Metabolism, and metabolic rate tend to feature pretty highly in literature related to dinosaurs and other reptiles. For instance it is often stated that reptiles have metabolic rates around 1/10th those of similar sized mammals and birds, but what exactly does that mean? Talks of thermoregulation focus heavily on the role of metabolism, while allometric studies focus on how metabolism is affected by size. Given the prevalence of metabolic terminology in dinosaur and reptile papers/books, I thought it might be best to quickly give a review of metabolism, metabolic studies, and what all of that means for real animals.
Metabolism is everything
Metabolism is defined as the sum total energy expenditure of an organism. That is to say metabolism is the total energy an organism uses during its life. It is often broken up into the chemical reactions that build up resources (anabolism) and the reactions that break those resources down (catabolism). The amount of metabolism, or energy expenditure during a specific interval of time (seconds to days) is referred to as metabolic rate. From bacteria to blue whales, metabolism is the measure of all the energy that lets these critters go, and metabolic rates determine how much energy that is going to take. It can be measured in a variety of ways from respirometry to doubly labeled water and heart rate telemetry. The diversity of metabolic rate measurements is reflected in the units used to measure metabolism; which can range from watts/hour to milliliters of oxygen per minute, and even to joules per second.
Specificity is important
A key thing about metabolic rates is that they are plastic. They change depending on the situation presented. For instance one could measure the metabolic rate of a sleeping cat, and then compare it to measurements from that same cat while playing, or after eating a big meal. Metabolic rates ramp up when energy demand increases, and then ramp down when that energy demand decreases, or when the environment demands drastic energy cuts (e.g. starvation). Thus when measuring the metabolic rate of an animal it is important to decide exactly what kind of metabolic rate you are trying to measure.
And boy, oh boy are there a lot of different flavours to choose from.
One can measure: BMR, SMR, RMR, MMR, AMR, and FMR just for starters.
Those are a lot of initialisms, and they are just the most common ones. The choice of metabolic rate that one decides to measure is also going to dictate the technique that will be employed. So what do all these things stand for, and what technique is best for what? Let’s find out. Continue reading → Post ID 709
The site has been pretty slow for the past couple of months as academic life has eaten up a lot of my free time. That should change in a few weeks (and I do have a doozy of a post I have been working on).
Whether, or not the crocodile meant to try this, or if this was an accidental predation attempt remains unknown. While this might be the first time this was caught on film, it is not the only account of this. Nile crocs have been known to attempt to take down large elephants by grabbing onto their trunks. The initial attack is almost always unsuccessful, but if the croc winds up doing enough damage it can result in the elephant dying days later (as a broken trunk pretty much limits all access to food). Not sure if the crocs ever benefit from this, but the fact that they can kill large adults is enough to make them a formidable threat to any thirsty elephant.
Continuing the series, let us now take a look at one weird turtle species in particular: Dermochelys coriacea, the leatherback sea turtle.
While the utter weirdness of D.coriacea is ultimately the main reason for why it wound up in this series, there is an ulterior motive. Having searched the internet for general information on the species I found myself rather disappointed with the amount of utterly generic / wrong info regarding leatherbacks. Its Wikipedia entry is particularly disappointing. So here’s hoping this influx of information can help alleviate that.
A turtle without a shell?
Yes, it’s true, leatherback turtles have lost their shells. Shell reduction is relatively common in turtles. It seems a little funny. After going through all the trouble of evolving impregnable armour, many taxa then went out and removed large chunks of it. We see shell reduction in snapping turtles (Chelydra and Macrochelys), soft-shelled turtles, and even other sea turtles. None of them, however, reduced their shells to the point of actually removing them.
In leatherbacks the “shell” is nothing more than a loose collection of osteoderms spread over the back and belly. There is no longer a definitive carapace, or plastron. In fact leatherbacks don’t even produce Beta-keratin (the hard component of reptile scales). Instead this has all been replaced by thick, leathery skin.
This is a long overdue follow up to my original Turtle Power article back in…yeah never mind the date.
As established previously, turtles are a strange, and highly diverse group of animals, but how did they come to be this way?
