Animals – and their eggs – are over two-thirds water. However, many species inhabit regions in which water is scarce, at least seasonally. This makes water a precious commodity, particularly during reproduction, when it is time to invest in the next generation. To ensure the best start in life for their young, animals often aim to synchronise egg hatching with the start of the wet season. But this is a double-edged sword as egg development has to begin towards the end of the dry season, when times are most challenging for the mother. As a result, she may have to get resourceful.
To investigate how seasonal breeders cope with egg production during periods of water scarcity, George Brusch from Arizona State University, USA, and his colleagues studied female Children's pythons, a species adapted to the wet–dry climate of northern Australia. The team monitored the snakes throughout their 3 week gestation period, during which egg development takes place. They deprived half of the egg-laden pythons of water, while providing the remaining snakes with an unrestricted water supply, and compared these snakes with a cohort of females that were not preparing to reproduce, half of which were also water deprived.
To assess dehydration, Brusch and colleagues took blood samples and measured osmolality, which represents the concentration of dissolved materials (solutes, such as salts). An increased osmolality means an increased concentration of solutes in the blood, which is a hallmark of dehydration. They saw that both reproduction and water deprivation independently increased blood osmolality and that water-deprived egg-laden pythons became the most dehydrated. This confirmed that water limitations are indeed a problem for these snakes during reproduction.
The team also measured the width – as an index of muscle size – and body mass of the snakes over the duration of their study. According to both metrics, dehydrated animals withered away more than animals with access to water, providing evidence that muscle was being broken down. Protein breakdown provides an excellent source of water, so this represents an efficient reallocation of resources. Earlier work had already established that snakes and other animals break down protein to provide nutrients during reproduction, but this was the first time it had been linked to water provision.
Finally, the researchers asked whether the snakes’ muscular sacrifice was enough to compensate fully for their lack of access to water. They found that the females that were deprived of water laid just as many eggs as those with water (about 12 in each case), but this wasn't surprising because the number of eggs carried by each female was probably determined before the experimental dehydration regime started. More importantly, however, the water-deprived females laid slightly lighter eggs than those provided with water. This may ultimately impact embryonic growth and hatchling performance, although the extent of this was not determined in the current study.
Muscle loss during reproduction has been documented from insects to birds and mammals, and although some of these animals inhabit dry environments, muscle had not been previously considered as a potential water source at times of drought. Of course, muscle loss is not trivial. In snakes, as well as other species, it is known that muscle loss during reproduction can negatively impact an animal's ability to move. Thus, it appears that successful reproduction may rely on a tricky compromise between resources for mother and egg, and this compromise is strongly influenced by the prevailing environmental conditions.