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Mammal reproductive output & body size; (a theory for) Why mammals can't grow as big as dinosaurs
Topic Started: Jul 5 2018, 05:41 AM (353 Views)
Caimännir
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I recently came across this paper, "Scaling of reproductive turnover in archosaurs and mammals: why are large terrestrial mammals so rare?", by Janis and Carrano.

It’s an incredibly interesting paper on the reproductive outputs of mammals vs dinosaurs (incl. aves), and how the higher reproductive outputs of dinosaurs may have been a significant factor in the large body masses they attained as compared to mammals - more significant than the oft cited biomechanical differences between the two clades.

Basically, they compared statistics like offspring per litter, litters per year, offspring per year, and total potential reproductive output (how many offspring an individual produces over their lifespan), to body mass. They performed this analysis on a range of mammals representing all sizes, comparing them to birds (they chose more terrestrial birds where possible) and dinosaurs where possible.

Here are the graphic data:
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What do you guys think of this data? I the trend is clear - as mammals get bigger, they produce less and less offspring; where birds and (though perhaps dubiously) dinosaurs maintain a similar reproductive output regardless of body mass.

The authors propose that “when viewed over evolutionary time, the presumed greater reproductive output of dinosaurs might make them less vulnerable to environmental perturbations than large mammals, and hence less vulnerable to extinction”. To be a large animal is a precarious position as is; perhaps the increasing inability for megafaunal mammals to reproduce enough to maintain or regenerate their population - as well as a decreased "evolutionary reaction time" - prevents them from sustaining themselves over a geological timescale; "the very few terrestrial mammals that have exceeded [5000 kg] in the fossil record (a classic example being the indrocotheriine rhinos) are known only from a very short period of geological time". This is the central theory of the paper.

What do you guys think? Why do you think that this discrepancy in reproductive output exists? Do you think this fact is the primary reason for a paucity of large mammals, or might other causes, like the biomechanical ones, be of greater significance? (ftr, the paper dismisses biomechanical causes.)

And what implications might this have for (speculative) evolution of dinosaur-sized mammals? How might mammals evolve to circumvent size-limiting reproductive factors? What might 70-ton mammals, and their 10-ton predators, look like?


My thoughts on the matter - exactly why mammals produce young at a lesser rate




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Flisch
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Caimännir
Jul 5 2018, 05:41 AM
Might mammals have cursed themselves by their viviparous, lactating, caring strategy?
Cursed? Evolutionary success is not based on body size. Body size is just another adaption that helps with predation, finding food and/or storing body heat.

If anything, the reduced potential for larger bodies is merely a side effect of another highly beneficial adaption: Broodcare. The evolutionary trend is clear: Species with intensive broodcare weather mass extinctions better than those without. Ammonites and Pterosaurs are extinct. And they're both thought to practise next to no healthcare. The text makes it clear that broodcare, at least the mammalian version, is very costly, but I suppose if you add together all the costs, the energy requirements do not outweigh the wasted energy from predated-upon juveniles.
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Sceynyos-yos
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To add on to that, in mass extinctions it's always the megafauna that goes away first. Hardly a curse, for those ones who survive anyway.
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Caimännir
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To add on to that, in mass extinctions it's always the megafauna that goes away first. Hardly a curse, for those ones who survive anyway.

Yeah, I chose "cursed" as applying for the palaeontologists who will never uncover the femur of a 60-ton indricothere (and the movie makers who will never make billions off a "Pleistocene Park" haunted by a 10-ton feline), not for the animals themselves. Mammals are plainly very successful, and their successful strategy seems to preclude large sizes for the better.

But I don't think broodcare and large sizes are necessarily mutually exclusive. They are exclusive in mammals today, where the adult parents nurture their offspring to adulthood; but - one might speculate - arrangements could evolve that involve both a mentoring-nurturing relationship between a capable older individual and a juvenile, and also allow for large sizes; a few ideas at the very bottom of the post.
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Velociraptor
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I think the fact that saurischian dinosaurs had a birdlike respiratory system and pneumatic skeleton is a more important factor in size limits than parental care. Ornithischians lack these traits and the largest ornithischian, Shantungosaurus, actually weighs less than the largest land mammal, Palaeoloxodon namadicus.
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IIGSY
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Hmm. What implications would this have for a terrestrial animal with a reproductive output more akin to some fish or arthropods, laying thousands of eggs? How big could they get?
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Canis Lupis
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Dinosaurs eat man, woman inherits the Earth.

I think this has much less to do with how many eggs/young you reproduce, but rather what your anatomical structure is like and whether a given environment/niche can support a large size. Anatomically, dinosaurs were just highly favored when it came to developing large terrestrial sizes. Mammalian structure is just not able to produce sauropod sizes. That has everything to do with bone and organ structure, and little to do with reproductive strategy.
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HangingThief
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IIGSY
Jul 5 2018, 04:23 PM
Hmm. What implications would this have for a terrestrial animal with a reproductive output more akin to some fish or arthropods, laying thousands of eggs? How big could they get?
Sauropods laid eggs that only weighed a few pounds. Incredibly miniscule compared to their adult body size. I don't know if there's anything known about how many eggs they laid, but it could've been a lot.
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Caimännir
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I'm not convinced that biomechanical factors are the constraining feature of mammal size. This paper, "The evolution of maximum body size of terrestrial mammals", does indeed find a maximum size reached by mammals across the cenozoic, but correlates these sizes with abiotic factors like continent area and temperature (Bergmann's rule), which suggests that size in mammals is limited by the resources mammals would require to sustain that size (and how mammals use the resources available to them).


This paper, "On the rareness of big, fierce animals: speculation about the body sizes, population densities, and geographic ranges of predatory mammals and large carnivorous dinosaurs", brings up the fact that food requirement (obviously) scales with body mass.

However, it goes on to claim that the largest theropod dinosaurs - if food requirements in carnivorous dinosaurs scales with the same proportion to body mass as it does in mammals - would have needed an incredible quantity of food (read: large herbivorous dinosaur prey) to sustain a tenable population; more than is evidenced in the fossil record, or would be realistic in the modern world.

The paper thusly postulates that dinosaurs (herbivores and/or carnivores, both would explain things) required significantly less food for the same body mass than mammals. It lists a few reasons, like ontogenic niche partitioning, and greater reproductive output of herbivores as mentioned by Janis and Carrano; as well as other possibilities, like mesothermy or efficient digestion, or a greater abundance of vegetation in the mesozoic.

Of course, air sacs and pneumaticised bones are weight-savers, which would undoubtedly be a big reduction in energy requirements per size - but I doubt that that's enough to account for the energetic sustenance of body sizes in dinosaurs that would be purely untenable for an ecologically modern mammal.

Edit 3: Either way, biomechanical constraints doubtlessly play a big role, but I wanted to pose an alternative concept of dinosaur size in juxtaposition to the usual air-sacs and hollow bones idea (and for the speculative potential, of course).
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Archeoraptor
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I think that as always all factors influenciate, also look at some whales they are on the sauropod size range, I know they are aquatic and their bauplan is less limited by that but that shows how truly the reproductive system can hold mammals back, I douth whales have less parental care than land ungulates or other big mammals
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