Extreme food-plant specialisation in Megabombus bumblebees as a product of long tongues combined with short nesting seasons

Megabombus bumblebees have unusually long tongues and are generally more specialised than other bumblebees in their choice of food plants. The phylogeny of Megabombus bumblebees shows that speciation was concentrated in two periods. Speciation in the first period (ca 4.25-1.5 Ma) is associated with the late rise of the Hengduan Mountains at the eastern end of the Qinghai-Tibetan plateau. Speciation in the second period (1.2-0.3 Ma) is associated with climatic cooling in the northern forests. The most extreme food-specialist species belong to the second period, which may point to climate as a factor in specialisation. These extreme specialist species occur either in the far north (Bombus consobrinus), or at high elevations (Bombus gerstaeckeri), in situations where long tongues coincide with the shortest nesting seasons. Species with the longest tongues but occurring further south (even at high elevations) use a broader range of food plants.