Waste Not; Want Not. Do Frogs Recycle Their Antifreeze?

Many advances in the biological sciences have come from what can be classified as—and funded as part of—agricultural research. Reproductive biology is one prime example but there are many others.

One advance the origins of which lie firmly in agricultural production is the recycling of urea. We were all taught at school that excess protein in the diet ends up as urea in urine with the nitrogen contained therein lost forever. But things are not that simple. Now it is known that in many animals, the urea is not simple dumped but can be broken down by bacteria in the gut and the nitrogen it contains recycled into protein.

It was the demonstration that urea, a form of non-protein nitrogen, added to the feed of farm ruminants, can be used within the rumen to make protein for growth or milk production that set off the interest in the recycling of urea under different nutritional and environmental regimes. It is not the ruminant’s tissues that are involved in breaking down the urea but the bacteria of the rumen. The bacteria then produce protein using urea as a source of nitrogen and this protein is digested and absorbed further down the alimentary canal. The history of the use of urea rather than expensive protein nitrogen sources was explained by my predecessor but one as Director of the Hannah Research Institute, James Andrew Buchan Smith (1906-2006), known throughout the world as ‘JABS’. Since those early days the recycling of urea nitrogen has been recognised in more and more organisms that produce urea as a nitrogenous waste product.

Amphibian species use high concentrations of urea in the blood for two main purposes: protection against dehydration and as an antifreeze. Some, notably the famous Crab-eating Frog (Fejervarya cancrivora) which can survive in sea water, build up high concentrations of urea in their body fluids in order to prevent the osmotic loss of water into the surrounding salty water. Other frogs that hibernate on land in sub-zero temperatures build up high concentrations of urea to act as a cryoprotectant. The high concentration lowers the freezing point of body fluids, preventing the formation of ice crystals.

In a paper published last year, a team in the USA produced preliminary evidence that the urea accumulated during hibernation in the Wood Frog, Rana sylvatica or Lithobates sylvaticus from northern North America, can be recycled into protein. Bacteria in the hind gut were found which could produce urease, an enzyme not produced by vertebrate tissues but essential for the recycling of urea nitrogen; more were present in winter than summer. Thus it is possible that as temperatures rise in spring, the bacteria become active, the urea is broken down and the products of that breakdown are converted into amino acids and hence protein.

Wood Frog
Brian Gratwicke [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)]

The findings on this frog indicate what could happen. However, there are some qualitative and quantitative pieces of evidence that would be more fully convincing of the first demonstration of urea recycling in amphibians. In mammals, specific urea transporters carry urea into the interior of the gut. Are they present in amphibians? Similarly, we do not know the magnitude and, therefore, the quantitative importance of any breakdown of urea to the overall nitrogen metabolism of these frogs at the end of hibernation. However, this could be another case of waste not, want not.  

Wiebler JM, Kohl KD, Lee RE, Costanzo JP. 2018 Urea hydrolysis by gut bacteria in a hibernating frog: evidence for urea-nitrogen recycling in Amphibia. Proceedings of the Royal Society B 285: 20180241. http://dx.doi.org/10.1098/rspb.2018.0241 

Time to kill off the term ‘brumation’ for hibernation in reptiles

Amateur herpetology circles and zoo keepers tend to obsess over using the term ‘brumation’ instead of hibernation for what reptiles and amphibians from temperate climes do in winter. In fact the internet is infested with those correcting others on what they see as an error of terminology.

"Bill" Mayhew
From here

Brumation was a word coined in 1965 by Wilbur "Bill" Waldo Mayhew (1920-2014) of the University of California at Riverside as a result of his work on hibernation in the Flat-tailed Horned Lizard, Phrynosoma mcallii (then known as P. m’calli). Mayhew was working in the heyday of research on thermoregulation and metabolism in lizards. He had arrived at the new campus of Riverside in 1954 after education (BA, MA, PhD) at Berkeley under the G.I. Bill. He had a distinguished war record as an air gunner in B17 (Flying Fortress) and B24 (Liberator) bombers in North Africa, China and Burma*.

Mayhew called his lizard an ‘obligatory’ hibernator because no matter what the outside temperature was, those kept in his laboratory ceased to eat and became torpid at the onset of winter. Other lizards—‘facultative’ hibernators could be kept active all winter by maintaining a high temperature. Mayhew recorded that metabolic rate (oxygen consumption) fell to an extent greater than that which would be expected from a fall in environmental temperature. For example, if the lizards were cooled during summer, metabolic rate fell but not to the extent seen in winter at the same body temperature. In other words, hibernation is some special physiological process that alters the normal relationship between body temperature and metabolic rate.

Phrynosoma mcallii
Photograph by Gary Nafis (CaliforniaHerps.com)

A decrease in metabolic rate independent of a decrease in temperature means of course less energy expenditure even if the temperature of the hibernaculum is high in the desert sunshine of autumn or early spring. In terms of running off body stores to maintain life during winter, a metabolic rate lower than that brought about simply by a fall in temperature would be of obvious survival value.

