[Archport] Did dinosaurs lack daughters?

[Graca Cravinho <graca@hermite.cii.fc.ul.pt>]

Title: Did dinosaurs lack daughters?

>
> Nature
> Science Update
>
> Did
> dinosaurs lack daughters?
> http://www.nature.com/nsu/040419/040419-11.html
>
> See-sawing climate may have fatally unbalanced sex
> ratio.
> 23 April 2004
> Helen
> Pearson
>
> Dinosaurs may have been forced into extinction partly
> because there were too few females, say researchers in the UK. The
> claim revives a venerable debate.
>
> The creatures died out roughly 65 million years ago, around the time
> that a huge meteor slammed into earth. Some scientists believe that
> the immense dust cloud thrown up caused swings in the climate that the
> dinosaurs were unable to survive.
>
> However, it is not clear exactly how the temperature change killed
> them off. Now David Miller of the University of Leeds and his
> colleagues are proposing in Fertility and Sterility1
> that if dinosaurs used temperature to determine the sex of their
> offspring, climate changes could have messed up the ratio of males to
> females.
>
> This idea is based on the reproduction of modern day reptiles such as
> crocodiles, to which dinosaurs are related. Crocodiles' sex depends on
> the temperature at which their eggs are incubated. Male crocs hatch in
> moderate temperatures, while females emerge if the heat rises or falls
> by a few degrees.
>
> In the case of dinosaurs, Miller suggests that changes in temperature
> after the meteor impact favoured the birth of males. Over time females
> would become rare, causing fewer young dinosaurs to be born and
> species to dwindle to extinction. "They'd probably have had it,"
> he says.
>
> Totally skewed
>
> The idea has been raised before, but few experts were convinced.
> Palaeontologists currently believe that dinosaurs started dying out
> around 10 million years before the meteor impact. This was accelerated
> by a swathe of volcanic explosions and sea level changes that upset
> the climate, although the details remain unknown.
>
> To strengthen the idea that a sex ratio unbalanced by temperature
> changes could have been the cause of the dinosaurs' demise, Miller's
> team built a mathematical model to show how fast a species might
> become extinct if it deviated from a 50:50 sex ratio, or one male for
> every female.
>
> If the sex ratio was skewed to 80:20, for example, the model shows
> that a population of 1000 animals would die out within 50 rounds of
> reproduction. That might represent only 500-1000 years, depending on
> the animal's fertile lifespan. "It is an interesting line of
> argument," says palaeontologist Norman MacLeod of London's
> Natural History Museum.
>
> But MacLeod says the idea is still a controversial one. The majority
> of dinosaur experts believe that the animals are most closely related
> to birds, which do not use temperature to determine sex. Either way,
> it is hard to confirm: "We do not have Triceratops or
> Tyrannosaurus rex eggs to incubate," he points out.
>
> Sex change
>
> Miller's analysis also has to explain why some animal groups that use
> temperature to determine sex survived the change in climate.
> Crocodiles, for example, lived through the same climate shift.
>
> Miller speculates that they may have been able to protect their eggs
> from temperature extremes because they lived near cooling streams, or
> were able to adapt to the changing conditions faster than the
> long-lived dinosaurs.
>
> He believes that other more robust ways of determining sex might have
> evolved partly because temperature-dependent sex determination is so
> risky. Today, most animals including humans use genes to determine
> sex, so that males inherit one set of sex chromosomes and females
> another. This ensures a stable sex ratio regardless of meteors or
> extreme weather.
>
> Some reptiles have clung to the more primitive mechanism. This could
> be because they live in climates in which their eggs are protected
> from large swings in temperature, suggests Bruce Lahn, a geneticist at
> the University of Chicago.
>
> But that leaves these species, which include long-lived turtles,
> vulnerable to future climate change from global warming. "It is a
> real concern," says Lahn.

References

1.    Miller, D., Summers, J. & Silber, S. Fertility & Sterility, 81, 954 - 964, doi:10.1016/j.fertnstert.2003.09.051 (2004).



© Nature News Service / Macmillan Magazines Ltd 2004