Low food availability and high energy expenditure are not exclusively found in cold winter climates, but can also occur in hot, dry climates in summer. Desert ecosystems face a shortage of water in the dry season, which is accompanied by a shortage of food for many animals. Most plants do not grow, seed or fruit in this period, which increases the difficulty an animal has to find food. Avoiding adverse circumstances like high temperatures can also shorten the time available for foraging.Some dessert mammals have therefore evolved a state similar to hibernation called estivation (old spelling: aestivation).
What is estivation?
Unlike hibernation estivation occurs by definition only in summer. It is characterized by a lowered body temperature to approximately match the environment and a reduction in metabolic rate (MR). Because estivation only occurs in summer, the body temperature during estivation is much higher than during hibernation. It seems that no qualitative metabolic differences exist between hibernation and estivation, but instead that all differences in body temperature, rate of heat loss, duration of arousals and potential energy saving arise because of differences in ambient temperature (Wilz and Heldmaier, 2000).
Advantages of estivation for desert animals
Estivation saves energy
Estivation allows mammals to save energy when food availability during drought in arid regions is low. When in estivation, the MR can be greatly reduced, resulting in substantial energy saving. For example in the edible dormouse (Glis glis) oxygen consumption is reduced by 85.3 %. This allows the animal to survive though the period of drought on its fat reserves (Wilz and Heldmaier, 2000).
Estivation reduces water needs
Estivation allows mammals to save water because low metabolism reduces its need for water.
Reduced risk of predation
An additional advantage of the inactivity is the reduced risk of predation. Retreating to underground burrows entirely protects small mammals from most avian predators, i.e. birds of prey such as owls (Radzicki et al., 1999).
Disadvantages of estivation
Harmful effects of estivation
Occurrence of estivation is mostly limited to periods with low food availability. This indicates that there are also detrimental effects associate with it. Because of the great similarity with hibernation, it is likely that the same detrimental effects are also associated with estivation. Read our page about hibernation here.
When comparing the energy saved by estivation with the energy saved by hibernation, the savings are lower during estivation. It is not clear why this is the case, but it probably involved the duration of the dormancy bouts and the arousal characteristics. The energetically expensive arousals take place more often when ambient temperatures are higher (Park et al. 2000). Why the arousals take place more often at higher temperatures is not certain, but some say it could be possible that this reflects the elevated costs of maintaining the metabolic needs of the cells without the protecting and preserving effects of cold temperature.
The effects of estivation on the edible dormouse Glis glis
Here we present one representative studies that illustrate the dynamics of estivation in one mammalian species and the energy saved by employing it.
Wilz and Helmaier (2000) studied the energy saved during estivation by the edible dormouse Glis glis. The edible (or fat) dormouse Glis glis is a small (90 – 150 gram) nocturnal rodent belonging to the family of Gliridae. It is found in woodlands and shrub zones in central, southern and eastern Europe. Their main diet is fruit and seeds, but also insects and leaf-buds are eaten (Fietz et al. 2004).
In this experiment estivation was defined as torpor bouts lasting > 1 day in summer. The average minimum energy expenditure during estivation is 0.031 ml O2/g/h at ambient temperatures ranging from 15 °C to 20 °C. During normothermia oxygen consumption is in the range of 180 to 230 ml O2/g/h in this range of ambient temperatures.
To estimate the energy saving per day by employing estivation, a comparison of the Average Daily Metabolic Rate (ADMR) during days in normothermia and days with estivation has been made. The ADMR of normothermic dormouse was estimated at 5.1 L O2/d. At days in estivation without arousals, ADMR was approximately 0.75 L O2/d. This is an energy saving of 85.3 %. Because the costs of arousal can have a large influence on the possible energy saved by employing estivation, an estimation has been made of these costs. During arousal for estivation at ambient temperatures of 16 °C, oxygen consumption is approximately 0.9 ml O2/g/h. Arousal duration at this temperature averages approximately 2 hours. One arousal bouts therefore costs 1.8 ml O2/g, for a dormouse of 130 g this is approximately 0.234 L O2. Estivation bouts lasted between 69 and 109 h. Taking this into account, ADMR during an estivation bout of 72 hours including the cost of arousal can be estimated to be approximately 0.828 L O2/d. This is an every saving of 83.8 % compared to normothermia. This indicates that arousal costs barely affect the amount of energy saved by employing estivation.
By employing estivation the edible dormouse Glis glis realizes a substantial energy saving compared to remaining euthermic.