There's nothing like a nice tall, cool glass of water on a hot summer's day, but for animals living in hot and parched deserts, free-standing water is hard to come by and these animals have to find other ways to keep hydrated. Some reduce movement altogether to save water lost from evaporation, and instead fend off dehydration by relying on fluid stored in their urinary bladders. Others are able to remain active and extract water from their diet. Given that most meaty meals contain up to 70% water, Christian Wright, along with his supervisor, Dale DeNardo, and colleague Marin Jackson, from Arizona State University, USA, wondered whether the carnivorous Gila monster, Heloderma suspectum, could rely solely on its juicy diet of vertebrate nestlings and eggs for all its water needs during the dry season (p. 1439).
Mimicking pre-summer conditions found in the wild, the team first allowed 12 adult Gila monsters to drink as much water as they wanted. As the team already knew that Gila monsters are able to use water stored in their urinary bladders, they first drained the Gila monsters' bladders before simulating the hot and arid summer conditions, keeping them at 30°C for the duration of the experiment. By taking weekly blood samples and measuring osmolality (a measurement of the ratio of solutes to water), the team could determine how dehydrated the lizards were. Upon reaching a mildly dehydrated state, some of the thirsty lizards were presented with a tempting juicy juvenile rat, in the hope that this would slow down their dehydration. However, they dehydrated just as quickly as their unfed friends, and after 32.5 days all the lizards had reached extreme dehydration – a state that they experience naturally by the end of the seasonal drought. Even the provision of another meal at this stage did not significantly rehydrate them.
Next, the team repeated the experiment adding in an extra meal at the beginning. This time, the team took blood samples 6 h, 24 h and 48 h after meal consumption to check for immediate changes in osmolality. While the extra meal allowed the lizards to last an extra 10 days before becoming extremely dehydrated, the team still found that water from food was not sufficient for the lizards' water needs. In fact, blood osmolality increased after feeding, suggesting that significant amounts of water were used during digestion. The water gained from digestion did not recuperate these initial water costs. However, when the team presented the parched lizards with 42 ml of water – the equivalent of the amount found in their final meal – osmolality significantly decreased, improving the animals' hydration. It is likely that despite their water-rich, meaty diets, Gila monsters rely solely on the water stored in their bladders after pre-summer rains, without which they dehydrate quickly.