Tortoises, like crocodiles, have been wandering this planet relatively unchanged since prehistoric times – far longer than us hominids. They tend to inhabit the hotter countries where vegetation is sparse, and have adapted to browsing intermittently whenever they find suitable food plants. In these countries the winters are short and cold, which is why some species hibernate. In the UK, we have mild autumns and irregular temperatures during the winter, which can precipitate periods of hibernation that are too long to allow survival, especially if tortoises are not physically prepared. In the natural state hibernating tortoises will bury themselves about a foot underground (sufficiently close to the surface to allow some oxygen to percolate, but away from predators and the extreme effects of the cold weather), and their metabolism will slow to almost zero but, crucially, there will besome energy (i.e. food reserves) used up over the ensuing months.
In the wild.
Tortoises that live in tropical climates never hibernate (they will die if they are forced to). Certain species that inhabit Mediterranean countries and Asia Minor do hibernate, although it is not necessary for them to do so in this country. Before deciding to allow them to hibernate it is essential to identify correctly which species of tortoise you own: examples hibernators kept commonly as pets include Hermann’s tortoise (Testudo hermanni), Horsfield’s (T. horsfieldi), and some of the Spur-thighed group (T. graeca).
Physical fitness. In order to survive hibernation tortoises must have built up sufficient body fat reserves, which assists in the provision of energy and the storage of water and vitamins that are required for the ‘long haul’. This food reserve must be in place by mid-August when the biological clock has started ticking, and available food takes longer to digest. There must be no food still in the gut when hibernation commences because it will decay and cause disease. It takes about 4-6 weeks for it to pass through at this time of year. Tortoises must be adequately hydrated beforehand however they do not seem to want to drink – the appropriate procedure is to place them in a small lukewarm bath (on a plank angled so that the head remains above water level); this encourages them to void urine and faeces, thereby emptying their system, and they may sip as well. A special formula (the Jackson ratio) has been devised to assist with checking whether one’s tortoise has sufficient body reserves to survive hibernation, but it is important that this is done correctly. In addition to the foregoing, attention should be paid to the effects of parasitism (worms) and other aspects of general health, including eyes (swelling, discharge), mouth (inflammation, blood spots, cheesy discharge), tail (smell, unusual excretion), legs (unusual swellings) and general demeanour, before considering hibernation.
Keeping them awake.
It should be emphasised that it is not necessary for tortoises to hibernate in this country and they can overwinter in a vivarium (also useful for bad weather days at other times of the year), which can be set at appropriate temperatures. During the day temperatures should range from 25-32degC, and allowing for a 5-7degC drop at night. Maximum-minimum thermometers should be positioned at either end and monitored continuously, and a good ultra-violet light source should be switched on for 12 hours every day (with no glass in between to filter important rays).
Hibernation period. Juveniles should not undergo hibernation, and the young tortoise’s first hibernation should be short (about 6 weeks), thereafter periods up to 3 months only should be allowed. People do push it longer but it is not recommended. Tortoises normally lose about 1% of their body weight per month whilst hibernating and it is perfectly acceptable, and sensible, to weigh them at intervals during this period; if weight loss is excessive, they must be brought out of hibernation. The temperature must be between 5-8degC: too low and they will freeze to death, too high and the body may become more active and dehydration may result (if for instance, during a check, you notice that the bladder has been emptied, the tortoise must be woken up). One of the frequently occurring problems of very low temperatures is blindness resulting from changes in water density in the eyes.
Preparation for hibernation. This is known as the cooling-off period. For the first week the tortoise should be kept at normal vivarium temperatures and enjoy daily bathing and feeding; during the second fortnight the tortoise should be brought to room temperature, feeding should stop but daily bathing should continue, and for the next two weeks bathing should stop and the tortoise kept at outside temperatures (i.e. in garage or similar). Once it has been weighed it should be placed on a plastic tray, or cardboard box and put into the hibernation unit. Hibernation units can be created using polystyrene insulation blocks, which will slow down the temperature differential (but not stop it), so mild winters may ruin the process. Dedicated fridges are a perfectly good alternative, but whatever system is used, reliable maximum-minimum thermometers must be in place. During this period, weekly checks are advised when this also allows for air exchange.
Post hibernation. Bring the vivarium up to the appropriate temperature, lukewarm bath daily and offer food the next day. Don’t expect your tortoise to eat straight away, but it should be active and eating within a fortnight, if not bring it in for a veterinary check-up.