Hotter summers, melting icecaps and intense rainfall are just a few of the effects of global warming. And now scientists claim we can add animal dwarfism to that list. The US researchers have found that mammals shrank significantly during at least two ancient global warming events. The finding suggests a similar outcome is possible in response to future climate change. Scientists have known for years that animals such as horses and deer became much smaller during a period of warming, called the Paleocene-Eocene Thermal Maximum (PETM).

This occurred around 55 million years ago when global temperatures rose by about 6 °C over a period of 200,000 years. Now University of Michigan palaeontologist Philip Gingerich have found evidence that mammalian ‘dwarfing’ also occurred during a separate, smaller global warming event 53 million years ago.

‘The fact that it happened twice significantly increases our confidence that we're seeing cause and effect, that one interesting response to global warming in the past was a substantial decrease in body size in mammalian species,’ said Dr Gingerich, a professor of earth and environmental sciences. Scientists believe a shrinking body size is a common evolutionary response by mammals to extreme global warming events, known as hyperthermals. They claim this may be ‘a predictable natural response for some lineages to future global warming.’

The smaller, later warming event analysed in the latest study, known as Eocene Thermal Maximum 2 (ETM2), lasted 80,000 to 100,000 years. Teeth and jaw fossils of early hoofed mammals and primates that spanned this later climatic event were collected in Wyoming's Bighorn Basin, and the size of molar teeth was used as a proxy for body size.

The researchers found that body size decreased during the later warming period, but not as much as the dwarfism seen in fossils for the earlier warming event. For example, the study revealed that a lineage of early horses the size of a small dog, called Hyracotherium, experienced a body-size decrease of about 19 per cent during ETM2. The same horse lineage showed a body-size decrease of about 30 per cent during the PETM. After both events, the animals rebounded to their pre-warming size. ‘Interestingly, the extent of mammalian dwarfism may be related to the magnitude of the hyperthermal event,’ said team member Abigail D'Ambrosia of the University of New Hampshire.

The ancient warming events may have been caused by the release of seabed methane clathrates, a kind of methane ice found in ocean sediments, though this topic remains an area of active research, Dr Gingerich said. The parallels between ancient hyperthermals and modern-day warming make studies of the fossil record particularly valuable, said team member Will Clyde of the University of New Hampshire. ‘Developing a better understanding of the relationship between mammalian body size change and greenhouse gas-induced global warming during the geological past may help us predict ecological changes that may occur in response to current changes in Earth's climate,’ said Mr Clyde.