That's an odd name for a post, and I'm aware that my amateur status opens me up for all kinds of criticism; I'm a dabbler, not a scientist. The reader could also infer a sort of "mammalian chauvinism", i.e. the concept of evolution that divides nature into "higher" and "lower" forms, with the "lower" forms all striving to become more mammal (read: human)-like. This is the sort of 19th-century thinking that produced the "evolutionary dead-end" fallacy about dinosaurs, arguing that the most successful and long-lived tetrapod family on the face of the earth was simply too dim-witted and sluggish to survive. We now know these creatures to have been intelligent, dynamic, and adaptable in their own right - in fact, they live among us today in bird form. Our mammalian ascendancy, when you get down to it, is down to sheer luck.
At the same time, as I survey the record of life, I can't help but notice some odd patterns emerging again and again - cases of convergent evolution that leave me scratching my head. To help develop my theme, I'd like to present a very special modern lizard called the Taegu, and specifically the Black-and-White species (Salvator merianae) currently running rampant in Florida.
I'm not going to mince words: the Taegu is a super-lizard, surpassing even the well-evolved monitor lizards for sophistication. The Taegu is extremely intelligent, to the point of problem-solving; as pets they enjoy human company (provided they're handled enough) and can even be housebroken. They are partially warm-blooded, in a process called "seasonal reproductive endothermy": they change their entire metabolism during the breeding season. And last but not least, they have heterodont teeth: grippers, rippers, and grinders. Taken altogether, one could say this is "The Lizard that Wanted to be a Mammal!" (cue doomful music).
This is not to say something like, "Mammals are the apex of evolution, and every species wishes it were a mammal". Rather, it would be better to say, "There are a suite of features commonly associated with Mammals, toward which many tetrapod species seem to be evolving." Clunky, true; but I think it gets at the point I'm trying to make. Though biological evolution is a completely blind, fumbling process, it still tends toward greater and greater complexity as genes are selected for survival and adaptability. To coin a phrase, evolution is the "Anti-Entropy". Take eyes, for instance: the damn things just keep evolving. Snakes, for instance, seem to have evolved from lizards that went underground and lost their eyes...and when they surfaced, they re-evolved their eyes. That's why snake eyes are so weird and unsophisticated. Granted, the genetic information was already there, but the fact that these genes were slowly switched off and then slowly switched back on is still pretty wondrous.
All this is a roundabout way of saying that certain complex features - for instance, large brains, endothermy, and heterodont teeth - keep being selected. Look at the precursors of mammals, for instance: the gorgonopsids et al. developed large brains, heterodont teeth, hair follicles, and even partially upright postures - all this before the age of the dinosaurs. The dinosaurs themselves are distinguished by upright postures; some also had large brains, follicles (feathers in their case) and heterodont teeth, and many were certainly endothermic. Had they not died out at the end of the Cretaceous, we might have ended up with "mammals" that actually had a dinosaur lineage.
And even our modern avian dinosaurs have a weirdly mammalian traitor: the kiwi. As many scientists will tell you, the kiwi hits several key mammalian points: flightlessness, extremely hair-like feathers, an acute sense of smell, whiskers, and ovaries in the female (a rare feature in birds).
The fact that the Mammalian taxon vacuumed up all of these features - plus its key distinguishing feature, milk production - hints at what might be termed, "The evolutionary grindstone". This is rather the opposite of the "Apex of life" fallacy; instead, we see mammals evolving under the direst of circumstances: first they nearly perish in the End Permian Exinction, in which fully 70% of all land lifeforms were extinguished (as well as 90% of sea life); then they have to contend with the Age of Reptiles, in which the archosaurs were busy fighting it out, Clash-of-the-Titans style, to see who would inherit the earth. For the next 135 million years they lived in the shadow of the dinosaurs. Our incredible sense of hearing and smell, and the keen night-vision of most mammals, are relics of this period. In this scenario, natural selection brutally punished any attempt at diversification, only rewarding minute, complex refinements of the features that already worked. As with empires throughout history, we mammals make the mistake of thinking we "conquered" our reptilian adversaries, when in fact we merely scraped by, surviving by the skin of our complex teeth.
So in short, the answer to the question, "Why do reptiles want to be mammals?" is, "They don't. They just want to survive." When the going gets tough, the suite of features considered "mammalian" are simply the ones selected for.