In the summary of his paper, Mayhew wrote:

The term brumation is proposed to indicate winter dormancy in ectothermic vertebrates that demonstrate physiological changes which are independent of body temperature.

In the paper itself he expanded that argument:

Results to date show that relatively complex physiological changes occur during or immediately preceding winter dormancy in some ectothermic vertebrates. To this extent, these animals are similar to hibernating birds and mammals. However, they differ from these heterotherms (see Cowles, 1962) in their inability to control their body temperatures. Consequently, it seems advisable to have one term to designate winter dormancy in heterotherms and another for such ectotherms. Hibernation has been used to denote this condition in heterotherms particularly, so it seems best to retain this term for that group of vertebrates. Therefore, I propose the term brumation (from bruma, L. winter) to indicate winter dormancy in ectothermic vertebrates that demonstrate physiological changes which are independent of body temperature.

Now, there are three points. Firstly, the similarity between ‘heterotherms’ and Mayhew’s ectotherms is greater than the difference. Both have mechanisms to lower metabolic rate, irrespective of the body temperature. Secondly, Mayhew’s definition limits it application to reptiles, or amphibians, or fish, in general since it applies to those ‘that demonstrate physiological changes which are independent of body temperature’. Mayhew did not know what happened in other reptiles—or amphibians—or fish. To apply ‘brumation’ to all would imply one adaptation for hibernation in all poikilotherms and that notion would need to be tested in a wide number of species. Thirdly, I have found nothing to suggest that Mayhew himself intended his term to apply to all reptiles or all poikilotherms. Therefore, if you use the word hibernation to describe winter dormancy, which makes no assumption about the physiological mechanisms involved, you will never be using the wrong term; with brumation you may well be wrong.

Others have questioned, indeed, slightly ridiculed, Mayhew’s now 50-odd year old neologism, as explained here in what seems to be a defunct blog.

This house gecko, Hemidactylus bowringii, I photographed
in Hong Kong in 1966

I was reminded of my long-standing objection to the use of brumation when I found in a pile of papers the notes for a talk I had given in May 1967 to the Hong Kong Natural History Society. I must have just read Mayhew’s paper because I asked the question not only what happened to reptiles during the winter in Hong Kong but also what happened to their metabolism. The reason I did so is because Hong Kong lies just within the tropics but does experience spells of relatively cold winter weather. Hibernation, I argued, would be short and perhaps interrupted. Indeed, when I was looking up John Romer’s early herpetological activities in Hong Kong during the late 1940s, I came across his observations:

In the colder weather [in Hong Kong] geckos either disappear altogether into hibernation or are very much less active. Here is my house there are several specimens of our very common house gecko, Hemidactylus bowringii and since their disappearance into hibernation last year I noticed two of these for the first time on the 15th February. I did not see either of them again until the 29th February [1948 was a leap year] when one of them was seen. They seemed to disappear again until the 7th March and were then seen quite frequently always in the same part of the house.

The question I asked then—and still have no answer now—is whether Hong Kong reptiles and amphibians faced with variable winters of relatively short duration behave like the North American desert lizards studied by Mayhew and others, in that they have a mechanism to lower their body rate independently of their body temperature, i.e. brumate by Mayhew’s definition, or do they simply lapse into cold torpor for a few weeks or even days, with their metabolic rate simply matching the decrease in temperature?

Golden Kukri Snake. Just off Route Twisk near the road to
the summit of Tai Mo Shan. 1968

But there is a second question. Do all Hong Kong reptiles and amphibians use the same mechanism to cope with low winter temperatures? House geckos living on, say, the tip of Kowloon near the sea could face a very different set of circumstances from the Golden Kukri Snake (Oligodon cinereus) we found in 1968 at about 500 metres altitude just off Route Twisk or the Mountain Pit Vipers (Trimeresurus monticola) found around 900 metres on Tai Mo Shan where frosts are sometimes recorded.

In summary, brumation: (i) unnecessary since it has no explanatory value (ii) can, possibly, be positively misleading when applied indiscriminately, (iii) time, therefore, for the term to join the dead parrot in the choir invisible.

My binning of brumation in no way detracts from the work that Mayhew did nor his reputation as a  physiological ecologist and academic field naturalist. He was a co-founder of the University of California’s Natural Reserve System for field studies. A dormitory for visiting scientists on the Philip L. Boyd Deep Canyon Desert Research Center is named in his honour, as is a new chair in the university.

*In his retirement he wrote Pictorial History of the 7th Bombardment Group Wing 1918-1995, published in 1998.

Mayhew WW. 1965. Hibernation in the horned lizard, Phrynosoma m'calli. Comparative Biochemistry and Physiology 16, 103-119.

A useful summary of the early work on hibernation in reptiles:
Bennett, A. F. and Dawson, W. R. (1976). Metabolism. In Biology of the Reptilia, Vol.5 (ed. C. Gans and W. R. Dawson), pp.127 -223. London, England: Academic Press.