The 2003 Harvest Mouse Release

This article first appeared in Zoo Life, Chester Zoo's magazine for members and adopters

Following the big success of last years pilot project to establish a release protocol for Harvest Mice Micromys minutus, we decided to take the project into a much larger second stage in 2003. Early in 2003 we carried out a series of live-mammal trapping sessions in the area in and around the 2002 release site, where 130 mice had been released last May. We were encouraged to find harvest mice, offspring of those we released last year, that had successfully over wintered and were in excellent condition. Given that we know that this is a species that is subject to huge natural winter mortality rates, this success prompted us to undertake an even more exciting project this year, which enabled us to encompass many of the lessons learned during 2002.

Just a reminder that the Harvest Mouse is the UK`s smallest rodent, weighing in at just 5 - 6 grams. Its very size poses enormous problems for researchers who have to find ways of following these minute creatures to establish how they survive, where they go, and what habitat they prefer. This information will help to establish the ideal conditions in which well-informed re-introduction programmes can be carried out. Needles and haystacks spring to mind but the team at Chester have taken up that challenge!

During the week commencing 9 June 2003, at the culmination of a huge amount of planning and a multi-disciplinary "team effort", 270 Harvest Mice, 50% males, and 50% females, were released from our stock onto the Zoo's farmland adjacent to the Shropshire Union Canal. Once again, the animals had not only been bred in the Zoo - at the off-show Harvest Mouse unit run by Mammal Keeper, Felicity Fair, but by several volunteers from as far away as Huddersfield and Windsor! Those bred by volunteers had been returned to the Zoo to join the Zoo-bred animals in time for a thorough veterinary screening carried out by the Zoo's vets prior to release. Each was in perfect condition thanks to significant husbandry efforts made during the preparation process. The mice were between 8 weeks and 12 months old, and every individual was micro-chipped so that we know their lineage, have exact information about where they were released, and can monitor their progress once they have been released.

On the 9 June we individually placed 135 mice out into secure mesh "soft release" enclosures which were then staked into place at allocated points around the release site. One of the significant findings from last years release was that we needed to design much better soft-release enclosures for this year if we were going to obtain a more accurate idea as to whether soft-release is beneficial, in comparison to hard-release. Paul Howse, the Twilight Zone's Team Leader was allocated this daunting (because Harvest Mice seem able to escape from almost anything!) task. He worked wonders, and thankfully, our 135 new soft-release enclosures worked brilliantly, the aim being to provide food, water, shelter, protection from predation and also the hope of establishing a small home-range prior to actual release 48 hours later. During the evening of the 11 June, a further 135 harvest mice were added through a hard-release, whereby animals are simply placed in suitable habitat, again in allocated places on the same site. By "joining" those already placed in position in their soft-release cages two days earlier, we effectively released animals in clusters of two males and two females on each of 67 release points covering a two hectares area. At the same time, the soft-release cages were "loosened" to allow animals to move out of them if they wished. It was like a military exercise! A team of nine staff and volunteers choreographed our move from one end of the site to the other carefully releasing animals in a set order to avoid the risk of damaging our precious habitat and of treading on animals we had just released!

Perhaps the most exciting, and certainly scientifically ground-breaking aspect of this years release has been our decision to radio-collar 20 individuals. This has never been attempted before, anywhere in the world, with such a tiny creature. We commissioned Biotrack in Dorset to make the minute collars for us, having thoroughly investigated any animal welfare issues involved with collar use.

They were able to produce a collar weighing just .35 of one gram, and with a fair amount of "tweaking" by extending the audio-pulse intervals used to track the animals, we were given a battery life of about 21 days. Collaring was carried out by Zoo Vet Shan Siah who anaesthetised the mice whilst MSc Student, Leon Barnes (who is a dentist in his "other life"!) attached the collars, using tiny instruments designed for orthodontic work! These tiny collars need soldering to activate them, and a willing member of our Maintenance Team, the Mechanic Keith Bragan, was roped in, with some trepidation, as he is used to welding rather larger subjects, to do the honours. Truly a joint effort! A team of three well-trained, committed, "trackers", Sarah Bird from Botanics, Eleanor Condon from our Research Department and Leon Barnes, an MSc student registered with the Zoo, then took over. With just the very occasional help from Roy Leigh, of the Cheshire Mammal Group, and Paul Howse from the Twilight Zone, Sarah, Eleanor and Leon took on the huge and complicated task of tracking these animals, over a 24 hour-a-day period for 20 days, in a series of shifts. The aim was to "track" each mouse twice during each of the four six-hour shifts, thus obtaining 8 "hits" per mouse, per 24 hour period, and to record its location, and habitat choice.

To maintain consistency, ten of the radio-collared animals (five male and five female) were placed in soft-release enclosures, and tracking started immediately. This may sound a little bizarre as technically we should know where they are if they are in a secure soft-release enclosure, but by a stunning piece of good luck, one of these collared animals found an escape route. We were able to track its behaviour with regard to its use of the soft-release enclosure and noted that he kept coming back to base from day one, and travelled a little further on each foray out. From this mouse alone we have confirmed research that was otherwise speculative. The remaining ten (5.5) radio-collared mice were hard-released with the others two days later and tracking started immediately. It will be extremely interesting to see what both the radio-tracking and all the other post-release monitoring data reveals when it has been analysed.

Our first post-release monitoring session of all the mice took place over three days of live trapping during the last week of June and we were thrilled by the number of pregnant Harvest Mice we were already re-capturing at that stage. We know that none of our animals were pregnant at the time of release so we now know that at least they were "finding each other" after they had been released!

Radio tracking was expensive and exhausting, but so worthwhile. The data we obtain from this, specifically about survival, dispersal and habitat choice, will not only be far more accurate than that achieved by any other method, but will very significantly impact on release and re-introduction research around the world. Scientists eagerly await our findings and Chester should be very proud of our part in such important re-introduction research.