Why dinosaurs were larger than today's terrestrial mammals
Why were some dinosaurs able to get so much larger than today's terrestrial mammals? It was hypothesized that large dinosaurs might have faced a lower risk of extinction under ecological changes than similar-sized mammals, because large dinosaurs had a higher potential reproductive output than similar-sized mammals (JC hypothesis). However, a new study suggests that reproductive strategy could have contributed to the evolution of the exceptional sizes seen in dinosaurs.
The research report authored by Drs. Werner and Griebeler at the University of Mainz, Germany was published on December 14, 2011 in the open access peer-reviewed journal PLoS ONE.
A higher potential reproductive output is advantageous when the size of a population is reduced, e.g., by a catastrophic event. While at equilibrium (at the carrying capacity of the population) mortalities are high in a population, because mortalities balance births; at lower population sizes mortalities are lower, and, thus, the number of surviving offspring per adult and reproduction event is much higher than at equilibrium. If population size is reduced and the species has a higher potential reproductive output, the species is able to increase population size faster than a species with a much lower potential reproductive output and a similar adult mortality. The faster a population reaches the equilibrium after a reduction in population size, the lower is its extinction risk and the higher is its chance to sustain viable populations over evolutionary time.
The researchers first tested the assumption underlying the JC (Janis and Carrano) hypothesis. They therefore analyzed the potential reproductive output (reflected in clutch/litter size and annual offspring number) of extant terrestrial mammals and birds (as “dinosaur analogs”) and of extinct dinosaurs. With the exception of rodents, the differences in the reproductive output of similar-sized birds and mammals proposed by Janis and Carrano (1992) existed even at the level of single orders.
Fossil dinosaur clutches were larger than litters of similar-sized mammals, and dinosaur clutch sizes were comparable to those of similar-sized birds. Because the extinction risk of extant species often correlates with a low reproductive output, the latter difference suggests a lower risk of population extinction in dinosaurs than in mammals.
In the research report, the researchers present a very simple, mathematical model that demonstrates the advantage of a high reproductive output underlying the JC hypothesis. It predicts that a species with a high reproductive output that usually faces very high juvenile mortalities will benefit more strongly in terms of population size from reduced juvenile mortalities (e.g. resulting from a stochastic reduction in population size) than a species with a low reproductive output that usually comprises low juvenile mortalities.
Based on their results, Drs. Werner and Griebeler suggest that reproductive strategy could have contributed to the evolution of the exceptional gigantism seen in dinosaurs that does not exist in extant terrestrial mammals. Large dinosaurs, e.g., the sauropods, may have easily sustained populations of very large-bodied species over evolutionary time.
Reproductive Biology and Its Impact on Body Size: Comparative Analysis of Mammalian, Avian and Dinosaurian Reproduction. Werner J, Griebeler EM. PLoS ONE .2011;6(12): e28442. doi:10.1371/journal.pone.0028442
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