The turtle bauplan has been a phylogenetic double edged sword. On the one hand, the unique shell design, and the necessary body contortions associated with it, make chelonians a very easy group to classify. However, it is these same peculiarities that keep us from finding the ancestor to turtles. To date, there are no “half-turtles.” No good transitionals between one reptile group to that of turtles. As such, the list of turtle ancestors runs all over Reptilia. Some paleontologists believe the origin lies at the base with reptiles like procolophonoids, and pareiasaurs. Others believe turtles are a bit more closely related to extant reptiles, and belong in, or alongside the sauropterygians (plesiosaurs, nothosaurs, and placodonts). There is even some evidence to suggest turtles are actually in the same reptile group as dinosaurs and crocodylians (Archosauria).
How can the list be this extensive? Read on to find out.
I would be remiss not to talk about this amazing discovery published last week in Science.
Farmer,C.G. & Sanders,K. 2010. Unidirectional Airflow in the Lungs of Alligators. Science. vol.327:338-340
The anatomical similarities of alligators and birds has been known for quite some time (at least 100 years), and this anatomical similarity extends down into the lungs. Though alligators lack the pneumatic carvings of the post-cranial skeleton (air sacs) that are seen in birds, saurischian dinosaurs and pterosaurs; their lungs and bronchi do share the same structural features.
Birds have a unique lung design that allows air to pass through it in a single direction. Unlike mammals, there is no “dead end” to the avian lung. This provides the benefit of a constant supply of highly oxygenated air to the lung tissue; which allows for more efficient gas exchange. Up until last week, this lung design was thought to be a hallmark of birds, and possibly saurischian dinosaurs, and pterosaurs.
Well it turns out that this unique avian synapomorphy is a heck of a lot older than we thought.
Dr. Colleen Farmer, and Kent Sanders M.D. of the University of Utah, considered the uncanny anatomical similarities of the avian and crocodylian lung, and wondered if these similarities extended to the physiology too. In other words: If it looks like a unidirectional lung, does it also function like one?
Farmer & Sanders set to work by removing the lungs of four dead alligators donated to her lab. They pumped air through them, and monitoring the direction in which it traveled (using flowmeters). They then surgically inserted flowmeters into anesthetized alligators, and measured the airflow direction in living animals. Lastly, to drive the point across completely, they filled up an excised lung with fluid that contained fluorescent beads, and proceeded to pump the water in and out. This last test was recorded, and three movies of it, were made available to the public. They can be viewed here. Three was probably overkill though, as once you’ve seen fluorescent beads move one way in a gator lung, you’ve seen them all. : )
The results showed conclusively that alligator lungs pump air through them in one direction only. The repercussions of this find are actually pretty enormous. For starters, the similarity in anatomy and physiology of avian and crocodylian lungs, suggests that they are homologous. This would mean that both groups inherited these lungs from a common ancestor. This means that it was highly likely that all dinosaurs, pterosaurs, rauisuchians, aetosaurs, phytosaurs and the myriad of other archosaurs that graced this planet some 200 million years ago, housed this particular flow-through style lung.
It also helps put to rest arguments about air sac functions. It has long been argued that the presence of a unidirectional lung, necessitates the presence of air sacs to “pump the air in.” (air sacs offer zero, or next to zero gas exchange potential, so there is no actual breathing going on in them). A lack of air sacs in ornithischian dinosaurs, has been used to suggest that their pulmonary physiology was more like mammals and lizards, than it was like birds (Ruben et al 1999). Data from previous research (O’Connor & Claessens 2005) has cautioned that the presence of air sacs does not guarantee the existence of a flow through system. These latest data now show us that a flow-through system can, and likely did, evolve without the “need” for an air sac pump.
Exactly how all of this works, is still not understood. The “hepatic piston” diaphragmatic pump of crocodylians is well known, and is likely the ultimate driver of respiration in these animals, but the nuts & bolts of how all this unidirectional flow takes place (the fluid dynamics of the lung) remains a mystery. One question that would be worthy of a follow up study (which the author’s have hinted at doing) is whether, or not a cross-current, or counter-current system (where deoxygenated blood flows perpendicular, or opposite the direction of highly oxygenated air) is present in crocodylians too. A cross-current system is found in birds. Is that unique to them, or was this also a phylogenetic “hand-me-down?” Hopefully now, with this new discovery, future research will be done on the crocodylian lung, to further understand how it actually works.
