Snakes don't shrink, but 'shrinkage' is an almost inevitable outcome of measurement error by the experimenters

The recent report of Galapagos marine iguanas showing bidirectional growth, i.e. decreasing in body size up to 20% of their initial size during periods of reduced food availability, has induced to speculate whether these 'shrinkages' are adaptive or not, and whether they are widespread or not in squamate reptiles living in seasonally variable environments. A first case-test with Australian water pythons provided no evidence of 'shrinkage'. I tested the same issue by using my mark-recapture data of a species-rich community of snakes (16 species) from the tropical forests of southern Nigeria, where the food availability varies seasonally (with productivity peaks during the wet months). I attributed a 'measurability' score to each species in relation to the easiness of their handling, taking into consideration a range of factors including the snake's body size and weight (hence the body vigour, as the large vigorous snakes are more difficult to measure correctly than small non-vigorous species), the temperament (aggressive species are more difficult to measure correctly than placid species), and the dangerousness (highly venomous snakes are more difficult to measure correctly than non-venomous species). Out of 6431 recapture data, 2.8% were cases of shrinkage, and 14 out of 16 species examined had at least a single case of shrinkage (the percentage of shrinkage cases was not significantly different among terrestrial, semi-aquatic and arboreal species). The percentage of shrinkage cases varied significantly in relation to the 'measurability' score of the single species (producing an exponential growth curve for the relationship between the two variables). The mean SVL reduction at the second capture event compared to the first (in percent terms) was trivial compared to shrinkage data for marine iguanas, but in very good agreement with data from water pythons. Both the frequency of occurrence of shrinkage events and the percent of decrease in SVL did not differ inter-seasonally. Hence, I conclude that shrinkage events are just apparent in snakes and are caused by measurement errors by experimenters. I also predict that future studies may observe (apparent) shrinkage events more frequently among very large and/or very dangerous species (i.e. large boas and pythons, cobras, mambas, huge vipers), and that the frequency of shrinkage cases will not be higher than 6-7% of the total recapture events.