C. M. (1994) Mustela nivalis. In: Best T. L., Cameron, G. N., Koopman, K. F. eds. Mammalian species 454. The American society of mammalologists. Pp. 1–10.
Search for Mustela nivalis in New Zealand science online
New Zealand Journal of Ecology articles containing Mustela nivalis.
Search NZ Royal Society journals for articles containing Mustela nivalis.
