Print Shortlink

Rut Gauge and the Fox Heat Cycle

Practical Operation

The Norwegian manufactured SiLi3 rut gauge has proved to be an excellent aid in checking heat in the fox, especially in connection with artificial insemination, provide it is used properly. Results may be poor with faulty or careless operation.

It is absolutely essential that the heat detector is kept clean. Slipshod cleaning and careless/incorrect use may result in serious injury and infections of the vagina and/or uterus (womb) and even fatal metritis (infection of the womb). Such infections if the sexual organs cause the vixen unnecessary suffering, as well as reducing the number of cubs born (many empty vixens). We wish, in this blunt manner, to emphasize that correct cleaning and operating procedures must be followed.

Remember to clean the heat detector

  • The heat detector (probe) shall be washed and disinfected between each vixen, and must be kept warm. (use the thermosystem and the recommended detergent and disinfectant agents).
  • The thermcontainer and cleaning system must be emptied and cleaned after each spell of work. Similarly, the rings and coupling system on the probe must be cleaned and dried.
  • The detergent and disinfecting fluids must be changed each day (after each spell of work).

Checking the vixen

The fox-vixen should be caught in a calm manner as possible. As the vixen is lifted out of her cage, make sure that the tail (vulva) is lowest so that urine can run out freely should the vixen urinate.

Clinical Assessment of the Vulva

x = slight (initial) swelling

xx = obvious swelling

xx\ = marked swelling and firmness

xxx = max. swelling, firm, darkening. Vixen begins to stand for the male.

xxxL = begins to soften, dark

External signs of a fox in heat - swelling of the vulva

External signs of a fox in heat – swelling of the vulva

Introduction of the recording probe

The vulva must be fairly well swollen and firm ( at least xx\) before starting to record. Prior to this stage, the genital opening and vagina are tight, and the introduction of the probe is not difficult, but may also injure the mucous membrane (especially in young females).

If the area around the genital opening is soiled, the vulva and surrounding area must be thoroughly wiped of (with a clean, dry paper) before the probe is inserted.

The vixen is held head downwards using the restrainer. The measuring probe is inserted at an angel of 45 degrees to the line of the back (about 1/4 of the probe), and then strained and pushed gently (without forcing) in as far as it will go, parallel to the line of the back. the trigger switch is pressed in, the display is read and noted down on the cage card (+ date).

NB! The measuring probe must not be used to forcibly, “open up” the vulva and vagina of the vixen!


  1. Do not begin to record before the vulva is properly swollen (xx\ – xxx).
  2. Readings must be evaluated in conjunction with other signs of heat (behavior/vulva). Do not rely blindly on measurements alone.
  3. If in, double inseminate/mate twice at an interval of 48 hours.
  4. Adult animals may often have a longer heat period than cubs/young animals. Late in the mating season, the period is shorter in all animals.
  5. It is the shape of the curve that counts, not so much the actual measured values. Maximum values may vary quite considerably from vixen to vixen (broken line in the figures *).
  6. Remember that we are measuring electrical resistance (not conductivity). If there is no connection between electrodes (rings) via mucus/mucous membrane = max. reading (no current passes between the rings). This means that if readings vary during recording, it is the lowest value which is the correct one. Urine (i.e. salt solution) is a good conductor of electricity – exerts little resistance – and may result in an abnormal fall in the reading in relation to external signs of heat and previous recording(s).
  7. Disregard variations in the display value of less than 50 ohms.
  8. As a biological material is involved, there will always be “exceptions to the rule,” and we may experience inexplicable variations in some cases.
  9. Maximum reading = DAY 0. The first day on which a fall in the measured value is registered = DAY 1, etc.
  10. When referring to blue fox, the following types are included:
  • Blue fox types: All mutations of species Alopex lagopus (Arctic fox, blue, shadow, white etc.)
  • Silver fox types: All mutations of the species Vulpes vulpes (red fox, silver, platinum etc.)

Reading and Assessment of the Right Time

Correct time for mating/insemination

Examples of Curve Shape

Assessment/Interpretation of different types of measurement curves (fox)

The arrows and shaded areas indicate the correct time.

In figures A-C P/I  P = Mating, I = Insemination

A. “Normal curve” for approx. 70% of vixens

  • Blue fox types:
    • Young animals – late on the first day, early on the second day
    • Adult animals – during the second day
  • Silver fox types – during the first day (young animals early, adult animals late)

B. “Normal curve” for approx. 20% of vixens

Slow development, but obvious change nevertheless: approx. one day later mating/insemination than in example A.

  • Blue fox types:
    • Cubs: Day 2
    • Adults: Late day 2/ day 3
  • Silver fox types: Late day 1/day 2

Check after about 48 hours. If you still get a relatively high measurement, repeat mating/insemination.

C. “Normal curve” for approx. 10% of vixens

Curve top flattened (plateau) for several days (or above 1000 ohms for several days using the old heat detector).

Mate/inseminate on the third day of the plateau. Check after 4-8 hours, if still at the maximum or above 200 ohms, repeat mating/insemination. Applies to both silver and blue types.

D. “False heat” (false 1 true 2) approx. 10-15% of all vixens

Most common early on in the mating season. Vixens which show indefinite external signs of heat and give indefinite readings (rarely high values), but which permit mating once (rarely standing for the male for two days running) may have a false heat. Such vixens must be checked after 5-7 days and will then often exhibit normal heat which is assessed as for A-C. The first heat was then false, and no ovulation occurred.

Ideal Curves

False Heat

False heat may occur. However, vulval swelling rarely develops fully. The vixen may permit mating, though a repeat is rarely successful. Readings rarely exceed 300 ohms. Such uncertain vixens should always be checked again about 5-7 days later as they may then be undergoing a new true heat. (The cubs of such uncertain vixens should not be kept for breeding stock).

False readings may arise with urine in the vagina. This produces a large abnormal fall in the resistance of the vaginal mucus. Inexplicable elevation or reduction in resistance may also occur in connection with severe stress or excitement in the bitch. The presence of petroleum jelly (Vaseline) in the vagina may result in very high values.

If in doubt, always carry out repeat measurements after 6-12 hours. (Daily fluctuations of about 50 ohms may occur, but are not of significance).

Right Time

Assumed best mating times: Blue fox 1-3 days after peak reading, silver fox 0-2 days after peak reading. In other words, shortly after a marked fall in recorded values is seen, day 1 = the day marked fall (at least 50 ohms) from peak value is measured, provided readings are taken every day during the relevant period. If recorded values and clinical signs indicate that the vixen is still strongly on heat, mating/insemination should be carried out every other day until the heat period is over (readings less than 250 ohms). When frozen semen is used, insemination should be performed every day.

Vet. Jan A. Fougner. Rut Gauge. A/S Lima: Norway. Norwegian Fur Breeders Association.