[ASC-media] Big babies and small families make evolutionary sense

Niall Byrne niall at scienceinpublic.com.au
Thu Jul 10 00:42:50 CEST 2008

Big babies and small families make evolutionary sense 

Thursday 10 July 2008

Why don't elephants (and humans) have thousands of little babies instead
of one big one?

 Sydney researchers have discovered and modelled the key factors
responsible for offspring and family size. 

Their model predicts what size offspring will be favoured by evolution.
It will help conservation biologists understand what makes species

And it may help explain why human families tend to become smaller,
rather than larger, as parents become richer and live longer.

The study will be published later this month in the journal The American

Why do mammals invest so much time and energy in their offspring? "It's
all to do with abundance of energy, safe environments, and intense
competition," says Daniel Falster, a doctoral student at Macquarie

He and his colleagues studied the family size for hundreds of animal and
plant species from dingos, to gum trees and whales. 

At the centre of Daniel's work is a mathematical model which mimics the
evolutionary process. By estimating the evolutionary pressure on a
particular characteristic, such as the level of parental expenditure on
each offspring, he can predict how that character will change. 

The model can make predictions across the plant and animal kingdoms from
butterflies to fish to gorillas and even humans. "Our model was able to
explain some of the major offspring size patterns seen across species,
such as why larger parents have proportionately smaller offspring, and
why mammals have offspring some 10,000 times larger than plants," Daniel
says. "That gave us a lot of confidence that the fundamentals of the
model were right."

The key process in Daniel's model is competition among rival offspring.
The better an offspring is cared for and fed, the larger it tends to be,
giving it an evolutionary advantage. 

The importance of such size-selective competition is already well-known
in plants, where height allows them to capture more sunlight, but Daniel
suggests it may have been overlooked in other groups. "Our work shows
that without competition, evolution would favour parents having many,
tiny offspring," Daniel says. "Elephants would be having thousands of
little babies instead of one large one".

In reality many species have small families, making their populations
more vulnerable to extinction. 

"A lot of endangered species also have very large offspring. They have
low population growth rates, and this makes it hard for them to endure,
or recover from, any sort of harvesting or loss."

Daniel hopes that his model will help biologists struggling to
understand what makes some species vulnerable to extinction.  

Daniel also thinks his model helps explain aspects of human nature.
Patterns in human demography are consistent with patterns seen among
wild species, he says. "As parents get richer, live longer, and their
environments become more benign, offspring become larger and family
sizes become smaller. Our model explains why this demographic transition
makes sense in light of evolution."

Daniel Falster is one of 16 early-career scientists chosen for Fresh
Science, a national program sponsored by the Federal and Victorian
governments. He is presenting his research to the public for the first

Media contacts: Daniel Falster on 0432 914 310, dfalster at bio.mq.edu.au;
Sarah Brooker on 0413 332 489; and Niall Byrne on 0417 131 977 or
niall at freshscience.org

Photos and background at





Niall Byrne

Science in Public


ph +61 (3) 9398 1416 or 0417 131 977

niall at scienceinpublic.com.au <mailto:niall at scienceinpublic.com.au> 

Full contact details at www.scienceinpublic.com


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