Econstudentlog

Dinosaurs past and present (1)

It’s a neat little book.

Some quotes from the first half:

“most trackways of extant mammal species seen in the wild record leisurely paces made during the slow, unhurried daily and seasonal routine […] Top-speed runs are very important in the evolution of limb structure, but maximum speed accounts for only a tiny fraction of the total footsteps taken by an individual animal during its lifetime. Fossil footprints should be viewed as the documentation of an average daily cruising speed, not of top speed.  […] dinosaurs had cruising speeds as high or higher than that of mammals with comparable body size and feeding habits. Mammoths cruised at speeds no higher than that of nodosaurid and sauropod dinosaurs. Moa cruising speed was no higher than that of duck-billed dinosaurs. Theropod dinosaurs cruised at higher speeds than that of modern mammals. Therefore we can conclude that the average everyday pace of dinosaurian locomotor activity was as quick as or quicker than that of the present-day Mammalia.

In addition, the footprint survey showed that the primitive reptiles and amphibians of the Paleozoic cruised at speeds far slower than that of dinosaurs and mammals. Life in the Carboniferous and early Permian must have been played out at a toad’s pace. A sudden and dramatic increase in average cruising speed coincided with the rise of the advanced mammallike reptiles (therapsids) and thecodonts at the beginning of the Triassic. And the Triassic acceleration of cruising speed coincides precisely with change in bone histology, documented by Ricqlès (1974)”

“MacArthur and Wilson (1967) argue that on a continuous scale of reproductive strategies there are two extreme kinds. One is “r selection” (r stands for rate of increase by reproduction) in which an individual has many offspring either by having a few offspring at frequent intervals or by having large numbers of offspring at one time. One characteristic of organisms that exhibit r selection is that they are small. A good example among mammals is mice versus elephants; mice show r selection, elephants do not. A pair of mice will produce many generations in a short time, while a pair of elephants have few young and each generation takes more than ten years. Elephants show “K selection” (K stands for the carrying capacity of the environment, which becomes the limiting factor for these animals). Two features of animals that exhibit K selection are large size and long generation time.
K and r selections are the two extremes of a range of reproductive strategies. K selection is especially suited to stable climates in which the full resources of the environment can be exploited safely. The tropics are a good example. […] In contrast, r selection is best suited to unpredictable environments, such as temperate and subpolar regions where the production of large numbers of offspring insures against environmental catastrophe, freeze, flood, or drought. Clutch sizes correlate inversely with body size (Calder 1983).” [To me this was just review of stuff I already knew, but I figured some of you didn’t know about r/K selection theory, and the tradeoff between quality and quantity of offspring is an important concept one should know about so I decided to include the quote in the post. Some of the stuff below is review as well, but again – it’s important stuff you should know and it doesn’t hurt to go over it again.]

“The larger the animal, the lower the metabolic rate per unit body weight. Although metabolic rate seems to be related to surface area, it is not. (Not all mammals are constantly warm; those that are not will lose heat to the air when the air is cooler than they are and gain heat when the air temperature exceeds their own. Yet, in heterotherms the larger the animal, the lower the metabolic rate and their metabolic rates also correlate with their surface areas.) Coulson (1984) theorizes that metabolic rate is determined principally by the distance blood has to travel from the heart to the capillary, and the greater the distance, the greater the resistance and the slower the flow through the capillaries and veins and return to the heart.”

“Stepping frequency is inversely related to body size or limb length (Calder 1984). Stridelength increases with size (Maloiy et al. 1979). The total energy cost of travel increases with size but is cheaper per kilogram (Bennett 1982, for reptiles; Taylor, Heglund, and Maloiy, 1982, for mammals).”

“For mammals, at least, size is a significant factor in determining a sense of hearing. Mice can hear more than two octaves higher than elephants at an intensity level of sixty decibels. A strong inverse relationship can be recorded between the high-frequency cutoff (at sixty decibels) and the time difference between the arrival of sounds at the two ears in mammals (Heffner and Masterton 1980; Heffner and Heffner 1980). Birds do not seem to exhibit such a relationship with the high-frequency cutoff, but extremely large birds have not been examined (Knudsen 1980; Dooling 1980).”

Volume is proportional to length cubed; surface area is proportional to length squared. If a simple geometric shape such as a cube doubles in length, it will acquire four times the original surface area and eight times the original volume. When we think of tiny dinosaurs, it is helpful to think of the converse of the consequences such scaling where volume is dramatically reduced relative to surface area […] The result is a tiny individual with a high surface to volume ratio. […] this most profoundly affects heat exchange and other exchange phenomena […] Tiny animals are more likely to seek shelter when stressed by temperature change.”

September 26, 2013 - Posted by | biology, books, Paleontology

3 Comments »

  1. Isn’t the r/K selection theory no longer in vogue?

    Comment by Miao | September 26, 2013 | Reply

    • That’s sort of true:

      “The theory was popular in the 1970s and 1980s when it was used as a heuristic device, but lost importance in the early 1990s as it was criticized by several empirical studies.[1][2] The r/K selection paradigm has been replaced by a life-history paradigm. However, this continues to incorporate many of the themes important to the r/K paradigm.[3]” […] “[the] new life-history paradigm has matured into one that uses age-structured models as a framework to incorporate many of the themes important to the r–K paradigm.”

      Of course there are a lot of other tradeoffs besides the quantity/quality one that also are important when it comes to determining the optimal strategy of reproduction, but I don’t think anyone would say that this tradeoff is irrelevant – and so I still think a simple r/K framework is a useful way to think about some aspects of reproductive patterns across species (and I definitely think it’s the kind of idea people should at least be familiar with). That said, which tradeoffs are most important is naturally to some degree an empirical question, and I’m not really familiar with the literature on this stuff as of today – but it seems perfectly obvious to me that r/K on its own cannot possibly explain all the variation in reproductive strategies that we might observe in biology, and that it probably gets a lot of things wrong. The simple r/K model is way too limited (“Although some organisms are identified as primarily r- or K-strategists, the majority of organisms do not follow this pattern”) but there are some important ideas here anyway. Whether to invest a lot or a little in each individual offspring is an important decision parameter, and it’s a decision parameter which will necessarily relate to some extent to how many offspring you eventually end up having. When to have offspring during your life cycle is also important, and how much it’s optimal to invest in any given offspring probably often depends on age as well. The gender of your offspring may potentially be very important. And so on and so forth. Life’s complex, and obtaining a complete model that ‘explains everything’ is not likely, so relying on simple models in order to understand important dynamics makes sense.

      Comment by US | September 26, 2013 | Reply

      • The context should also be kept in mind: We’re implicitly comparing behavioural patterns of Sauropoda with those of Compsognathus in the discussion here. Did individual Supersauri have hundreds of offspring every year? We can’t say for sure – we weren’t there – but they almost certainly did not.

        Comment by US | September 26, 2013


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