Exceptionally Low Metabolism of the Panda Linked to Mutation in the Thyroid Production System

There are few animals on the planet as familiar as the Giant Panda. Its role as the logo for the world wide fund for nature, the perilous nature of its existence in the wild, and the fact that they have been exported worldwide as a symbol of Chinese political friendship for decades, continue to sustain its iconic status. Yet, despite this phenomenal popularity, the political and conservation status of the panda makes doing research on them extremely difficult. There are many, and justified, hurdles to jump to do scientific work on animals: to do research on pandas those hurdles are both more abundant and substantially higher. Therefore many fundamental measurements that have been made on other species are lacking for pandas.

 

Scientists from the group of Dr. John Speakman, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, are starting, however, to fill in these gaps, some of which are detailed in a paper published in Science, (Nie et al 2015 Science 349: 171-175, DOI:10.1126/science.aab2413) which summarises some work they have been doing on the energy metabolism of these amazing animals, in collaboration with Professor WEI Fuwen from the Institute of Zoology, Chinese Academy of Sciences.

 

Perhaps one bit of panda biology everyone knows is that it is a Carnivore that became a vegetarian. The mammalian order Carnivora includes several families of animals including the canids (wolves, dogs and foxes), felids (cats such as the lion and tiger), mustelids (weasels etc), pinnipeds (seals, walruses and sea-lions) and ursids (bears). All these groups apart from the bears subsist by killing and eating other animals. Because meat is easily digested they are characterised by having short alimentary tracts. Apart from the polar bear, many bears include various amounts of vegetable material into their diets. The panda has taken this to its ultimate extreme: eating almost only bamboo. Although pandas have many adaptations for eating bamboo (like an extra ‘thumb’ to help hold the shoots) these do not include a modified alimentary tract. The panda has the guts of a lion: ideal for digesting meat, but very inefficient for digesting bamboo. So they have to eat lots of it; perhaps as much as 10-20 kgs per day.

 

Scientists have long speculated that to survive on such a low quality food pandas must have a low rate of metabolism. However, until this paper in Science nobody had managed to measure exactly how much energy they use. They applied a technique called the doubly-labelled water method which measures the rate at which animals eliminate stable isotopes from their bodies to measure the metabolism of 5 captive pandas at Beijing zoo and 3 wild pandas living in the Foping nature reserve. The answer is that their metabolic rate is exceptionally low. Corrected for their body weight (which for the animals we measured was 92 kg) the panda has substantially lower metabolism than almost all other mammals. In fact the metabolism of the panda is closer to what would be predicted for a 90 kg reptile, than a 90 kg mammal.

 

How they achieve such low rates of energy use was the focus of the second part of this paper. Much of the energy that our bodies use is burned up in relatively few organs – like our brains, kidneys, heart and liver. Using historical autopsy data they found that pandas have small organs for their body size. Their brains are only 82% of the expected size, their kidneys only 74.5% and their livers a remarkable 62.8% of the expected size for a 90kg mammal. Plus if you ever went to see a panda in a zoo you will know that they are not the most active of animals. Indeed using GPS loggers they found that in the wild pandas move on average at just 26.9 metres per HOUR!

 

A key physiological system involved in regulating people’s metabolism is the thyroid hormone system. The scientists suspected, given their very low metabolic rates, that pandas might have something unusual going on with their thyroid hormones, and this hunch turned out to be correct. Pandas have very low levels of the main thyroid hormones T4 and T3. These low hormone levels were able to be traced to a unique mutation in the panda genome which affects a critical gene involved in thyroid hormone synthesis. People who have low thyroid hormone levels often complain that they feel cold. This is potentially because their lowered metabolic rate is insufficient to keep them warm. Having low metabolism raises a similar problem for the panda – how does it manage to keep warm. The answer is that despite living in semitropical habitats the panda has a really thick fur coat. This serves to trap what little heat their metabolism produces inside their bodies to maintain their body temperature. A direct consequence of this is that the surface temperature of the panda (measured using a thermal imaging camera) is about 10^oC cooler than the surface of other black and white animals like the zebra. Pandas it seems are literally cool!

 

Email: j.speakman@genetics.ac.cn

 

 

 

Figure 1. Thermal image of giant panda eating bamboo (Image by Yonggang Nie)

 

 

Figure 2. Captive Panda in Beijing zoo eating bamboo (John Speakman)