Judas technique for feral goat control

Feral goats (Capra hircus) are an ecological problem worldwide, particularly in closed systems, such as islands, where native plants, animals and insects have evolved in the absence of large mammalian herbivores. Due to their grazing habits, goats alter plant species composition and hinder regeneration of vegetation. Goats can also cause soil erosion and reduce the habitat of native fauna that share a similar environmental niche.

Eradicating feral goats from island ecosystems is important to prevent extinctions and restore ecosystem function. For eradication to succeed, detecting and removing fragmented herds at low densities is essential during the final stages of control. The Judas goat technique can help detect these remaining goats.

“Judas goats betray their feral companions by revealing their location via a radio tracking device”

Judas goats are:

  1. captured
  2. desexed
  3. radio-collared
  4. strategically released to associate with and reveal the location of feral goats in the control area.

The underlying principle of the technique is that goats are averse to isolation and seek out other goats (or conspecifics) when isolated. The method is also used as a monitoring tool to confirm eradication1.

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Since its conception in the early 1980s, Judas goats have been increasingly and successfully used worldwide in Judas goat with radio collar fitted before release.

Since its conception in the early 1980s, Judas goats have been increasingly and successfully used worldwide in eradication programs even on large islands, such as Pinta Island (Galápagos), San Clemente Island (California), Lana’I Island (Hawaii) and Raoul Island (New Zealand)2.

The Judas goat method has been utilised in the eradication program on Kangaroo Island, South Australia, initiated in 2006 by the Kangaroo Island Natural Resources Management Board (KINRMB) and is now nearing completion (see Judas goat case study3). Both locally captured and imported feral goats from the mainland were used. Feral goats are better Judas animals than domesticated goats because they are more successful at searching for other mobs.

Although Judas goats are normally released in the final stages of an eradication campaign, on Kangaroo Island they were released in the early stages so they had time to become familiar with the terrain and location of resources and other goats. Imported Judas goats, in particular, rely on feral herds to teach them local survival knowledge, such as the location of watering points; shelter belts; caves and feeding grounds.

Young adult goats are generally preferred because older ones may suffer from capture stress and complications during transportation, and may not be as social and interactive as younger ones. All-white Judas goats were selected for the Kangaroo Island program because they contrasted with the local darker feral goats, and were easier to spot at a distance or in thick vegetation, which helped identification and prevented accidental shooting. Pre-release procedures on Kangaroo Island included acclimatisation and quarantine in a pen for at least one week for imported Judas goats to safeguard against importing weeds and prevent deaths from stress. Judas goats were also photographed, measured, weighed, ear tagged and fitted with a radio tracking or satellite collar.

To increase the efficiency of the Judas goat technique, all animals were sterilised at a local veterinary clinic4. Problems often arise with unsterilised Judas goats, such as pregnant females leaving associate goats to give birth and males inseminating remnant feral females. Sterilisation of female Judas goats induces a prolonged oestrus effect, by which they actively seek out bucks and increase travel distances or are more likely to associate with remnant goats. Female Judas goats sterilised with a fallopian tubular transection leads them into oestrus every 21 days during the breeding, which tends to occur all year round. Sterilised females remain social in contrast to intact, breeding females, which remain solitary for one month to a year after giving birth2. Males are sterilised by vasectomy, which has no negative effects on social and sexual behaviours. Interestingly, there was no significant difference between sterilised male and female Judas goats in locating feral goats on Kangaroo Island.

The number and density of Judas goats to be released should be determined by the distribution and density of the feral goats. If the ratio of Judas goats to feral goats is too high, the Judas goats may mob together and stop searching for other mobs. In this case, Judas goat mobs need to be reduced to encourage searching behaviour again. If Judas goats mob together for long periods of time, it is important to destroy them, particularly in the monitoring phase.

VHF collars are often used as they are more affordable and have longer battery life. They are particularly useful where Judas goats are released and checked regularly in easily accessible terrain. In large areas in inaccessible locations or thick vegetation, satellite/VHF collars may be better as Judas goats can be tracked and located on the Internet. This means regular checks are not necessary and control officers can initially locate Judas goats before control operations begin, saving time and resources.

Two techniques were used to destroy feral goats in the Kangaroo Island program. Originally, a hunting team of two to three people systematically walked sections of the Island’s western and north coast in a line, 50 m apart, within radio contact until numbers were low. The last few goats were destroyed by focusing on mobs with Judas goats instead of systematic coastal walk. Once eradication was thought to be complete and all known goats were eliminated, monitoring using Judas goats continued for a further 12 months. During this time Judas goats were regularly located to check they had not joined with feral goats, and the coastline was regularly searched for remaining animals. After 12 months, the Judas goats were destroyed and a further 12 months of searching for scats and tracks was undertaken. The community were also asked for sightings of goats.

The Judas goat technique has accelerated the number of successful goat eradications worldwide and contributed to recovery of endangered native plants.

Further information:

  1. Sharp T and Saunders G (2012). Standard Operating Procedure – Use of Judas Goats (GOA005). NSW Department of Primary Industries, Orange.

  2. Campbell KJ and Donlan CJ (2005). Feral goat eradications on islands. Conservation Biology 19(5):1362-1374.

  3. Campbell KJ, Baxter GS, Murray PJ, Coblentz BE, Donlan CJ and Carrion GV (2005). Increasing the efficacy of Judas goats by sterilisation and pregnancy termination. Wildlife Research 32:737-743.

  4. PestSmart Case Study: Feral goat eradication on Kangaroo Island (2013). PestSmart Toolkit publication, Invasive Animals CRC.

  5. PestSmart toolkit for feral goats: https://www.feral.org.au/pestsmart/feral-goats/

Invasive Animals Ltd has taken care to validate the accuracy of the information at the date of publication [April 2013]. This information has been prepared with care but it is provided “as is”, without warranty of any kind, to the extent permitted by law.

Author Centre for Invasive Species Solutions
Year 2013
Publisher Centre for Invasive Species Solutions
Pages 2
ISBN/ISSN PestSmart code: GTFS1
Control method Judas Technique
Region Australia - national
Documents PestSmart Factsheet: Judas technique for feral goat control [300 kb PDF]
Links PestSmart Toolkit for feral goats:   www.pestsmart.org.au/pest-animal-species/feral-goat/