We don't all grow and develop at the same rate, but if you are a fish a delayed growth spurt can turn you into a wimp especially when it comes to attracting a mate.

Scientists at the University of Glasgow have discovered that male fish can compensate for a poor start in life, by catching up in size so that they end up looking identical to those that have grown at the normal rate. However, looks can be deceptive: late developers are almost always subordinate to their well-fed brethren. The results of the study have been published today in the Royal Society's scientific journal 'Proceedings".

The team from Glasgow of Dr Nick Royle, Dr Jan Lindstr￶m and Prof Neil Metcalfe carried out the study on green swordtails, which are small tropical fish closely related to the guppies often found in pet shops. The 'sword' that gives them their name is a long and colourful extension to the tail that males grow when they become sexually mature. By altering the food supply of the fish when they were young, the research team examined the long-term effects of different growth patterns - a subject that is of great current interest given the recent evidence that early nutrition can have surprising effects much later in life in a range of animals including humans.

Using these small short-lived fish enabled the scientists to control their growth rates accurately from soon after birth for the rest of their lives.

Families were divided so that some males received generous rations throughout and grew quickly to the size at which they could become sexually mature, while their brothers were initially fed at a lower rate so that they grew more slowly, but were then given extra food so that they could catch up in size. When all the fish had become sexually mature they were measured in detail, and then were tested for their ability to become dominant.

'We were surprised at how completely the late developers managed to catch up in size,' explains Dr Royle. 'After only a few months on the extra food rations they were identical in weight, size and appearance to fish that had been well fed throughout their life, and we couldn't tell them apart. And the females seemed to find them equally attractive.'

However, while they might have looked fine to the females, their success in competition with other males was a different matter.

As Dr Royle explains: 'On virtually every occasion that a late developing male was made to compete with a size-matched male that had grown normally, the late developer ended up the loser.'

The fish took the opportunity to compensate for a poor start in life by growing fast when conditions improved, but this came at a cost. Still, it may be a cost worth paying: if the late developers can end up being large and attractive to females then they stand a chance of breeding and passing on their genes.

Media Relations Office (media@gla.ac.uk)

For fuller details please contact the University Press Office on 0141 330-3535, or call Professor Neil Metcalfe directly on 0141 330 5968 or 0777 170 4231.

The study was funded by the Biotechnology and Biological Sciences Research Council, and will be published on September 22 as: Royle, N.J., Lindstr?m, J. & Metcalfe, N.B. 2005. A poor start in life negatively affects dominance status in adulthood independent of body size in green swordtails Xiphophorus helleri.

First published: 22 September 2005