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Who cares about mouse lemur hands?

Updated: Sep 28, 2018

-> This Girl <-



I’m proud to say that when you google “mouse lemur hands and feet” as one does, a 2017 article published by myself, Emilienne Rasoazanabary,[1] and Laurie Godfrey is the first entry to pop up. And while I know it’s a niche market, I expect to get a lot of professional mileage out of this one. After all, who wouldn’t want to read an article about what is indisputably one of the cutest group of species around (totally not biased)? Point in case:


Given they come from a clade in dire straits (95% of all lemur species are critically endangered), this Mötley Crüe of 20 (or maybe 23? Or maybe 22? We don’t really know) tiny species seems to be doing reasonably well, though that depends on the species. They’re adaptable little creatures, which is necessary when your teeny-tiny size (30 paper clips) and general lack of bitey-pointy bits [2] means you’re on the menu for every hungry snake or raptor in the forest. As a result, these guys have evolved a number of mechanisms to cope with high rates of predation, including having more-than-typical numbers of young each year (so if one or two get eaten it ain’t no thing), fast life cycle, and learning not to be choosy. They aren’t choosy when it comes to food, they aren’t choosy when it comes to location, and they aren’t choosy when it comes to setting up a homestead. To put this in human terms, a mouse lemur on the house market would tour the Biltmore Estate one day, and a roofless, earthquake-damaged tiny house the next, and be like "Meh, either one'll do." In fact, their only habitat requirement seems to be that they like trees, the actual kinds of which matter little. You’ll find these guys skittering up and down anything from thick, spiny-trunked Alluaudia trees of the spiny forest to unstable, almost vine-like branches of the gallery forest. Mouse lemurs make the best of the lot they’re given, something to which we all should aspire [3].


Meet the reddish-gray mouse lemur:

In 2010, my colleague Emilienne Rasoazanabary spent several months studying this species. For over a year she tracked, trapped, and tagged hundreds of individuals, took a suite of phenotypic (physical) and behavioral data, and released them with a chunk of banana [4] as thanks for their involuntary assistance. From this tedious data collection she helped to generate a comprehensive profile of this little-studied species from Southwest Madagascar, including how it copes with habitat changes wrought by local human populations.


In a subsequent investigation issuing from this work, she and Laurie Godfrey hit upon an interesting pattern. They noticed that the individuals captured in the Spiny forest type had markedly different hand and foot form [5] than those captured in a nearby, yet entirely different, Gallery forest type. On its surface this doesn’t seem that impressive—after all, it would make sense that the suite of traits needed to survive one place aren’t necessarily the best for survival in another [6], but what was interesting was a prior study had already looked at genetic variation across mouse lemurs, including these two groups specifically. They discovered that our subpopulations they were genetically indistinguishable. This suggested that these groups interbred freely and were therefore the same species [7].



If hand and foot form was developmentally (i.e., genetically) regulated, this poses a problem for evolution-based science. Under traditional models, interbreeding between these populations should also intermix the genes for hand and foot traits and balance them between the forests over successive generations, but that didn’t seem to be the case here. In fact, we would later find that with 5 simple finger and toe measurements we could determine which forest an individual came from with 98.7% accuracy, so clearly the differences between them were significant. So what was going on? Were hands and feet super plastic, meaning they could change form when an individual migrated from one forest type to another? Was the genetic similarity simply a fluke? Were we witnessing the earliest days of speciation in action (meaning these populations were diverging to become separate species)? Was there something in the each population’s environment that might explain this tidy pattern? We had many questions. Incidentally, projects with many questions are my favorite kind.


This is where I, then a starry-eyed graduate student, was brought on. Laurie was my PhD committee member and knew that I’d invested a lot of time and extra training over in the Biology department learning “Evo Devo Bio,” or the branch of biology that investigates how developmental mechanisms guide the path of evolution. Given my interests and this nifty pattern, it seemed logical that we collaborate. So we did.


Having neither robust DNA data nor a cache of mouse lemur embryos from which we could study development, we decided to investigate this conundrum using multiple lines of evidence, not unlike circumstantial cases in forensics. First, we verified that the pattern they'd detected prior was legitimate by conducting a more expansive analysis. Then, we invested whether the pattern was detectable in infant and juvenile specimens. This is important because young individuals weren’t old enough to migrate between forests, so the patterns of their hands and feet might indicate genetic/developmental tendencies. Third, we used Emilienne’s meticulous behavioral data to investigate whether there were differences in how mouse lemurs in each location interacted with their environment and, further, if these coincided with differences in hand and foot form. Finally, we strengthened our circumstantial argument by delving into the development and population genetics literature to see if there was any precedent for this in other species.


