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The Journal Gazette

Tuesday, May 15, 2018 1:00 am

Room for doubt

Intellectual honesty proves refreshing in dinosaur extinction debate

Christer Watson

I've spent the last couple of years avoiding a lot of political writing. Part of that avoidance is probably because I haven't liked the news much. Another part of it is that I am getting tired of the tone of many debates. There doesn't seem to be any room for honest, open-minded, public consideration of how to solve problems.

As a refuge, I prefer reading movie reviews and scientific debates. For example, I recently heard some scientists propose that the dinosaurs died off because of poisonous plants.

The standard story for how the dinosaurs died is that there was a large asteroid impact. Poisonous plants seemed like something out of left field.

So I found and read the paper. It was nicely constrained. The authors explained their idea but didn't try to oversell it. In their conclusions, they made special effort to emphasize the value of the traditional explanation and, at the same time, advocate for considering other perspectives.

Now for their idea. The dinosaurs mostly died around 66 million years ago. The widely accepted explanation is that a roughly 6-mile asteroid collided with the Earth. Most of the asteroid would have evaporated as it moved through the air, leading to a lot of soot in the atmosphere and a radically different environment.

Observations about things so many years ago are difficult to make precisely, however. There is some evidence that the dinosaurs actually died off over the course of 7 million years. That does not seem consistent with the very quick changes that an asteroid impact would make.

Hence the search for other, more gradual changes. In this paper, the scientists propose that toxic, flowering plants may have first developed around this time. These plants may be partly or mostly responsible for the death of the dinosaurs.

This seems like a crazy idea. Could dinosaurs really, as a whole group, eat themselves to death? There are several reasons to think this may have been possible.

First, many dinosaurs relied mostly or solely on plants for food. Because of their large size, many dinosaurs could not be very selective in the plants they ate. Those dinosaurs that ate meat relied heavily on the plant-eating dinosaurs, so their fates may have been closely tied together.

Second, and more interesting to me, the scientists propose that dinosaurs were especially bad at developing taste aversion. The idea is that the dinosaurs were incapable of realizing which plants were making them sick, so they would continue to eat them.

One way to test this idea is to examine currently living, closely related species. Crocodiles, alligators and their South American counterpart, caimans, are all somewhat close relatives to dinosaurs. A study in the 1980s analyzed caimans' ability to develop a taste aversion. Two different groups of caimans were purposely made sick with a drug. One of the groups, however, was given chicken to eat for the first time right before receiving the drug. The idea was to see whether the caimans could associate a new food, chicken, with being sick.

After the caimans recovered, both groups were given chicken. The groups ate the chicken equally. That indicates that the caimans were not learning to avoid food that would appear to be making them sick.

As a contrast, mammals have a strong ability to develop a taste aversion. Rats, for example, are notoriously difficult to poison because rats will frequently sample any new food and, if they become sick, avoid the new food systematically.

In a crazy-sounding (to me) study in the 1970s, scientists introduced a new flavor to rats, fed them a drug to induce sickness, then anesthetized the rats. The idea was to have the rats asleep while they were sick. Being asleep might prevent the rats from learning to avoid the new flavor.

Nope, it didn't. The rats, after recovering, would avoid the new flavor. Somehow, subconsciously (if that even makes sense for rats), they had developed a taste aversion.

So I don't actually know whether toxic plants killed the dinosaurs. Neither do the scientists who published this article – and they basically admit it in the article. That makes the debate a lot more interesting and gives me a lot more faith in the participants. That is refreshing.

 

Christer Watson, of Fort Wayne, is a professor of physics at Manchester University. Opinions expressed are his own. He wrote this for The Journal Gazette, where his columns appear the first and third Tuesday of each month.