Mating patterns of Gammarus lacustris (Amphipoda: Gammaridae) from three populations across a gradient on the Tibetan Plateau

Abstract

The large-male mating advantage and size-assortative mating are two different size-based patterns which deviate from random mating in Gammarus species. The size-assortative pairing in gammarid amphipods may have arisen for microhabitat segregation, loading constraints, sexual selection, or a combination of them. This study investigated the mating patterns of Gammarus lacustris from three populations along an altitudinal gradient in the northeastern Tibetan plateau. We demonstrated that the species shows a size-related non-random mating pattern. There is a strong sexual size dimorphism (SSD) in G. lacustris, which does not vary along the altitudinal gradient. The ultimate cause of male-biased SSD is likely the stronger selection acting on male than on female body size, either because large males have a higher mating success than smaller males (large male advantage at the highest elevation population), or because they mate with larger females with more fecundity in three populations, or both, which results in size-assortative mating of G. lacustris. Additionally, the loading constraint hypothesis (size-assortative patterns potentially increased their fitness in fast-flowing water) cannot be excluded.