Ultimately that is the biggest piece of news to come out of this paper. For well over 100 years, the crocodylian lung was just assumed to be a “dead-end” space that worked in a manner similar to that of mammals. It wasn’t until someone actually thought “what do we really know about this structure” did we find something quite the opposite taking place. This is hardly the first time that this has happened either (for instance). As I have mentioned (ranted/harped on) before, reptiles tend to get the short end of the stick when it comes to a lot of biological and paleontological studies (especially if they involve comparison between broad animal groups [classes]). I’m always amazed (though rarely surprised) when a study that actually looks into commonly held assumptions about these critters, finds said assumptions to be quite off the mark. Here’s hoping that we continue to see future studies like this, go on.
In the end, all of this brings us closer to the truth about how life really works; which is why we do all of this stuff in the first place.
Farmer,C.G. & Sanders,K. 2010. Unidirectional Airflow in the Lungs of Alligators. Science. vol.327:338-340
O’Connor, P.M.& Claessens, A.M. 2005. Basic Avian Pulmonary Design and flow-Through Ventilation in Non-Avian Theropod Dinosaurs. Nature. Vol. 436:253-256.
Ruben, J.A., Dal Sasso, C., Geist, N.R., Hillenius, W.J., Jones, T.D. 1999. Pulmonary Function and Metabolic Physiology of Theropod Dinosaurs. Science. Vol.283(5401):514-516.
[Editor’s note: A response from the authors can be found here. It answers many of the questions I had about the paper, though I feel the biggest question remains open for debate. I appreciate the authors taking their time to answer my questions, and PLoS ONE for allowing this type of open communication.]
This post has taken an inordinate amount of time to write up. Mostly because it required finding enough free time to sit down and just type it out. So I apologize ahead of time for bringing up what is obviously old news, but I felt this paper was an important one to talk about, as it relied on a old, erroneous, but very pervasive, popular and rarely questioned hypothesis for how automatic endothermy (mammal and bird-style “warm-bloodedness”) evolved.
Back in November, a paper was published in the online journal: PLoS ONE. That paper was:
Using muscle force data for the hindlimbs of theropods, and applying it to a model based on Pontzer (2005, 2007), the authors were able to ascertain the approximate aerobic requirements needed for large bipedal theropods to move around. Their conclusion was that all but the smallest taxa had to have been automatic endotherms (i.e. warm-blooded).
Time to stop the ride and take a closer look at what is going on here.
In 2004, John Hutchinson – of the Royal Veterinary College, London UK – performed a mathematical study of bipedal running in extant taxa. He used inverse dynamics methods to estimate the amount of muscle that would be required for an animal to run bipedally. He then tested his models on extant animals (Basiliscus, Iguana, Alligator, Homo, Macropus, Eudromia, Gallus, Dromaius, Meleagris, and Struthio). The predictive capacity of his model proved to be remarkably substantial and stable (Hutchinson 2004a). A follow up paper in the same issue (Hutchinson 2004b) used this model to predict bipedal running ability in extinct taxa (Compsognathus, Coelophysis, Velociraptor, Dilophosaurus, Allosaurus, Tyrannosaurus and Dinornis). Results from this study echoed previous studies on the running ability of Tyrannosaurus rex (Hutchinson & Garcia 2002), as well as provided data on the speed and agility of other theropod taxa.
Meanwhile in 2005, Herman Pontzer – of Washington University in St. Louis, Missouri – did a series of experiments to determine what was ultimately responsible for the cost of transport in animals. To put it another way: Pontzer was searching for the most expensive thing animals have to pay for in order to move around. One might intuitively assume that mass is the ultimate cost of transport. The bigger one gets, the more energy it requires to move a given unit of mass, a certain distance. However experiments on animals found the opposite to be the case. It actually turns out that being bigger makes one “cheaper” to move. So then what is going on here?
