I thumb my nose at Darwinism

Carnac Island has no free water for most of the year. Rainfall collects in puddles on rocks but the water is highly saline and no good to drink. Clever island tigers have learned to drink water directly off their own bodies to minimise the intake of salt. When I did this simple experiment in 2004 this behaviour had only been documented on one other occasion in Bothrops.

A tale of two tiger snakes

Ecophysiological comparisons of the two conspecific populations of the tiger snakes in Western Australia shows that semi-arid Carnac Island (CI) snakes respond opportunistically to water when it is available, and the stimulus for this response appears to be the detection of rainfall. Carnac Island snakes can also discriminate salt water from fresh water, drinking only the latter, and they employ a range of water collecting tactics, such as the ability to drink water droplets from various surfaces, including their own body surface. The same behaviour was not observed for Herdsman Lake (HL) snakes, but could potentially be expressed under the appropriate environmental conditions.

Rates of water loss vary between the populations and HL snakes lose water at a higher rate than CI snakes. Total evaporative water loss (EWL) comprises mainly cutaneous water loss, and the epidermis of CI snakes is less permeable to water than the epidermis of HL snakes. There is a strong positive and linear relationship between temperature and total EWL loss in CI snakes and this, coupled with burrow and surface temperature data from the field, has revealed that an average sized male CI snake (approx. 350 g) can save approximately 65 ml of water over 45 days in summer.

Dehydration elicits thermal depression in snakes from both populations. However, this behaviour is more pronounced in semi-arid CI snakes. Carnac Island snakes suffer some degree of seasonal dehydration in the field, causing elevated concentrations of plasma [Na+] over summer (Ladyman and Bradshaw, 2003). Hypernatraemia causes thermal depression in both populations, with a weak negative relationship between plasma [Na+] and temperature selection that was significant for CI snakes and close to significance for HL snakes. Nevertheless, HL snakes do not appear to experience dehydration or elevated circulating electrolytes in the field during summer, which is expected for a snake inhabiting a perennial freshwater swamp. In CI and HL snakes, laboratory-induced hypernatraemia elevates circulating concentrations of the neuropeptide arginine vasotocin (AVT). Despite the positive correlation between AVT and both plasma [Na+] and plasma osmolality for laboratory snakes, field samples from CI snakes indicate that circulating levels of AVT may be influenced more by plasma osmolality than plasma [Na+]. Whatever the case, CI snakes injected with AVT also showed thermal depression, indicating that many physiological variables can influence temperature selection, rather than one variable alone (Ladyman et al., 2006).

Body mass (BM) increased and plasma electrolyte levels decreased following cloacal enema, indicating that the snakes could rehydrate from fluid in the colon. Electrolyte loading and water loading increased body mass, and observations of fluid storage in the colon of dehydrated and salt-loaded snakes suggest that this BM increase is due to fluid storage in this region of the digestive tract. Concentrations of electrolytes (hypoosmotic to plasma [Na+]) in the urine drawn out of excreter were low. This creates a passive gradient for water reabsorption into the body, suggesting that both island and mainland tiger snakes are capable of colonic reabsorption.

All of the traits examined indicated physiological or behavioural modifications of the fundamental reptilian ‘bauplan’ to economise water handling for CI snakes. Several of these traits were also available to HL snakes, though it appears that in the absence of the appropriate environmental conditions (i.e. aridity) these traits are not expressed. Nevertheless, possession of these traits allows at least semi-arid island dwelling snakes to survive in an environment lacking in a major resource: water.