"Change in body length is considered to be unidirectional in vertebrates1, but we have repeatedly observed shrinkage in the snout-to-vent length of individual adult iguanid lizards. In two studies, one lasting 18 years and one 8 years, of two island populations of Galápagos marine iguanas (Amblyrhynchus cristatus), we found that individuals became shorter by as much as 20% (6.8 cm) within two years. This shrinking coincided with low availability of food, resulting from El Niño events. Body length increased again during subsequent La Niña conditions, when algal food was abundant. We found that lizards that shrank more survived longer than larger iguanas during harsh periods because their foraging efficiency increased and their energy expenditure decreased.
Shrinking in marine iguanas may be an adaptive response to low food availability and energetic stress. Measurements of a cohort of adults more than 300 mm long during the strong 1992–93 El Niño event show that individuals that shrank more survived significantly longer (Fig. 2b). The mechanisms that determine whether and to what extent an individual shrinks during El Niño events remain unclear. Reduction in body length has been observed previously, and growth rates set to zero by definition, but to our knowledge this is the first report of shrinkage in adult vertebrates" (Wikelski and Thom 2000: 37)
[Note: The decrease in body size is the main strategy, no matter how it occurs. However, the paper mentions reabsorption of bone as the possible mechanism.]