Reproductive biology and its impact on body size: comparative analysis of mammalian, avian and dinosaurian reproduction.

Jan Werner,Eva Maria Griebeler

doi:10.1371/journal.pone.0028442

Abstract

Janis and Carrano (1992) suggested 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). First, we tested the assumption underlying the JC hypothesis. We therefore analysed 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. Second, we 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 our results, we 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.

Highlights

Body size is one of the most fundamental attributes of any organism [1,2]
While body size maxima for some organisms can be directly studied in living species, the largest terrestrial animals that have ever existed on earth, the sauropod dinosaurs, must be studied from the fossil record
Because the underlying evolutionary model could be violated and/or the phylogenetic tree utilized by phylogenetic comparative method (PCM) could be inaccurate, we performed additional standard analyses based on subsets of species with different body size classes by comparing their means

Summary

Introduction

Body size is one of the most fundamental attributes of any organism [1,2]. While body size maxima for some organisms can be directly studied in living species, the largest terrestrial animals that have ever existed on earth, the sauropod dinosaurs, must be studied from the fossil record. Their thesis is supported by the observation that body size influences nearly every aspect of the biology of currently existing organisms and that many life history variables correlate with body size [4,5,6] Variables such as mortality, age at sexual maturity, size or number of offspring are important for understanding life history strategies and population extinction risk, because such factors are directly related to the fitness of an organism [7,8,9,10]. We first tested the underlying assumptions of the JC hypothesis by comparing the potential reproductive output (reflected in clutch/litter size and annual offspring number) of terrestrial herbivorous mammals, birds and dinosaurs. We discuss the JC hypothesis in the context of our results and of extinction risk studies on recent animals

Materials and Methods

Results

Discussion