Black or white? The effect of tank background on the welfare of laboratory frogs (Xenopus laevis)

The African clawed frog (Xenopus laevis) is a common model organism for developmental and genetic research, but few studies have investigated its welfare needs and current husbandry guidelines aren’t supported by quantitative evidence.

The visual environment is particularly important for species such as X. laevis, because they are adapted to live in murky waters and use cryptic camouflage as a fundamental defence mechanism, where their mottled green and brown skin provides disguise from predators. In contrast to conditions in the wild, Xenopus in laboratories are housed in clear water that is regularly cleaned to maintain frog health and allow visual inspection.

As a compromise, darkened or opaque tank sides and floors have been suggested as a means of better replicating the wild environment in captivity and providing the animals with a greater sense of security. However, until now no empirical evidence existed to confirm this, and many laboratories continue to use clear or white tanks.

A new study recently published in Applied Animal Behaviour Science has demonstrated that laboratory housed X. laevis kept in tanks with dark (‘ecologically relevant’) backgrounds show fewer signs of stress compared with those kept in tanks with white backgrounds. The work was led by Dr Lottie Hosie from the University of Chester and funded by the NC3Rs.

The researchers assessed the frogs’ physiology non-invasively, by measuring levels of corticosterone (amphibian stress hormone) in the water. The team also recorded the behaviour of the frogs, including atypical behaviours that could be signs of negative welfare (such as walling - persistent swimming along a tank wall and scrabbling at the wall with front legs, which can lead to sores on the snout). They found that a white background produced higher corticosterone release rates in females, a greater proportion of atypical locomotion behaviours in both sexes, and a greater drop in body mass, compared to a black background.

These findings show for the first time that tank background is an important aspect of frog housing and welfare. This work is particularly important as X. laevis are long lived in captivity and may be housed in the same environment for many years, extending the impacts that refinements to captive housing can have on individual welfare.