Pontzer tested a variety of options for what could be happening; from extra mass, to longer strides. In the end Pontzer found that the effective limb length of animals, was ultimately the limiting factor in their locomotion. Effective limb length differs from the entirety of the limb. Humans are unique in that our graviportal stance has us using almost our entire hindlimbs. Most animals, however, use a more crouched posture that shrinks the overall excursion distance of the hindlimb (or the forelimb). By taking this into account Pontzer was able to find the one trait that seemed to track the best with cost of transport in animals over a wide taxonomic range (essentially: arthropods – birds).
This latest study combines these two technique in order to ascertain the minimum (or approx minimum) oxygen requirements bipedal dinosaurs would need in order to walk, or run.
As with the previous papers, the biomechanical modeling and mathematics are elegant and robust. However, this paper is not without its flaws. For instance in the paper the authors mention:
We focused on bipedal species, because issues of weight distribution between fore and hindlimbs make biomechanical analysis of extinct quadrupeds more difficult and speculative.
Yet this did not stop the authors from applying their work on bipeds, to predicting the maximum oxygen consumption of quadrupedal iguanas and alligators. No justification is ever really given for why the authors chose to do this. Making things even more confusing, just a few sentences later, it is mentioned (ref #s removed to avoid confusion):
Additionally, predicting total muscle volumes solely from hindlimb data for the extant quadrupeds simply assumes that the fore and hindlimbs are acting with similar mechanical advantage, activating similar volumes of muscle to produce one Newton of GRF. This assumption is supported by force-plate studies in other quadrupeds (dogs and quadrupedal chimpanzees)
The force plate work cited is for quadrupedal mammals. However, mammals are not reptiles. As Nicholas Hotton III once mentioned (1994), what works for mammals, does not necessarily work for reptiles. This is especially so for locomotion.
In many reptiles (including the taxa used in this study) the fore and hindlimbs are subequal in length; with the hindlimbs being noticeably longer and larger. Most of the propulsive power in these reptiles comes from the hindlimbs (which have the advantage of having a large tail with which to lay their powerful leg retractor on). The result is that – unlike mammals – many reptiles are “rear wheel drive.”
The last problem is by far the largest, and ultimately proves fatal to the overall conclusions of the paper. The authors operated under the assumptions of the aerobic capacity model for the evolution of automatic endothermy.
It is here that we come to the crux of the problem, and the main subject of this post.
Recently went through the hoopla involved with getting WordPress upgraded (needed to upgrade the MySQL database). Everything appears in working order, barring some strange “?” symbols. I’ll have to figure that out later.
This blatant case of slander has raised the question of what one should do in this situation. It has also brought up the broader question of how scientists should handle the media. Should we just sit back, hoping that the interviewers will present the facts as best they can, and then deal with any possible blowback if/when that fails? Should scientists demand tighter editorial control over what is shown in videos like these? We are their scientific consultants after all. Theoretically they need us for legitimacy; which gives us a bargaining chip.
I don’t know what the right answer is. The least I can do is help Matt pass this info along so future researchers who are asked for an interview, can ask the production crew for assurances that they won’t be slandered in the final product.
Continuing my trend of “catching up,” an article in the November issue of Natural History magazine, talks about a new study in the Quarterly Review of Biology, that finds group nesting to be very common place among extant reptiles.
That study would be:
Doody, J.S., Freedberg, S., Keogh, J.S.? 2009. Communal Egg-Laying in Reptiles and Amphibians: Evolutionary Patterns and Hypotheses. Quart. Rev. Biol. Vol.84(3):229-252.
In the paper, Doody et al (no laughing) did a massive search through the herpetological literature (both technical journals, and hobbyist magazines) to look at instances of communal egg laying in reptiles and amphibians (herps). I’m not being hyperbolic here either, as the paper states:
In total, our assembled database was gathered from 290 different sources, including 176 different scientific journals, 72 books or book chapters, 29 unpublished reports, and 13 unpublished theses. We also have included a number of reliable personal communications from herpetologists.
What the authors found was that group gatherings of herps are vastly more common than previously believed. Group egg laying was found to be present in 345 reptile species. Now you might be thinking 345 really isn’t all that much for a group composed of some 8700 species.
