Female mosquitofish select males who had a better start in life, but researchers aren't entirely sure how they can tell the difference. Pictured is a male mosqutiofish (Gambusia holbrooki) trying his luck with a female while another male looks on.
Credit: Andrew Kahn
ABERYSTWYTH: Females fish prefer to associate with males that experience normal development over those that suffer an early period of food restriction - despite appearing identical in every way.
In a new study of mosquitofish (Gambusia holbrooki), Australian researchers have found that females are able to identify males that, due to a period of reduced food intake, underwent a period of accelerated growth in early development. Despite appearing the same to us, females were able to identify these fish and avoid selecting them as mating partners.
"This is the first time that that compensatory growth has been shown to influence sexual attractiveness in animals. What was really interesting was that the females were picking up on differences in the males we couldn't see ourselves," said Andrew Kahn from the Australian National University in Canberra, lead researcher of the paper published in Biology Letters today.
The cost of catching up
Restricted access to food in early life can affect development and growth. However, if feeding conditions improve then in many species, individuals are able to catch up with the development of others with accelerated growth, or by delaying sexual maturation. This can result in animals that, despite appearing superficially identical, have very different developmental histories.
As one of the first few studies into development and sexual selection, this research is the first of its kind to identify a link between compensatory growth and female mate choice. Previous studies on zebra finches and sword tales did not detect any similar connection.
"We asked: can females tell the difference between a male who has undergone normal, consistent growth and one who's suffered a period of poor growth, followed by compensatory growth? We found that females could indeed discriminate between the two - even when they were the same size - and spent, on average, more time with the normal-growth males," said Kahn.
Judging a better start
The researchers tested 635 newborn male mosquitofish fry, 311 of which were placed on a restricted diet for 21 days. These fish were then left to reach an age of maturity before beginning the trial. The trials were conducted in an aquarium with three compartments, with one male placed in one compartment each and a female in the middle compartment.
After a five-minute acclimation period, the barriers between the cylinder were lifted and photographic samples taken to measure the interest displayed by the female to each of the males.
Although the males appeared identical to researchers, the females were able to pick up on cues to identify males that had undergone this period of compensatory growth. The authors argued that this behaviour may help females to avoid disease and to select a genetically superior mate. "This suggests that sometimes animals that are assumed to be equally attractive by researchers (because they are they are the same size) might not be when you actually let the animals choose," said Kahn.
Compensatory growth or poor nutrition?
According to the researchers, there is a difficulty in identifying if the results of their study are directly linked to compensatory growth, or if an early period of poor nutrition on its own effects male attractiveness. If the later is true, this could suggest that an early period of stress is expressed throughout life and effects attractiveness.
Although it is currently not known for sure how females are able to make these assessments - some early suggestions include differences in locomotor performance, or movement, and the length of genital organs (called gonopodia). Research has indicated that the gonopodium length can be influenced by food shortages in early life.
Brian Langerhans from North Carolina State University in the U.S., who was not involved in the study, agreed with the researchers that this may be a clue into understanding how female fish are able to see what researchers cannot. "How females are able to assess past histories in the males without obvious visual cues or known social histories is currently a mystery but the first places to look are probably the size of the male's gonopodium (male genital organ) and their body shape", he said. "Both of these may be altered by poor nutrition early in life and are known to influence female mating preferences."
