Humidity in Testudo graeca graeca habitats

Weisner and Iben assert that “faster growing hatchlings live under the cover of grass... providing a higher level of ambient humidity”. In fact, in many Testudo habitats there is little or no grass present. In areas where it is, for example in Southern Spain and parts of North Africa where Esparto grass (Stipa tenacissima or Lygeum spartum) is an important feature of Testudo graeca graeca habitats measurements recorded over a 12 month cycle by the present author using a series of SL52TH temperature and relative humidity data loggers revealed that the typical relative humidity within clumps of grass occupied by tortoises infrequently exceeded 50% during daytimes and was typically below 40%. Levels of 30% were common. The only instances of very high humidity recorded invariably coincided with rainfall or immediately following episodes of precipitation. In total, more than 18,000 data points were recorded and in only 2% of these did relative humidity exceed 80%. There is a rise in moisture overnight, usually at approximately 3-4 am as dew begins to form. This persists for a very short time, and usually peaks at around 65% RH.

Extensive observations were made of wild juvenile and hatchling Testudo graeca graeca activity at various sites in Murcia and Almeria, Southern Spain, and miniature data loggers were temporarily affixed to their carapaces to record changes in both carapace surface temperature and relative humidity every 30 seconds. The results consistently indicate that juveniles are frequently exposed to ambient relative humidities as low as 20% for extended periods during browsing and basking. Tortoises were monitored over a 12 week period in early spring 2010 and at no time were they exposed to levels of relative humidity approaching 100% except during or immediately following rare episodes of precipitation as a consequence of thunderstorms. This data clearly demonstrates that while it is true that clumps of Esparto grass do provide a “higher” level of relative humidity microclimate that is exploited by tortoises, it is typically well below <50% RH deep within the grass clump compared to <30% in free air, which is a very long way from the levels of 90% to 100% RH suggested by some authors.

These images show the habitat studied.

Recording relative humidity next to a juvenile Testudo graeca graeca in early Spring 2010.

The same location in context.

Planting a miniature data logger in the microclimate used by juveniles.

Typical results of logger over a 14-day cycle in this habitat. The peak in RH (red line) to 90% coincided with an early summer thunderstorm with torrential rain.
There is typically an overnight rise in RH, normally peaking at circa 65%. During the day, RH averages circa 35%.