Well then aren’t you a Debbie Downer?
Seriously though, the authors address this by mentioning:
Although the difficulty in locating nests hampers our ability to determine the actual frequency of communal egg-laying among species, we can better estimate this proportion by dividing the number of known communally egg-laying species by the total number of species, excluding those for which eggs have not been found. We conducted such a calculation for the three families of Australian lizards known to include multiple communally egg-laying speciesâ€”Gekkonidae, Pygopodidae, and Scincidaeâ€”as gleaned from the Encyclopedia of Australian Reptiles database (Greer 2004). Proportions of these lizard families known to lay communally were 4â€“9%, but, when we exclude species for which nests are not known, these values rise dramatically to 73â€“100%
The biggest take home message to get from Doody et al’s review, is just how much we don’t know about extant reptiles.
…the present review highlights our inadequate knowledge of the nests and/or eggs of reptiles. For instance, the eggs or nests are known in only 7% of Australian lizards of the three families that commonly lay communally (N = 411 oviparous spp.) (Greer 2004).The extent of this knowledge for Australian lizards is probably similar to that for reptile eggs on other continents, particularly South America, Africa, and Asia, where the reproductive habits of reptiles are poorly known. This is in stark contrast to other vertebrates such as birds, for which complete field guides to the eggs and nests are available for several continents
Indeed, just by doing the brief research run needed to compile this blog post, it was apparent that communalism is much more common in reptiles than anyone ever thought. However, because so many of these reports are either anecdotal, or buried in obscure journals, it is easy to miss all the many cases where it is known.
This discovery lead the authors to the inevitable follow up question of: “why?” What benefit do mothers gain by nesting communally?
Numerous hypotheses for why animals nest communally, have been proposed.
Saturated habitat (only so many suitable nest sites)
Sexual selection (choice of males that live in a particular area)
Artifact of grouping for other reasons
Attack abatement (easier to hide a bunch of eggs in one site, than in multiple sites. Less chance that your eggs will be the ones that are eaten).
Maternal Benefits (save time and energy finding a suitable nest site by “freeloading”)
Reproductive success (if the nest site worked once before…)
The authors reviewed all of these possible reasons for communal egg laying in herps. In the end, they found evidence for both the maternal benefits hypothesis, and the reproductive success hypothesis, though they felt a mixed model better explained things.
Sadly, though the authors mentioned how a lack of information on the natural history of most reptiles is largely responsible for this sudden revelation about their nesting behavior, they nevertheless make repeated mentions of how “social interactions are generally less complex in reptiles and amphibians than in other tetrapods” or how herp sociality forms “relatively simple systems“.
The reality is that the old view of simplistic “loner” reptiles that only come together to mate, is not accurate. This is especially true for parental care in reptiles.
The popular view (among the public, and the scientific community) is that reptiles are? “lay’em and leave’em” types when it comes to reproduction. Despite all the herpetological knowledge to the contrary that has been acquired in the past 50 years, it is still popular to spout the party line about reptiles being “uncaring parents.”
Zoologist Louis Somma took issue with this view of reptilian (in particular, chelonian and lepidosaurian) parenting. He conducted a literature search to see how often mentions of parental care in reptiles are recorded. In the end he wound up finding 1400 references to parental care in reptiles (Somma 2003)!
Somma’s survey covered various aspects of parental care. He found reported evidence of nest building and / or guarding in tortoises like Manouria emys (McKeown 1999), Gopherus agassizii (Barrett & Humphreys 1986) and 4 other species of chelonian.
Turning to lepidosaurs, Somma found parental behaviour to be present in 133 species of lizards and 102 species of snakes. Even a species of tuatara (Sphenodon punctatus) is known to guard its nests (Refsnider et al. 2009). Though these numbers appear small compared to the total amount of species that have been described; much like the Doody et al. paper, this is just based off of species whose nesting behaviours we do know. That these taxa all span a wide phylogenetic range, suggests that parental care is more commonplace than initially thought.
Nest guarding is usually a maternal trait, but some squamates exhibit nest guarding behaviour in both parents, such as some cobra and crotaline snakes (Manthey and Grossman 1997) , as well as tokay geckos (Zaworski 1987).
