Naked mole rats &c (was: eaten words and food for thought)

[ccp@okway.okstate.edu]

{my comments in brackets}

Subject: eaten words and food for thought 
Author:  <cadams@hh.gpz.org> at SMTP
Date:    6/11/98 6:27 AM
     
You say the mole rats have a metabolic rate "less than half" that of other 
rodents.  At a body size of <1 kg, an ectotherm would be expected to have a 
resting metabolic rate <20% that of a comparably sized endotherm.  I wonder 
where the mole rats fall.  Also, you say that their body temperature 
approximates ambient.  But what about their metabolic rate?  Does it decrease 
steadily with decreasing ambient temperature?

{The basal metabolic rate (BMR) of the naked mole rat (NMR) is 25.3 mL O2/h; 
at a body mass of 39.5 g, which is 43% of that expected from the allometric 
prediction equation for rodents generally. The Namib golden mole (NGM) has a 
BMR of 13.6 mL/h, which is only 22% of the predicted BMR for an insectivoran 
with a body mass of 26.1 g (but the prediction equation includes data for 
shrews). For comparison, predicted standard metabolic rates for squamate 
reptiles weighing 26.1 and 39.5 g at a body temperature of 33C (which is the 
body temperature of both NMRs and NGMs when at BMR) are 3.17 and 4.42 mL/h, 
respectively, so both mammals are still tachymetabolic relative to classical 
ectotherms.
        Both NMRs and NGMs do display a limited endothermic response to 
decreased ambient temperature, that is, at environmental temperatures below 
the zone of thermoneutrality their metabolic rate increases (to a point; at 
temperatures below about 20C metabolic rate decreases). However, their 
thermal conductances are so high that this increased heat production is 
insufficient to maintain body temperature homeothermically, and body 
temperature drops (though it may be a few degrees above ambient). 
        McNab (1966) uses the ratio of BMR to conductance as an index of 
endothermic homeothermy. If we standardize this ratio for a typical mammal at 
1.0, NMRs have a relative ratio of 0.14, compared to 0.04 for a typical 
lizard. McNab: "In terms of energetics the naked mole-rat has gone 2/3 of the 
way towards ectothermy."
        So these weird mammals are not really ectothermic (their rates of 
heat production are high enough to affect body temperature at least a little 
bit), but on the other hand they're pretty pisspoor endotherms. They are 
definitely not homeotherms, and they don't thermoregulate (at least not very 
well), so they are poikilothermic thermoconformers...almost.
        To me this fits in nicely with the Bennett/Ruben idea that high 
resting metabolic rates evolved originally not for thermoregulation, but to 
support higher rates of sustained activity; both of these mammals are 
fossorial and engage in a lot of energetically expensive burrowing 
locomotion, which helps to explain their higher-than-squamate BMRs despite 
their abandonment of metabolic temperature regulation.

References
NMR:    McNab 1966 Ecology 47:712-733
        Lovegrove 1986 Oecologia 69:551-555
NGM:    Fielden et al. 1990 Journal of Arid Environments 18:221-237.
        Seymour et al. 1998 Journal of Zoology 244:107-117.                   
other relevant sources that I can't find or don't have are cited therein}

     
Why don't birds walk at lizard speeds?  I don't mean egrets stalking 
crawfish in the water.  I mean WALKING, getting from point A to point B. 
After all, most of them are quite capable of getting around without 
walking.  So why do they insist on walking at "endothermic" speeds? For 
the same reason that humans do not walk at 0.1-0.5 kph.  Go ahead, try it. 
Walk at 2 meters per minute.  Yes, you can do it.  But it feels very 
unnatural.  Your body is not designed for that speed.

{What's your point? The difference between extant endotherms and 
ectotherms is not maximal speed or minimal comfortable speed. The 
difference is in aerobically supportable speed. A lot of lizards (of 
whatever body size) could outsprint you from point A to point B, but they 
fuel their muscles anaerobically and therefore fatigue quickly. Nobody 
would argue that an endotherm can sustain a wider range of behaviors and 
speeds than an ectotherm, but I still say that, causally, that has little 
or nothing to with posture.}

I was not arguing that as an endotherm a sauropod must devote a much 
greater portion of its energy budget to muscular contraction because it 
cannot sit down.  I was arguing that if it were an ECTOTHERM, spending all 
day on its feet would require it to consume much more energy.  This defeats 
the greatest advantage of ectothermy. 

