One of the general trends seen throughout the animal kingdom, and especially within Class Mammalia, is a scaling of longevity to body size. Elephants live longer than horses, which live longer than mice. There have been many, many studies which attempt to parse out the reasons for this, ranging from metabolism and fecundity to intracellular transport and skeletal biomechanics. As with many phenomena in biology, however, there are indeed exceptions to the rule, and these cases can be extremely instructive.

This brings us to bats. One of the many fascinating aspects of bat biology is that their lifespans are much higher than would be expected for their body size. Most bats are comparable in mass and volume to small rodents, and yet they live much longer than their terrestrial friends. For example, Tadarida brasiliensis_, a common North American species, weighs 10-20 grams, approximately the same as a lab mouse. And yet, while mice are usually lucky to reach their third birthdays in captivity, captive bats have been known to live for up to 20 years. Anecdotally, at one field site in Mexico we saw vampire bats (_Desmodus rotundus) that had been tagged (as adults) nine years ago.

Why the discrepancy? Why do bats violate the scaling rule?

A new study has recently added a new piece to the the puzzle of chiropteran longevity. Salmon et al. examined extracted proteins from liver samples of T. brasiliensis and Myotis velifer, and then treated the proteins with chemicals known to induce mistakes in protein folding. They then compared the proteins to samples from mouse livers that had been exposed to the same chemicals.

The results were fascinating: the bat proteins showed significantly less misfolding and damage than the mouse proteins, suggesting that bat proteins have some adaptation that allows them to maintain their structural integrity under conditions that warp the proteins of other mammals. The bat samples had lower levels of both protein ubiquitination (which functions to label proteins for degradation by proteasomes) and overall proteasomal activity, in addition to lower levels of carbonylation, the signature damage caused by oxidation. All of that, in a nutshell, means that the bat proteins were much hardier under stress. The authors suggest that bat longevity “is correlated with resistance to protein oxidation and enhanced protein homeostasis.”

It will be very interesting to follow this vein of research. How significant is this phenomenon in other bat species? In the study, T. brasiliensis showed more significant damage resistance than M. velifer_. Bats, comprising 20% of all mammal species, are incredibly diverse in both morphology and lifestyle, so it would be interesting to look at representatives from each family. What about megachiropterans, the large fruit bats which some have suggested may actually be more closely related to primates than microchiropterans? (This hypothesis doesn’t have widespread support, but it makes for an interesting argument, see this booksGLXltMC&printsec=frontcover&dq=bats+biology+and+behavior for a well-rounded discussion of the evidence that supports and refutes the idea)

I am not an ornithologist, but I have often wondered about exceptions to the scaling rule amongst birds, with small parrots often having much longer lifespans than eagles. It would be interesting to see a study similar to Salmon et al. on different vertebrate taxa.

And of course, further work can look more closely at what mechanisms exactly allow bat proteins to resist oxidation and therefore prolong senescence. Some journalists are already making many loud claims about the study uncovering a “fountain of youth,” implying that a human therapy could result from a better understanding of this phenomenon. This is a true possibility, but it seems a bit early yet to start making promises.

Still, you have to appreciate the sheer luck of some species: bats get to fly AND eat the equivalent of their entire body weight every night, and they have extended lifespans to allow them to enjoy these pleasures for many years. Sigh.

References:
Altringham et al. 1998. Bats: biology and behaviour. Oxford University Press.

Salmon et al. The long lifespan of two bat species is correlated with resistance to protein oxidation and enhanced protein homeostasis. The FASEB Journal, 2009; 23 (7): 2317 DOI: 10.1096/fj.08-122523*

Photo credit: S. Pederson, Nevis Biodiversity Project