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

Abstract

Societal Impact StatementToday, expansive C4 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 C4 grasses were only a minor part of hippo diet. This, in turn, suggests that C4 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 Extinct hippopotamuses (Hippopotamus spp.) were part of Madagascar's megaherbivore guild. Stable carbon (δ13C) and nitrogen (δ15N) isotopes in radiometrically dated bone collagen track spatial and temporal variation in diet and habitat. If hippos helped maintain C4 grassy biomes, then they should have regularly consumed C4 grasses, which have high δ13C values. However, if expansive C4 grassy biomes are anthropogenic, then forests would have been more extensive in the past, and hippos would have predominantly consumed C3 plants with low δ13C values. Nitrogen isotopes can clarify foraging habitat (moist or dry). We assessed δ13C and δ15N 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. Carbon isotopes suggest (1) limited C4 consumption by hippos in the central highlands, dry deciduous forest, and succulent woodland ecoregions; and (2) moderate consumption of C4 resources in the spiny thicket. Nitrogen data indicate that hippos foraged in wetter habitats than sympatric lemurs in all regions. Malagasy hippos did not regularly graze C4 grasses in dry, open habitats, even in regions blanketed by C4 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. C4 grassy biomes were spatially limited in extent in the past and only expanded after the Late Holocene introduction of domesticated ungulates.

Can rewilding with giant tortoises increase woody habitat and limit fire across Madagascar's grasslands?

“…Recent findings discount Malagasy hippopotamus (Crowley et al, 2023) and elephant birds (Hansford & Turvey, 2022), but Aldabrachelys may represent just such a grazer. Present-day Madagascar is largely grassland with limited mosaics of closed-canopy dry forest and open-canopy tapia woodlands, across the historic range of Aldabrachelys.…”

“…If the first hypothesis were correct, extinct Malagasy hippos would have fed in open grasslands dominated by C 4 grasses (and have higher δ 13 C values), and browsing of C 3 woody vegetation in woodland and forested habitats would have been uncommon and atypical. Isotope data now clearly find that Malagasy hippos were mostly C 3 feeders, never predominant grazers, lived mostly in forest and browsed in moist and wooded habitats, even in regions that today are vast grasslands (Crowley et al, 2023), corroborating earlier studies that showed there never were pure C 4 grazing hippos (Crowley et al, 2021;Godfrey & Crowley, 2016;Hansford & Turvey, 2022;Joseph et al, 2024a). As Madagascar's hippos lived predominantly in forests and wetlands, their feeding strategy has been identified by various authors as similar to that of forest-limited African pygmy hippopotamus, Choeropsis liberiensis (Crowley et al, 2023;Hansford & Turvey, 2022), which does not form obligate C 4 grazing lawns .…”

Fire incongruities can explain widespread landscape degradation in Madagascar's forests and grasslands

“…Recent evidence suggests large scale tree-loss has likely already taken place across much of the vast treeless grasslands (Crowley et al, 2023;, and because transformation transpired ca. 1000 CE, this landscape-scale historic deforestation does not reflect when evaluation considers only current tree-loss.…”

“…This lens is important, because palaeoecological evidence suggests that much of Malagasy tree-loss has already occurred and that large aspects of Madagascar's treeless grasslands may be fire-induced anthropogenic formations that formed ca. 1000 CE (Burns et al, 2016;Crowley et al, 2023;Godfrey et al, 2019;Hixon et al, 2021;Joseph & Seymour, 2022b;Razafimanantsoa, 2021).…”

“…Biodiversity has been lost at a range of scales, with multiple extinctions ensuing in the wake of this habitat degradation (IUCN, 2020;WWF, 2021). For example, at human settlement, forestdwelling hippos ranged across areas that are today vast grasslands (Crowley et al, 2023) created through historic deforestation (Burns et al, 2016;Crowley et al, 2023) followed by ongoing fires that have degraded rangelands to the extent that they can no longer support cattle year-round, without supplementary feeds and dry-season winter burns . This is important, because novel derived grasslands arising from forest clearance can impact climate, by reducing rainfall and facilitating drought (Chapman et al, 2020;COP26, 2021;Hoffmann & Jackson, 2000;Lawrence & Vandecar, 2015;Smith et al, 2023), which in Madagascar can precipitate famine UNICEF, 2021;WFP, 2021).…”