Katie L. Hector

Resveratrol has shown evidence of decreasing cancer incidence, heart disease, metabolic syndrome and neural degeneration in animal studies. However, the effects on longevity are mixed. We aimed to quantify the current knowledge of life extension from resveratrol. We used meta-analytic techniques to assess the effect resveratrol has on survival, using data from 19 published papers, including six species: yeast, nematodes, mice, fruitflies, Mexican fruitflies and turquoise killifish. Overall, our results indicate that resveratrol acts as a life-extending agent. The effect is most potent in yeast and nematodes, with diminished reliability in most higher-order species. Turquoise killifish were especially sensitive to life-extending effects of resveratrol but showed much variation. Much of the considerable heterogeneity in our analysis was owing to unexplained variation between studies. In summary, we can report that few species conclusively show life extension in response to resveratrol. As such, we question the practice of the substance being marketed as a life-extending health supplement for humans.

Summary: 1. ‘Compensatory growth’ and ‘catch‐up growth’ are often used interchangeably to describe the faster than optimal growth that occurs following a period of dietary restriction in the development of many animals. Concerns about the statistical analysis of these studies have drawn attention to the risk of false detection in reports of compensatory and catch‐up growth.2. This study aims to quantify the degree to which these compensatory responses occur across the animal kingdom. In addition, this study distinguishes the two terms, ‘compensatory growth’ and ‘catch‐up growth’, to clarify the fitness consequences of rapid growth. Compensatory growth refers to a faster than usual growth rate, while catch‐up growth implies attainment of control size.3. Eight meta‐analyses and meta‐regression analyses were conducted on data extracted from 88 papers, including 11 taxonomic classes. The results confirmed that both growth tactics (i.e. compensatory and catch‐up growth) occur across a wide range of taxa and result in decreased direct fitness components.4. Importantly, the meta‐analytic methods used made it possible to identify the specific experimental techniques that most successfully promoted rapid growth after restriction and key differences in the responses of the four major groups (mammals, birds, fish and arthropods) to dietary restriction. Endotherms are more likely to show a compensatory growth response because of their determinate growth; in contrast, the indeterminate and saltatory growth tactics of fish and arthropods reduce the pressure to rapidly achieve a large size.5. Among the first meta‐analyses to be conducted in this field, this study provides valuable support for the premises of compensatory and catch‐up growth and also discusses weaknesses in experimental design, and possible solutions, in compensatory growth research. For example, we recommend conducting the experiment within the most linear phase of an animal’s growth to avoid analytical complications arising from size‐dependent growth, and our results indicate that dietary dilution more closely resembles quantitative restriction than clutch size and intermittent feeding restriction methods when normal quantitative restriction is not possible.

Compensatory growth, or catch‐up growth, occurs when an organism grows faster than the optimal rate after a period of growth restriction. The evolved optimal growth rate maximizes an animal's fitness potential while preserving tissue quality. Rapid compensatory growth allows the organism to achieve an adult size closer to that of an unrestricted conspecific. However, this accelerated growth may come at the cost of impaired fitness later in life due to accumulated cellular damage. Amphibians are an interesting, yet neglected, group in which to observe the effects of compensatory growth because of their flexible life history and the importance of large size for reproductive fitness. We investigated the effects of early nutritional restriction on the growth, morphology and three fitness‐related behavioural traits of brown tree frog tadpoles Litoria ewingii before and after metamorphosis. Tadpoles were fed reduced rations for two weeks, c. 35% of the control group's larval period, before being returned to the diet of the controls. The dietary treatment caused a significant difference in pre‐ and post‐metamorphic survival between the groups. The tadpoles on the restricted diet exhibited faster weight gain upon refeeding and reached a final size significantly larger than the control tadpoles. However, the larval period of the restricted group was extended by c. 5 days, compared with the control group. Early nutritional restriction also negatively affected the pre‐metamorphic fitness‐related behavioural trait of swimming speed. The restricted group showed an unexpected advantage in both post‐metamorphic fitness‐related behavioural traits of feeding latency and hopping ability. These results contrast with previous work on compensatory growth in tadpoles because nutritional restriction affected the developmental rate and also resulted in ‘over‐compensation’ of growth. Our experiment provides support for the prevalence of compensatory growth in diverse taxa and a unique observation of the post‐metamorphic consequences in amphibians adapted to ephemeral environments.