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The Sex of the Dinosaurs

According with a new provocative theory the giant reptiles were extinguished because (and for no other reason than) they were left without females to mate.

by Matias Loewy

Newsweek En Espanol
December 7, 2012

About 65 million years ago, the dinosaurs, which had dominated the earth for a period of 70 times longer than the human being has been on this planet, disappeared in a very silent way. The cause of this catastrophe is one of the major enigmas of all times and has intrigued Dr. Sherman Silber since he was kid of 5 years of age, he is now 71. “I was fascinated, all the kids that I knew wanted to know what happened to these monsters?,” Silber said to Newsweek, this physician specialist in infertility who has pioneered transplants of testicle and ovary in humans. He as written about the first microsurgery to reverse a vasectomy and who currently directs the Infertility Center of St. Louis, Missouri, where he has taken care of business executives and rock stars, astronauts and even kings, from all over the world.

Although many scientists consider that the trigger of this extinction was the cause of an asteroid about 10 kilometers in diameter that impacted in the actual area of Yucatan and resulted in an incredible number of fires, earthquakes, tsunamis, volcanic eruptions, acid rain and something similar to a “nuclear winter”, the question for Dr. Silber remains: If this was true, why it is that the other animals that were supposed to be living at that time, like crocodiles, turtles, mammals and birds did not disappear at the same time?

While he was investigating other genetic factors that may affect female infertility, Dr. Silber found a possible explanation. His theory, which he formulated for the first time in 2004 and just recently presented at the “Buenos Aires XV Argentina Congress of Reproductive Medicine”, is as provocative as it is intriguing: The dinosaurs were extinguished simply because there came about a devastating imbalance in the selection of the sex of the newborns and as a result the male ended up without having enough females to mate. The effect was dramatic, ascertained Dr. Silber, “In the course of three generations they were all extinguished”.

Susana Gimenez’ comment that his proposal brings about an astonishing deduction; In the absence of this particular circumstance – the predominance of males instead of females – such giant creatures like the Tiranosaurius Rex may have survived the crisis and they may still be among us, still alive!

Dr. Silber’s theory is based on the fact that there are no reasons that will allow explanations for the dinosaurs becoming the only victims of the calamity that was associated with the celestial events that were initially described. It’s true the lack of food (as the result of the drop of temperature and the death of the plants) may have affected the largest of the reptiles. However at the same time the small dinosaurs that were as small as a chicken also disappeared. If instead, toxic substances may have been present in the environment, all forms of life will have suffered the same. It doesn’t seem logical to assume that the dinosaurs didn’t have the appropriate capacity to accommodate to the changes in the environment because they very likely were very adaptive creatures, emphasized Dr. Silber.

What happened to them? According to Silber the dinosaurs had a weak spot in their sex determination system, quite different from other mammals and birds that determine their sex by a 50-50 assignment of their sexual chromosomes during fertilization. The early offspring of these big reptiles turned out to be either males or females as a result of one of the most primitive of the accidents of nature affecting evolution? The temperature of incubation of the eggs.

The mechanism denominated “determination of sex by temperature” or TSD is still employed by certain reptiles. One year ago Dr. Silber traveled to visit the Galapagos programs dedicated to recover the population of giant turtles and iguanas. He confirmed that the local scientists incubate the eggs above 29 or 28.5 degrees °C, that way they obtain a larger population of female offspring. By doing that they maximize the reproductive capacity of the species. Below those temperatures, instead, the male descendants will prevail. With the crocodiles occurs something similar, although it is the heat and not the cold, the one that favors the descendants in favor of the masculine sex.

According with Dr. Silber, the dinosaurs were unlucky. The climatic changes were of such a large magnitude that persisted longer than in other previous times. This resulted in a situation where probably 9 out of every 10 of the new pups were males and not females. “We have built mathematical models and confirmed that such a rate predicts that the extinction of the species so effected will occur in approximately 3 or 4 generations”. As the physician emphasized. “All of this makes sense”.

Not everybody is as convinced as he is. Brian Switek, a scientific writer and author of the blog Dinosaur Tracking from the Smithsonian Institute, emphasized “there is no clear indication that the sex of the dinosaurs was determined by variations in temperature”. In fact, he added, the majority of the birds, descendant directly of the dinosaurs with feathers, established the sex of their newborns by genetic mechanism and not by thermic changes. Why not to think that the dinosaurs could have done the same? Silber counters convincingly that birds, or avians, were in fact dinosaurs that evolved genetic sex determination ZZ-ZW system 150million years ago, and that is exactly why they are the only dinosaurs that survived.

One other objection Silber brings up is the fact that the crocodiles and turtles used TSD, they continue to use it and they managed to survive this catastrophe. “How do you explain this?” Silber again pulled more aces from his sleeve: “Both turtles, and crocodiles, are capable of balancing the temperature of incubation. For example, moving away, looking for areas more warm or more cold, or hiding their eggs under the sand”, notes Dr. Silber. Other possibilities are that contrary to the dinosaurs, the alteration in temperature promoted for the arrival of newborn female turtles or, as result of their longevity could reproduce themselves even at very late age and with that having enough time to recover from the drastic high temperatures they experienced.

Additional arguments against the proposed theory are those provided by Sebastian Apesteguia, paleontologist from CONICET at the Felix Azara Institute of the Maimonides Unversity. “Is common thinking that about 65 million years ago the dinosaurs were the only ones that cease to exist, however at the same time many other animals like the amonites (marine molucs) and several different descendants of birds disappeared as he pointed out to Newsweek. However, Silber points out that these whole genera that did not survive did not have a genetic sex determining system.

Silber sees none of these objections as contradicting his theory that is based on the fact that sex determining chromosomes like X and Y or Z and W evolved over and over again for 500,000,000 years, and are entirely different in genera. The only reason for this recurrent evolution of genetic sex determining genes is to protect animals from the extinction that always results from the earth’s environmental temperature changes that always occur every 100,000 years.

Sitting at the conference rooms at the Hotel de Retiro, a few minutes after his conference, Dr. Silber smiled. He argued that the fact that he is not a paleontologist makes for a difficult acceptance of his proposal by paleontologists. However he believes that when all the new evidence is put together (for example when we learn about a method to determine the sex of the fossils of the dinosaurs) he will receive a report in favor of his point of view. “This could be maybe in 20 years, it doesn’t matter”, he says defiantly. “I don’t believe that there is any other better theory that can explain what happened to the dinosaurs”.


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