suddenly died out because they gave birth
to too many males as a result of climate change.
This is the theory put forward by David Miller
of medicine and Jonathen Summers of mechanical
believe that dinosaur populations died out
because the sex of their offspring was determined
Miller said: “The theory does not contradict
ideas that extinction was caused by a giant
meteorite striking the earth or that there
was a long-term change which dinosaurs could
not cope with – it elaborates on these
many creatures, such as humans and other mammals,
sex determination is genetically controlled
to ensure an approximate 50/50 male to female
some reptiles and fish use temperature-based
methods to determine sex of offspring. They
produce roughly 50/50 ratios under ideal conditions
but a variation of temperature brings a skew
towards males or females, with a skew towards
males leading to population decline.
particular, all crocodiles, which are essentially
similar to dinosaurs, use temperature sex
determination, and Dr Miller argues that this
could mean the extinct giant reptiles used
the same mechanism.
Miller hit upon the idea during research into
the evolution of the Y chromosome in humans,
which he is carrying out in collaboration
with Dr Sherman Silber of St Louis Infertility
Center in the US. Dr Summers brought in mathematical
techniques to model the effects of variations
in male/female ratio on populations.
Neill Alexander of the school of biology,
who is an authority on dinosaurs and has advised
the BBC, said: “Here we have an old
suggestion about dinosaur extinction being
examined in very new ways. Dr Miller and his
colleagues cannot prove that the suggestion
is correct, but they have gone a long way
towards showing that it is feasible.”
complex modelling is planned to explore the
effect of temperature change on different
sized dinosaurs and eggs.
Miller said, “We think that there was
an abrupt and prolonged change in the environment
and that the dinosaurs could not adapt to
it in time to save themselves. A temperature
change made them produce too many males and
they died out as a result."
Dr Miller said that temperature control of
sex determination may have been the original
mechanism for animals.
said: "Birds, which probably descended
from dinosaurs over 170 million years ago,
may have been early adopters of genetically
determined sex and hence immune from the environmental
vicissitudes that affected their dinosaur
cousins. Nevertheless, temperature based sex
determination continues to exist because it
may offer an effective means of rapid niche
exploitation favouring one sex over the other.
But it works best in reasonably stable conditions
and that was one luxury that the dinosaurs
didn’t have 65 million years ago when
they vanished from the fossil record."
For more information see: Miller, Summers
and Silber, 2004. 'Environmental versus genetic
sex determination: a possible factor in dinosaur
extinction?' Fertility and Sterility,
81, pp 954-964.