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Size Sexual Dimorphism in Dinosaurs - A Hypothesis
What is Sexual Dimorphism?
Sexual dimorphism as defined by wikipedia is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, size, ornamentation, and behavior. An example of ornamentation would be the striking color patterns of the males of many bird species such as peacocks and pheasants, while the females have a much duller color appearance. A morphological example would be the large, straight, erect dorsal fin on a male killer whale versus the hook shaped fin of the female. A behavioral difference would be the lower, more transient degree of social behavior in a male lion versus the tight pride of the females.
When scientists refer to sexual dimorphism and do not specify, more often than not, they are referring to size. Size sexual dimorphism is termed from the perspective of the human species. Hence, species with females being the larger of the two is referred to as reverse sexual dimorphism (RSD), due to the male of the human species being larger than the female. Species where there is a very large difference in size is termed strong sexual dimorphism.
What Causes Males or Females to Be Larger?
The short, simplified answer to this question is what gives the best chance of most efficiently propagating the species. One uniform constant in the animal kingdom is that the male of each species has a biological urge to pass on his genetics. However it is not a uniform constant that the female of every species takes an interest in her young. In every species that is social, the female will have a hand in raising the young. Some social species, the young will be raised by both parents. In solitary animals however, some young are parented, others are orphaned. Orphaned in this article does not mean that both parents are dead, it simply means the young are left to fend for themselves the moment they are born or hatched. Thus, any animal that enters the world does so in one of three states:
1. Social and parented.
2. Solitary but parented until adulthood or semi-adulthood.
3. Solitary and orphaned.
In animals that fall into the first category, the males almost always tend to be larger. Species that fall into the second category can go either way, and with species that fall into the third category, the female is almost always the larger sex.
Social and Parented Species
With most social animals, the male is the larger sex. This rule crosses families, orders, and classes. While most species of freshwater fish, and in a significant number of saltwater fish, the female is larger...it is most likely due to the fact that they lead a solitary lifestyle. In fish that travel in schools, such as barracuda, the males are larger. Virtually all social mammals and social terrestrial vertebrates, the male is larger. A rare exception is the spotted hyena, where the females are only slightly larger than males.
A primary reason for this manifestation of size dimorphism is the female is most often the primary caretaker of the young. Since she takes an interest in the well-being of her offspring, she wants them to have good genetics. She will select a large, strong, virile male. Males become as large, strong, and develop as much prowess as they can, in order to compete for the privilege of mating with females.
Another reason is that in most social animals, due to the fact the female is the primary caretaker of the young, the males protect the herd as a whole. It makes sense for the herd's benefit for them to be the strongest line of defense.
Solitary and Parented
Further strengthening the case that lifestyle plays the key role in sexual size dimorphism, we will investigate the variations in the animals that are solitary and parented. In this section we will take a look at one example of each outcome.
The big cats, except for the lion, are solitary. They also raise their young. In all of the species, males are significantly larger. Contrast that with the rorquals, or baleen whales. In all rorquals, females are slightly larger than males.
The big cats have a few offspring at a time, a relatively short gestation period, and are available to gestate in relatively short periods of time. The female has an interest in having healthy offspring, so the force of the males having to obtain size and prowess to compete prevails, thus the males are larger.
Rorquals on the other hand, reproduce very slowly, have long gestation periods, have only one calf at a time, and do not gestate as regularly. Like the cats, the female also has an interest in having a robust, healthy calf. However there is another force at work. The male wants to maximize the passing on of his genetics. But, he will not be around to raise the calf with the female. Baleen whales have one dangerous potential predator in the ocean - the killer whale. The defense baleen whales have to survive orca attacks are their sheer size and strength. But calves, not having attained full size, are vulnerable to being killed should the mother and her calf encounter pods of transient orcas. Therefore, the male wants the calf to be born as large as possible, and also to grow as fast as possible. A larger female generally gives birth to a larger calf, and carries more milk to enable the calf to consume more, growing larger at a potentially faster rate. That force, when paired with the opposing force of the female's desire for a male's prowess, results in a very close size variation, with females slightly larger. The strength of those opposing forces affects not only the outcome, but the degree of the outcome. The sexual size dimorphism is not strong in baleen whales, whereas it is stronger on the opposite side in the big cats.
Solitary and Orphaned
Animals that are solitary and not raised by either parent generally have a very high infant/juvenile mortality rate. Therefore, these species give birth to a vast number of young at one time.
A prime example are solitary insects and arachnids, and in the vertebrate world, snakes. Since the female will not care for her offspring after they are born or hatched, she cares much less about the prowess of the male than the male cares about passing on his genetics. A larger female gives birth to more offspring. Since most of his offspring will not live to adulthood, a male wants to mate with the largest females that he can find. This driving force prevails easily over any potential competition between males. A notable exception to this rule is the king cobra, where the males are slightly larger than the females.
What Type of Lifestyle Did Most Dinosaurs Have?
There is strong evidence of social behavior in many types of dinosaurs, especially sauropods, ceratopsians, ornithopods, and some theropods.
An example of such evidence are trackways containing many individuals of the same species. A couple ways to verify that they were traveling together at the same time would be to look at their size variation and also directional indications. Large degrees of size variations were found, indicating that adults and juveniles were traveling together. There was also indication of uniform direction change. Changing direction at the same time is a strong indicator of herd behavior.
Another example of evidence for social behavior are fossils from many individuals found in one place. In these cases of mass mortality, these herds most likely perished from a severe natural disaster.
There have also been eggs of developed dinosaur embryos found without teeth. This is evidence that they could not obtain food on their own at the time of their birth, and had to be fed by a parent.
The evidence of social behavior suggests that the majority of the species of dinosaurs fell into the first of the three situations - social and parented. This would indicate that in the world of the dinosaurs, in the majority of species, the males were larger. It is improbable that it is universally true, but a good bet that it was the case more often than not.
There were almost certainly variations of degrees of social behavior, but given the probable gregarious nature of many in the dinosaur clades, the dinosaurs as a whole were complex and developed that even any solitary species most likely parented their young.