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Madagascar's ephemeral palaeo-grazer guild: who ate the ancient C 4 grasses?

2016; Royal Society; Volume: 283; Issue: 1834 Linguagem: Inglês

10.1098/rspb.2016.0360

1471-2954

Laurie R. Godfrey, Brooke E. Crowley,

African Botany and Ecology Studies

Madagascar today is 65% grassland.Its grassy biomes were widely interpreted as 'anthropogenic' before Bond et al. [1] challenged that view, claiming that grasses colonized Madagascar during the Miocene.Through an analysis of grass clade endemism and specialization, Vorontsova et al. [2] have marshalled compelling support for this argument.Still, many questions about Madagascar's grasslands remain, and they are critical to debates regarding why so many forest-dwelling animals disappeared after human arrival.We know that forests declined and grasslands expanded, but we do not know precisely when, why or to what extent.Did significant pre-contact expansion occur [3]?Did post-contact aridification trigger this transformation [4]?Did forests disappear as people shifted their subsistence strategy from ephemeral foraging to dedicated agro-pastoralism [5]?Did early human colonizers severely reduce Madagascar's natural grazer community through hunting, radically changing the fire ecology and ultimately destroying forests [6,7]?Normally, grasslands are maintained by natural fire and by grazers, usually large-bodied herbivores.But Madagascar's modern grazers (e.g.cattle and goats) were recently introduced.If pre-contact grasslands were extensive, we would expect to see a significant palaeo-grazer guild.The prime candidates are pygmy hippopotamuses and giant tortoises-animals that, along with giant lemurs and elephant birds, comprised Madagascar's now-extinct megafauna.According to Vorontsova et al.[2], previously published stable carbon isotope (d 13 C) data suggest that (i) Madagascar's hippopotamuses consumed a 'high proportion of C 4 plants' (i.e.grasses) [8] and (ii) its giant tortoises had diverse diets [9].We were motivated to write this comment for three reasons.First, Crowley et al.[8] never claimed that Madagascar's hippopotamuses ate a 'high proportion of C 4 plants'.We did note that subfossil hippos, tenrecs, carnivorans and the endemic rodent Hypogeomys australis had higher d 13 C values than subfossil primates (except Hadropithecus [10]), but we never defended a high proportion of C 4 plants in the diets of any of these animals.Second, Vorontsova et al.[2] omitted important published isotope data on hippos and tortoises from Madagascar [8,11,12].Third, we believe that the literature would benefit from greater discussion of these data.C 4 grasses (including those measured in Madagascar [11]) typically have d 13 C values between 211 and 214‰, while C 3 plants have much lower values, averaging between 226.5 and 231.5‰ for different Madagascan forests [13].Assuming an offset of þ5‰ between herbivore bone collagen and plant diet (reviewed in [14]), pure C 4 consumers should have d 13 C values more than 29‰, those with pure C 3 diets should have d 13 C values less than 221.5‰, and those that consumed a relatively even mixture of C 3 and C 4 plants should have values of 214 to 218‰.Thus, herbivores with values between 218 and 221.5‰ likely consumed more C 3 than C 4 plants.Carbon isotope values for herbivores should approximate the relative abundance of C 4 grasses in a particular habitat provided that those herbivores are adapted to eating grasses and other C 4 plants, and that grazers are not selective in the grasses they consume.Clearly, d 13 C values for some vertebrates, such as ratites (which tend to select small fruits) and lemurs (which tend to browse on C 3 fruit