Animals could start shrinking because of global warming, scientists have predicted, after discovering that mammals became 'dwarfed' in a similar episode of climate change 50 million years ago.
Paleontologists discovered the fossil teeth belonging to an early ancestor of modern horses as well as a rabbit-sized hoofed mammal.
The teeth revealed that during Eocene Thermal Maximum 2, a warming event of around 3C that occurred approximately 53.7 million years ago, the animals shrank in size by 14 per cent.
Climate researchers currently estimate that the planet will warm between 2C and 4C by 2100.
Evidence of mammalian dwarfing during the largest warming event on record, the Paleocene-Eocene Thermal Maximum (PETM), is already well-documented and some creatures shrunk by as much as one third as temperatures rose between 5C and 8C.
But the impact of less extreme ancient global warming events on animals has remained unclear until now.
"Dwarfing appears to be a common evolutionary response of some mammals during past global warming events, and the extent of dwarfing seems related to the magnitude of the event," said lead author Dr Abigail Ambrosia, of the University of New Hampshire.
"Abrupt perturbations of the global carbon cycle during the early Eocene are associated with rapid global warming events, which are analogous in many ways to present greenhouse warming."
The researchers analysed tooth enamel from mammalian fossils collected in the Bighorn Basin of Wyoming including Arenahippus pernix, an ancestor of modern horses. The teeth provided information on ingested water and consumed vegetation as well as acting as a proxy for body size.
The authors propose several warming-induced mechanisms that could have caused the mammalian dwarfing, including lower nutrient availability.
"Body size change during periods of climate change is commonly seen throughout historical and geological records among mammals and other organisms," added Ambrosia.
"Nutrient availability along with, and as a consequence of, rising temperatures and drought, may also have a direct effect on body size. Drought conditions have been known to lead to smaller offspring.
"This suggests that dwarfing will be a likely natural response of some mammals to future global warming."
The research was published in the journal Science Advances.