So what did we find? We discovered that the pattern was indeed legitimate and that it was detectable both in adults and in the youngins, meaning hand and foot form was fixed early in life. We also found marked differences in how these guys moved through each forest that aligned well to their physical differences. Specifically, individuals from the spiny forest, with it’s thick-trunked trees, patches of bare earth and rocky outcroppings, had short fingers and long thumbs and big toes that would give them the best of two worlds: short fingers and toes make it easier to run across open ground before being devoured, while very long thumbs and big toes make it easier to grasp and climb thick-trunked trees with ease [10]. Conversely, individuals in the gallery forest had long digits and short thumbs/big toes that make it easier to cling flexible, unstable branches found high in the treetops.


These physical differences were corroborated by the behavioral data, which unsurprisingly showed spiny forest individuals far more frequently traversed the open ground, spent more time at lower levels and clinging to vertical (upright) substrates like tree trunks, and that tree saps, which come from the trunk of trees, [11] were a dietary mainstay. Gallery forest individuals rarely had need to wander down to the ground because they could simply travel from treetop-to-treetop via branch bridges, and the gallery forest’s closer proximity to water meant there was greater variation in diet, like flowers, fruits, bugs, small reptiles, and other tasty things commonly found in treetop branches. In short, gallery forest mouse lemurs chose the lush high rise life, while the spiny forest ones took what they could get. The fact that two such different forests are found within a short walk of one another is a testament to Madagascar’s amazing diversity...indeed it is unparalleled in that respect.


If you’re interested in the nitty-gritty (by which I mean the scientific hypotheses we used to frame our work, and our own views about whether these patterns were due to developmental genetics or skeletal plasticity), give our article a gander.

Footnotes


[1] OXFORD COMMA TIL DEATH!


[2] While I’m not a field researcher, I’ve been told by actual field researchers that mouse lemurs are particularly tenacious and will bite the crap out of you if given the chance. But being lemurs, they don’t have the super pointy teeth you might see on, say, your cat, an especially cranky badger, or xenomorphs because that's just not a lemur thing.


[3] I tell myself this a lot, as a 35-year-old postdoc with no home and limited prospects of prosperity.


[4] Like humans, most primates are food motivated. And while I haven’t tested it explicitly, I suspect this is a diagnostic feature of Order Primates. This may be taken to an extreme by our mouse lemur cousins, who would occasionally allow themselves to be recaptured and subject to all manner of invasive measuring simply for more banana. Humans seem to have some self-respect, at least. I love a good BBQ dinner, but I’m certainly not gonna volunteer for experimental lab testing to get one....at least not more than once.


[5] In early drafts, Laurie described these differences as hands that were either “pincher-like” and “clamp-like,” which naturally made me think of Zoidberg and Clamps from Futurama. I’m a little sad those descriptors didn’t make it into this paper.


[6] Incidentally, this is why Gina doesn’t camp or scuba dive.


[7] Yes, I know I’m drawing heavily from the biological species concept here. If any enthusiastic Evolutionary Bio grads want to have an intellectual arm wrestle over the relative merits of other species concepts, I’m free for a debate on February 30th. But I’m gonna lay a few ground rules down first: [1] you’re not allowed to invoke hybrids. Like at all. [2] That’s pretty much it. No discussion of hybrids.


[8] Mouse lemurs could also probably palm an exercise ball. This, coupled with their incredible speed and jumping prowess makes me think the next NBA team should be “The Mouse Lemurs.”


[9] Fun fact! Mouse lemurs (in fact, many lemurs) have squishy, bulbous fingertips just like geckos do. This enhances grip for climbing and clinging. Convergent evolution FTW! Caveat: geckos have other anatomical and structural features that take this even further, so could probably out-Spiderman lemurs when it comes to smooth, glassy surfaces.


[10] Imagine trying to climb a baobab tree without any gear to help you. It wouldn't go very well. That's because your thumbs and big toes are short and wimpy and can't wrap far enough around the trunk to give you leveraging power. But a size-scaled mouse lemur would have no struggle here. Don't feel bad about it, you can walk on two legs habitually! No other (living) primate can do that! You're special (and not just because your grandma says so).


[11] The next time you’re lamenting the fact you’re stuck eating ramen noodles and ketchup for a few days between paychecks, remember that some mouse lemurs are stuck eating tree sap nine months/year….marmosets too. Then stop whining and eat your simple starches.

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