We know that many species of tortoises do aestivate (summer dormancy) over the heat of summer, but there has long been uncertainly as to precisely which environmental factors 'trigger' this response. High temperatures? Lack of food? Lack of water? Or all three? We decided to take a closer look and gather direct observations from several contrasting sites in the same general area so that, as far as possible, we could isolate the influence of these various factors.
We looked at three entirely 'natural sites' with little or no nearby human activity and with no artificially introduced modifying factors. We then looked at other areas that directly bordered agricultural and domestic sites where there was enhanced vegetation growth due to irrigation. We also gathered reports from local observers who made observations of a) when tortoises were active and b) the nature of the activity (feeding, drinking, etc.).
These observations took place throughout July, by which point the tortoises in the 'natural' sites were well-established in their aestivation cycle. We recently released a new video that gives a good overview of these site, and discusses the hypometabolic state that aestivation involves:
The very first thing we noted was that during this period ground surface temperatures in this area (Almeria, Southern Spain) attained incredibly high levels on an almost daily basis: 60 degrees Celsius (140 degrees Fahrenheit). This is far in excess of the critical thermal maxima for Testudo graeca graeca. If exposed to such temperatures for more than a few minutes fatal overheating would result. It is therefore no surprise that tortoises need a strategy to escape such conditions. There is no source of fresh water available at all (with the exception of very rare episodes of rain), and there is also little to no suitable graze available. The area is incredibly dry and barren.
Tortoises increasingly seek shady retreats as ambient (and surface) temperatures rise, but eventually, these are insufficient and the tortoises must bury deeper into the substrate to escape potentially fatal overheating.
If we look at temperature readings from a data-logger with the probe recording surface temperatures in an exposed position, these are typical results.
Temperatures by early afternoon regularly attain 60 Celsius. Overnight temperatures are typically 20-22 Celsius. It is important to note that these overnight temperatures are in no way typical of overnight temperatures at other times of year when tortoises are active. These can reach surprisingly low levels, just above, or even slightly below, freezing.
At the beginning of this period, while tortoises in the natural study sites had ceased all activity, tortoises continued to be active in the early mornings (up until circa 9.30 am when ambient temperatures began to surpass 33 Celsius) in those areas where human activity provided a source of both food and - sometimes - water. Tortoises were observed, for example, drinking from leaking irrigation pipes, and grazing upon 'weeds' growing in cultivated margins.
As temperatures increased as the month progressed, diurnal activity even in these areas activity finally ceased. It is not hard to see why, as when we look at logger data recorded within the deep shade and shelter of a clump of Esparto grass we again find exceptionally high temperatures, although considerably lower than those in a fully exposed position. The peak temperatures recorded here approach 50 Celsius, but more typically hover just above 40 Celsius. Again, these are temperatures well outside of the tolerance of active, exposed tortoises. Tortoises respond by digging down and covering themselves with a depth of substrate. This is very effective at offering protection from these inhospitable conditions as discussed in the video.
We simultaneously recorded relative humidity in these shaded Esparto grass retreats, and we note that diring the day RH is incredibly low (11.9% was the minimum during this measurement cycle). Averages during the day are typically between 20-30%. Overnight, of course, as air temperatures fall, RH increases. It is very important to understand exactly what is being measured here, and why this happens.
The observer's results are very interesting. These may be summarised as follows:
From late May through mid June tortoises continued to approach irrigated areas that offered a source of food and water, even as tortoises in undisturbed areas were 'shutting down' and ceasing both feeding and activity, transitioning to continual aestivation.
Reports included tortoises approaching dripping garden taps to drink, and grazing upon wild flowers close to irrigated fields or golf courses. These activities almost exclusively occurred in the early mornings, before ambient temperatures escalated, though a few observations were made in the mid-evening period as temperatures began to fall.
As diurnal ambient temperatures regularly began to surpass 34 Celsius, however, activity even in these areas dramatically reduced, and then ceased entirely during daylight hours, despite a source of food and water remaining available.
No activity whatever was recorded at any site once daytime temperatures exceeded 35 Celsius, and tortoises entirely ceased daily appearences, even very early in the mornings. However, there are some indications that nocturnal activity might occur at such times provided water and food remains available.
UPDATE (August 3 2024): We have now established that where a source of fresh food and/or water is avaiable, fully nocturnal activity does occur in some locations. In this situation the tortoises bury down in a shaded area during the day, but emerge at various times during total darkness or in the dawn twilight period to eat or drink. This strongly suggests that even in conditions of extreme temperatures (one one day when this was recorded ambient air temperatures were 36 Celsius) provided that they can find a sufficiently sheltered 'hide' to escape the heat of the day, this is sufficient to overcome the need to fully aestivate.
Conclusions:
The primary 'driver' for aestivation appears to be excessively high temperatures combined with a lack of food and water.
Even where fresh food and a source of water remains available ,however, once temperatures cross a certain threshold tortoise activity ceases almost entirely during daylight hours, but as noted above, some nocturnal or twilight activity may take place.
Shaded retreats alone do not provide adequate or complete protection from such inhospitable conditions, burying down, or burrowing (in some species) is required in addition.
Aestivation may be temporarily interrupted by a rare summer thurderstorm that delivers heavy and sustained rains, during which tortoises might emerge for a brief period before burying down again. This can prompt fully nocturnal activity, with tortoises emerging to drink in the early hours of the morning in total darkness.
Such events might occur no more than once over the summer season, and in some years, not at all.
Full emergence from aestivation typically occurs in mid-late September provided that the seasonal rains associated with autumn arrive.
Nests laid in late April-May simultaneously hatch at this time.
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(c) 2024 A. C. Highfield/Tortoise Trust
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