What can hippopotamus isotopes tell us about past distributions of C4 grassy biomes on Madagascar?

Societal Impact Statement Today, expansive C-4 grassy biomes exist across central, western, and northern Madagascar. Some researchers have argued that the island's now-extinct pygmy hippopotamuses belonged to a megaherbivore grazing guild that maintained these grasslands prior to human arrival. However, the chemistry of hippo bones indicates that C-4 grasses were only a minor part of hippo diet. This, in turn, suggests that C-4 grasses were present but not widespread when hippos were alive and that grasses expanded only after Malagasy people shifted from hunting and foraging to agropastoralism approximately 1000 years ago. These results have important implications for environmental reconstructions and biodiversity management. Summary center dot Extinct hippopotamuses (Hippopotamus spp.) were part of Madagascar's megaherbivore guild. Stable carbon (delta C-13) and nitrogen (delta N-15) isotopes in radiometrically dated bone collagen track spatial and temporal variation in diet and habitat. If hippos helped maintain C-4 grassy biomes, then they should have regularly consumed C-4 grasses, which have high delta C-13 values. However, if expansive C-4 grassy biomes are anthropogenic, then forests would have been more extensive in the past, and hippos would have predominantly consumed C-3 plants with low delta C-13 values. Nitrogen isotopes can clarify foraging habitat (moist or dry). center dot We assessed delta C-13 and delta N-15 values for hippos from different ecoregions of Madagascar and compared these with data for extinct herbivorous lemurs from the same ecoregions. We further explored the effects of wet/dry transitions on isotopic trends for hippos from the central highlands and spiny thicket ecoregions. center dot Carbon isotopes suggest (1) limited C-4 consumption by hippos in the central highlands, dry deciduous forest, and succulent woodland ecoregions; and (2) moderate consumption of C-4 resources in the spiny thicket. Nitrogen data indicate that hippos foraged in wetter habitats than sympatric lemurs in all regions. center dot Malagasy hippos did not regularly graze C-4 grasses in dry, open habitats, even in regions blanketed by C-4 grassy biomes today. Malagasy grasses are adapted to grazing and fire, but these are likely ancient adaptations that accompanied grasses when they initially spread to Madagascar. C-4 grassy biomes were spatially limited in extent in the past and only expanded after the Late Holocene introduction of domesticated ungulates.