Still, it'll be pretty cool to see - if we manage to live long enough as a species - what the Taegu lizard and other weirdly mammal-like creatures evolve into. For the Taegu, I predict its Seasonal Reproductive Endothermy will evolve into full endothermy (and could provide an explanation for how mammalian endothermy evolved in the first place). Its brain will continue to develop as it explores new habitats, and its teeth will become more mammal-like and sophisticated - the four sets of "canines" on each side of the mouth will eventually become one large I-tooth. Follicles will be a different matter altogether; do hair/feathers naturally evolve alongside endothermy? At the least, it might develop a way of raising its scales to allow heat dissipation. Live birth is not out of the question, as skinks are viviparious; but remember that several mammalian branches, including the modern-day monotremes and the hypersuccessful but sadly extinct multituberculates, laid eggs. Upright posture is likely, but the hip- and shoulder girdle-modifications required would take a very, very long time. Milk production is the least likely adaptation to occur...but you never know.
Rick Out.
(PS: For an interesting article on the date of endothermic development in the mammal line, visit Brian Switek's excellent Laelaps, here).
At the same time, as I survey the record of life, I can't help but notice some odd patterns emerging again and again - cases of convergent evolution that leave me scratching my head. To help develop my theme, I'd like to present a very special modern lizard called the Taegu, and specifically the Black-and-White species (Salvator merianae) currently running rampant in Florida.
 |
| Argentine Black-and-White Taegu |
I'm not going to mince words: the Taegu is a super-lizard, surpassing even the well-evolved monitor lizards for sophistication. The Taegu is extremely intelligent, to the point of problem-solving; as pets they enjoy human company (provided they're handled enough) and can even be housebroken. They are partially warm-blooded, in a process called "seasonal reproductive endothermy": they change their entire metabolism during the breeding season. And last but not least, they have heterodont teeth: grippers, rippers, and grinders. Taken altogether, one could say this is "The Lizard that Wanted to be a Mammal!" (cue doomful music).
 |
| Taegu skull - wikipedia. |
This is not to say something like, "Mammals are the apex of evolution, and every species wishes it were a mammal". Rather, it would be better to say, "There are a suite of features commonly associated with Mammals, toward which many tetrapod species seem to be evolving." Clunky, true; but I think it gets at the point I'm trying to make. Though biological evolution is a completely blind, fumbling process, it still tends toward greater and greater complexity as genes are selected for survival and adaptability. To coin a phrase, evolution is the "Anti-Entropy". Take eyes, for instance: the damn things just keep evolving. Snakes, for instance, seem to have evolved from lizards that went underground and lost their eyes...and when they surfaced, they re-evolved their eyes. That's why snake eyes are so weird and unsophisticated. Granted, the genetic information was already there, but the fact that these genes were slowly switched off and then slowly switched back on is still pretty wondrous.
 |
| The "Mammal-like Reptile" Sauroctonus parringtoni (Wikimedia Commons) |
 |
| Pictured: Traitor. (Wikimedia) |
The fact that the Mammalian taxon vacuumed up all of these features - plus its key distinguishing feature, milk production - hints at what might be termed, "The evolutionary grindstone". This is rather the opposite of the "Apex of life" fallacy; instead, we see mammals evolving under the direst of circumstances: first they nearly perish in the End Permian Exinction, in which fully 70% of all land lifeforms were extinguished (as well as 90% of sea life); then they have to contend with the Age of Reptiles, in which the archosaurs were busy fighting it out, Clash-of-the-Titans style, to see who would inherit the earth. For the next 135 million years they lived in the shadow of the dinosaurs. Our incredible sense of hearing and smell, and the keen night-vision of most mammals, are relics of this period. In this scenario, natural selection brutally punished any attempt at diversification, only rewarding minute, complex refinements of the features that already worked. As with empires throughout history, we mammals make the mistake of thinking we "conquered" our reptilian adversaries, when in fact we merely scraped by, surviving by the skin of our complex teeth.
So in short, the answer to the question, "Why do reptiles want to be mammals?" is, "They don't. They just want to survive." When the going gets tough, the suite of features considered "mammalian" are simply the ones selected for.
Still, it'll be pretty cool to see - if we manage to live long enough as a species - what the Taegu lizard and other weirdly mammal-like creatures evolve into. For the Taegu, I predict its Seasonal Reproductive Endothermy will evolve into full endothermy (and could provide an explanation for how mammalian endothermy evolved in the first place). Its brain will continue to develop as it explores new habitats, and its teeth will become more mammal-like and sophisticated - the four sets of "canines" on each side of the mouth will eventually become one large I-tooth. Follicles will be a different matter altogether; do hair/feathers naturally evolve alongside endothermy? At the least, it might develop a way of raising its scales to allow heat dissipation. Live birth is not out of the question, as skinks are viviparious; but remember that several mammalian branches, including the modern-day monotremes and the hypersuccessful but sadly extinct multituberculates, laid eggs. Upright posture is likely, but the hip- and shoulder girdle-modifications required would take a very, very long time. Milk production is the least likely adaptation to occur...but you never know.
Rick Out.
(PS: For an interesting article on the date of endothermic development in the mammal line, visit Brian Switek's excellent Laelaps, here).
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