{Well, OK, I see your point (yes, standing all day might require more 
energy expenditure than lying down--but not necessarily, as shown by 
horses), but I don't follow the logic. Accepting for the moment the premise 
that sauropods could not lie down, they had to stand around all day 
whatever their metabolic rates were. Are you suggesting that because 
sauropods had to stand all the time they had to have been endotherms? I 
just don't buy it.}
     
As for monotremes, Greg has long since pointed out that the lack of a fully 
erect posture does not require ectothermy.  

{But a fully erect posture does "require" endothermy? Why should it?}

I will be very interested indeed to look more closely at how mole rats 
stand and locomote.
     
     {They are fully fossorial, spending their entire naked lives in 
     tunnels of their own construction (they dig with their incisors). My 
     impression is that they stand seldom and only weakly.}
     
And incidentally, the oft-repeated mantra that correlation does not equal 
causation is misleading. ANY correlation implies a causal link.  It does 
not imply that A directly causes B.  It may be a "spurious" correlation, 
as they say.  Perhaps unmeasured factor C causes both A and B.  Or perhaps 
A causes C causes D causes B.  Or something more complex.  But every 
correlation implies causality.

{Nah. A truly spurious correlation is just that; it means nothing. Two 
variables could be empirically and statistically correlated that have 
absolutely nothing to do with each other. The indirect causations you 
describe are probably more likely; in fact my preferred explanation for 
the putative correlation between posture and metabolic rate among extant 
animals is that both are correlates of selection for greater locomotor 
performance. (I think you were proposing a much more direct link, i.e. 
erect posture "forces" endothermy.) Logically my scenario does not 
preclude the independent evolution of metabolic rate and posture in 
response to different selection pressures. I'd guess that it's much better 
from an engineering standpoint to support a huge mass with erect columns 
than with cantilevers and cables. Sauropods coud have had erect posture 
for load-bearing reasons that have nothing to do with locomotion or 
metabolic rate.}
     
     All living ectotherms on this planet fall along a fairly narrow 
     regression line of metabolic rate vs. body size.  Why?  Why should an 
     insect and a lizard the same size have similar metabolic rates?  Not 
     one ectotherm has a resting metabolic rate approaching that of an 
     endotherm of similar size. Why not?  Similarly, birds and mammals, two 
     groups that have no common ancestor less than 250 million years old, 
     both have metabolic rates well above ectothermic and their regression 
     lines are virtually indistinguishable.  Why should a house mouse and a 
     warbler have similar metabolic rates?  One is forced to wonder whether 
     there is something universal going on.  
     
     {Well, the evolution of endothermy, and the evolution of metabolic 
     rates generally, are huge open questions with lots of fascinating 
     complexity. Let me try to address some of your interesting 
     thought-food, though I certainly don't have all the answers.
        First, you're talking here about resting (basal, standard) 
     metabolic rates, which are a useful comparative but of questionable 
     ecological relevance and definitely only part of the story. Why do all 
     ectotherms have similar metabolic rates? Because from a reductionist 
     standpoint they are very similar at the cellular level. Cells have 
     stuff to do that requires energy, all ectotherm cells have to do 
     pretty much the same stuff, and therefore all ectotherms use about the 
     same amount of energy at rest. ABOUT the same; there realy is a fair 
     amount of variation within any given body size class (severalfold). We 
     can surmise that the ectotherms with the lowest metabolic rates are 
     especially low-energy, high-efficiency machines, and that their 
     metabolic rates represent the minimal energy expenditure necessary for 
     life. The benefits of higher resting MRs are unclear and conjectural.
        Endothermic homeotherms are another story. As you point out, the 
     state has evolved independently at least twice, with strikingly 
     similar results (but still with considerable, as-yet-unexplained, 
     variation). Passerine birds (and hummingbirds) have the highest 
     resting metabolic rates (with other birds and most eutherians lower, 
     marsupials and some eutherians lower still, and monotremes lowest); 
     whatever the benefits of increased metabolic rates they have 
     apparently reached the point of diminishing returns, and yet higher 
     MRs are probably not supportable ecologically.
        The real question is, why is there essentially no overlap among 
     extant animals? Intermediate metabolic states _must_ have once 
     existed. Maybe dinosaurs had 'em.}
     
     That's why I'm very interested in the mole rats. Obviously there has 
     to be a period of intermediacy.  Are we catching mole rats in the 
     process of evolving ectothermy?
     
     {Maybe what molerats show us is the metabolic level consistent with 
     high sustainable rates of activity, but minus the extra needed for 
     physiological thermoregulation.}