HOLOCENE LAND SNAIL BODY SIZE CHANGE IN ALGERIA, NW AFRICA, AND ITS POSSIBLE CAUSES

Body size change of organisms can potentially inform about variations in ecological, climatic or even anthropogenic pressures. In particular, land snails are known to be highly sensitive to numerous external forces that often result in quantifiable changes in shell size. In the present work, we quantified body size fluctuations of the land snail Helix melanostoma throughout the Holocene archeological record of eastern Algeria, NW Africa, and explored possible driving mechanisms.

Archeological shells were retrieved from chronologically, well-constrained layers ranging in age between ~8500 to present. Over 1,000 adult shells were measured and results indicate that archeological shells significantly increase in body size from the past to recent. Overall, modern shells are the largest measured individuals, whereas all early Holocene shells exhibited significantly smaller shell sizes.

This apparent increase in land snail body size through time could be explained by at least two plausible hypotheses, i.e., climate change and/or human predation intensity variations. Larger snails are often better-adapted to drier conditions because of the reduced water loss due to their lower surface to volume ratio. Present-day drier conditions than those during the early Holocene (i.e., African Humid Period) could in part explain the observed patterns. Alternatively, smaller snail size of archeological shells could reflect a higher intensity on human harvesting practices, resulting in reduced snail sizes during the early Holocene, when snail populations were heavily harvested by Capsian people. In any case, this study indicates that snail size seems highly sensitive to external forces and could potentially be used as a proxy for climate change and/or human predation intensity.