Not only active guarding of the nest, but actual brooding of the eggs is also commonly reported in squamates such as various python species (Harlow & Grigg 1984, Lourdais et al. 2007), and skinks (Hasegawa 1985, Somma & Fawcett 1989). Some species are even known to groom their newly hatched young (Somma 1987).
More interesting still are various reports and observations of parental feeding in some reptile species, such as the skink Eumeces obsoletus (Evans 1959), and the cordylid lizard Cordylus cataphractus (Branch 1998). Not to mention recent evidence of parental feeding in captive crocodylians.
This leads me to the only reptile group where parental care is well publicized: that of the 23 extant crocodylian species. I could, at this point, list references for parental care in crocodylians. However because this behaviour is so well documented for this group, it would seem unnecessary. It is? better to shed light on the many (MANY) examples of parental care in other reptile species. I also didn’t include related examples like placental evolution in the skink genus Mabuya, or instances of egg binding in captive reptile mothers; due to a lack of appropriate substrate to lay their eggs.
In the end, the paper by Doody et al. adds to a growing body of evidence which suggests that the “lay’em and leave’em” reptile species of the world, are the exceptions? and not the rule.
Next time: Biomechanics of running suggest “warm-blooded” dinosaurs. Or: why the aerobic capacity model needs to die already.
Barrett, S.L. & Humphrey, J.A. 1986. Agonistic Interactions Between Gopherus agassizii (Testudinidae)
and Heloderma suspectum (Helodermatidae). Southwestern Naturalist, 31: 261-263.
Branch, B.. 1998. Field Guide to Snakes and Other Reptiles of Southern Africa. Third revised edition. Sanibel Island: Ralph Curtis Books Publishing.
Doody, J.S., Freedberg, S., Keogh, J.S.? 2009. Communal Egg-Laying in Reptiles and Amphibians: Evolutionary Patterns and Hypotheses. Quart. Rev. Biol. Vol.84(3):229-252.
Evans, L.T. 1959. A Motion Picture Study of Maternal Behavior of the Lizard, Eumeces obsoletus Baird and Girard. Copeia, 1959: 103-110.
Harlow, P and Grigg, G. 1984. Shivering Thermogenesis in a Brooding Python, Python spilotes spilotes. Copeia. Vol.4:959?965.
Hasegawa, M. 1985. Effect of Brooding on Egg Mortality in the Lizard Eumeces okadae on Miyake-jima, Izu Islands, Japan. Copeia, 1985: 497-500.
Lourdais, O., Hoffman, T.C.M., DeNardo, D.F. 2007. Maternal Brooding in the Children’s Python (Antaresia childreni) Promotes Egg Water Balance. J. Comp. Physiol. B. Vol.177:560-577.
Manthey, U. and W. Grossman. 1997. Amphibein & Reptilien S?dostasiens. Natur und Tier Verlag, M?nster.
Mckeown, S. 1999. Nest Mounding and Egg Guarding of the Asian Forest Tortoise (Manouria emys). Reptiles, 7(9): 70-83.
Refsnider, J.M., Keall, S.N., Daugherty, C.H., & Nelson, N.J. 2009. Does nest-guarding in Female Tuatara (Sphenodon punctatus) Reduce Nest Destruction by Conspecific Females? Journal of Herpetology. vol.43(2):294-299.
Somma, L.A. 1987. Maternal Care of Neonates in the Prairie Skink, Eumeces septentrionalis. Great Basin Naturalist, 47: 536-537.
Somma, L.A. & Fawcett, J.D. 1989. Brooding Behaviour of the Prairie Skink, Eumeces septentrionalis, and its Relationship to the Hydric Environment of the Nest. Zoological Journal of the Linnean Society. Vol.95: 245-256.
Somma, L. 2003. parental Behavior in Lepidosaurian and Testudinian Reptiles: A Literature Survey. Krieger Publishing Company. 174pgs. ISBN: 157524201X
Zaworksi, J.P. 1987. Egg Guarding Behavior by Male Gekko gecko. Bulletin of the Chicago Herpetological Society, 22: 193.