Smaller Brains Help Toads Survive a Harsh Life atop Mountains

With low temperatures, low oxygen, and high levels of UV radiation, life at 3,000 metres above sea level should be unforgiving to an animal that can’t produce its own body heat.  But the Asiatic toad has evolved an interesting adaptation to cope with harsh conditions: a smaller brain.

“For me, when I’m walking at 3,000 metres, I can feel the burning.  I often get sick.  But if you look at these little frogs, they live there all the time,” says Dr. Jinzhong Fu, a professor of evolutionary biology in the Department of Integrative Biology.  “These animals rely on external heat sources to heat themselves, but still you see them coming out and being active at night.  This, to me, is truly amazing.”

According to Fu, high elevation adaptation is “kind of a hot topic right now.”  And the Asiatic toad makes an ideal study subject.  It can be found living in habitats ranging from 400 metres to over 3,000 metres in a single river drain system on the side of a mountain in western China.  Few species exhibit such a wide range of variation in altitude within a short distance, providing an ideal opportunity to explore how the animal has adapted to survive life at high altitudes. 

In a recent study, Fu and other researchers found that a shift in brain size may be one such adaptation, with the overall brain size of Asiatic toads decreasing at higher altitudes. 

Like other amphibians, Asiatic toads are indeterminate growers.  Their bodies – and brains – continue to grow throughout their lifespan. This growth slows down once they reach sexual maturity, because the animals allocate more of their energy towards reproduction.  But Fu’s study shows that other changes in energy allocation are also occurring at high altitudes.

“The brain is very energetically expensive organ.  Sacrificing the size of the brain to meet the challenge of other energy demands seems to make sense,” says Fu.

There was, however, one unexpected result.  The portion of the brain that is thought to be associated with spatial learning became relatively larger at higher altitudes. 

Fu and his colleagues speculate that this difference may be due to the scarcity of breeding sites.  Greater spatial learning capability may be essential for the frogs to successfully navigate and reproduce at high elevations.

Could this pattern be seen in other species?  According to Fu, similar trends are likely to be observed in other reptiles and amphibians, but not in mammals.  This is because mammals can generate their own heat, whereas amphibians cannot, leading to differences in energy allocation between the two groups.

A follow-up study with the Asiatic toad is in the works to examine the relationship between the size of different body organs and the size of the brain region that communicates with them.  Reduction of organs in other species of frogs living in high altitudes has already been observed.  As Fu explains, a reduction in organ size likely means a reduction in the size of the associated brain region.

“For example, sometimes the middle ear bone completely disappears. It has been argued that at high elevation, hearing function is becoming less and less important.  If that is the case, then we expect the brain region associated with hearing to be reduced in size as well.”

Though the trade-off between brain size and good views may seem extreme for most, the Asiatic toad is providing new insights into evolution and the connection between habitat and energy allocation. 

And for Fu, that is an important piece of knowledge.  “I’m an evolutionary biologist.  Understanding adaptation is probably always going to be the most interesting part of the process.”

This study was funded by the National Natural Science Foundation of China and the Natural Sciences and Engineering Research Council.

Read the full study in the journal Ecology and Evolution.

Read about other CBS Research Highlights.