Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS
SAF Archives  search ask the scientists in the classroom cool science
scientists from previous shows
cool careers in science
ask the scientists

Photo of Howard Snell Howard Snell as seen on Voyage to the Galapagos: Paradise Lost?

Click on Howard's photo to read a brief bio.

q Why does the temperature in which the eggs are kept determine the sex of the tortoise hatchlings? (Question sent by Lorie and several other viewers)

A If we look at all animals in the world, the sort of sex determination that we think is normal (were sex is determined by genes - called genetic sex-determination) is rare! Many insects and other invertebrates, as well as many fish and reptiles have environmentally determined gender. That means that whether an individual is male or female depends upon an environmental cue. In the case of tortoises and almost all other turtles that cue is the temperature at which the egg incubates during the second third of development.

The temperature appears to affect the production of hormones that control the development of tissues which eventually become ovaries or testes. If the temperature is low, the hormones produced influence the tissues in a manner that leads to testes; and if the temperature is high, the influence leads to ovaries.

q Does the captive breeding program you are operating mean that the Galapagos tortoises are out of danger? Because there were so few tortoises left before you began the program, I am wondering if there is enough genetic diversity to sustain a healthy population. What's the outlook for this species? (Question sent by M. Pruitt)

A We wish that they were out of danger! Actually the genetic diversity is probably of little importance in this system. Animals that live on oceanic islands may be products of colonization events where only a few individuals founded the whole population in the first place. It is also common for such populations to go through "crashes" caused by severe environmental fluctuations (droughts, for example). The combination of initial low genetic diversity (few founders) with the frequent bottlenecks caused by population crashes yields populations that are resistant to low amounts of genetic diversity. In the tortoises we've seen no signs of detrimental effects potentially caused by low diversity.

Populations of Galapagos tortoises grow very, very slowly. We estimate that it could take 200 years for the population of tortoises on Espanola to reach their natural densities if we were to stop the repatriation program today. That's with nearly 1000 tortoises already returned to Espanola. On the other hand, if we keep working and increase our efficiency via research and improvements in the system we might reach a natural density in 40 to 60 years. Still a long time.

However, there are other populations of tortoises that need protection and research. That is probably the biggest danger. We might not be able to recover some populations in time because of the cost and effort involved.

q My question is about your experiment to reduce the rat population and increase the penguin population. What have you found out since this show was filmed? Are you hopeful this program will be a model for other islands in the Galapagos? (Question sent by Susan)

A The results of the experiment are not what we expected. There has been no detectable difference in the reproductive success of penguins on the islands where rats remain compared to the islands where rats were removed! Now - this doesn't mean that rats can't be a problem for penguins. There are several reasons why we could get this result. Penguins may have had poor reproduction everywhere - with and without rats - last year. Or the rats could have been ignoring penguins in preference of other prey on that particular site. What it does mean is that there is a possibility that rats are not the primary cause of low numbers of penguins we are seeing now.

We now have to do two things. The first is continue the project you saw on television and try to replicate those results. Replication is an important, but sometimes boring, component of science. As I mentioned above, an experimental result can be caused by many factors, and some might not have been appreciated by the researchers. If the experiment is repeated, and repeated again, each time yielding the same results, then our confidence in the results increases. Our interpretation of the results could still be wrong, but the results themselves become less suspect.

The second is that we have to begin looking for other causes of population declines in penguins. Other feral animals, diseases, natural climatic fluctuations, are all possibilities that we must investigate.

Even if our final results show that rats aren't the culprit for penguins, we will still investigate mechanisms for removing rats from islands. We know (from previous good experiments) that rats are a problem for many native and endemic Galapagos organisms and that they should be removed whenever possible.

The point of this experiment was to measure their specific effect on penguins. The difference between a general knowledge that "rats are bad" and measuring their specific effect on penguins illustrates the challenge of managing general environmental threats (rats) and specific cases of declining populations (penguins). The general problem is not always the specific cause.

q I enjoyed the show on the Galapagos, especially how it looked back to Darwin's trip to the islands, when many species seemed to thrive, and forward to the serious implications of increased tourism and the impact of introduced species. As someone trying to protect the Galapagos, do you believe tourism to the islands should be limited? (Question sent by Dennis)

A Tourism in the Galapagos is a model of control. Tourists are limited to specific visitor sites, they can only go ashore if accompanied by a guide licensed by the Galapagos National Park Service, and the number of boats available to carry them among the islands is controlled by the Galapagos Park. Thus the direct impacts of tourism are very, very small and probably no threat at all to the biological diversity of the islands. The very system of licensed guides and vessels effectively limits tourism.

However, tourism has had significant indirect effects on the Galapagos. The increased standard of living in the islands that resulted from the economic benefits brought by tourism became a magnet for immigration from the continent of Ecuador. The population of Galapagos grew at a rate several time that of Ecuador in general, and the increased human population required more goods and services from the continent. More goods and services meant more materials transported by boats and planes and this means more opportunities to introduce species.

A larger population of humans also requires more economic opportunities. The potential resources in an archipelago are very limited, and the new economic opportunities exploited by people might not be as environmentally compatible as tourism.

Thus tourism has had few, direct effects on the biological diversity of Galapagos. However, its indirect effects have been less controlled and resulted in significant problems. Those indirect effects would have been hard to anticipate thirty years ago. However, now that we know about them, they are being addressed by the Galapagos National Park Service and the Charles Darwin Research Station.


Scientific American Frontiers
Fall 1990 to Spring 2000
Sponsored by GTE Corporation,
now a part of Verizon Communications Inc.