Correct answer: 3, Females, unlike males, don’t have to be large to reproduce
Among Nile tilapia (Oreochromis niloticus), the males grow faster and become larger than females. Why is that?
According to Claus Wedekind, a professor at the Department of Ecology and Evolution at the University of Lausanne, “Since males must fight with each other to win a female, and a large body size can be an advantage in such fights, it makes sense for males to grow faster than females.” Because larger males have a better chance of reproducing, it pays off for them to invest their energy in growing big. On the other hand, being large does not carry an advantage for females.
Organisms can expend their energy on different things: survival, growth, maintenance, defense and reproduction. More energy for a particular function means less energy for another. As a species evolves, it learns to make the optimal compromises. Female tilapias invest more in reproduction than in growth.
Following the same logic, Wedekind has developed another theory to explain the allocation of energy among the fish’s various organs: “For fish, investing in ovaries and eggs is usually more important than investing in testicles and spermatozoids. Females therefore allocate more energy to their gonads than males, which impacts their overall growth.”
Did you know?
Tilapia is an ideal fish for farming. Not yet widely known in Europe, it’s one of the main farm-raised species in Asia, Africa and South America. It’s even the second most farmed fish in the world, after carp.
Tilapia fish farmers have long taken advantage of the size difference between males and females. Since the 1970s, tilapia farms have been made up almost exclusively of males, which are more profitable. To achieve this gender imbalance, some farmers induce sex reversal in the fish by adding androgens, or male hormones, to fish feed when the recently-hatched fry are in the sexual differentiation phase. As a result, fish with a female genotype end up having a male phenotype, and therefore grow to be bigger.
One special characteristic of the Nile tilapia is the way females incubate their eggs. First, the male digs a small hole in the seabed where the female lays her eggs. Once the eggs are fertilized, the female puts them in her mouth and incubates them for one or two weeks. One consequence of this is that tilapia females produce much fewer eggs than other species: only up to 1,500, compared with around 8,000 for salmon. Another consequence is that the females aren’t able to eat much during incubation.
“Le tilapia, le poisson le plus consommé au monde,” ConsoGlobe, 24 January 2013
Christian Ouedraogo. “Analyse comparative, physiologique et moléculaire des effets de trois traitements masculinisants chez le tilapia du Nil, Oreochromis niloticus, et recherche de marqueurs de traçabilité liés à ces approches.” Biologie Animale. Université Montpellier II-Sciences et Techniques du Languedoc, 2014. NNT: 2014MON20048.
Phelps, R.P. and T.J. Popma. 2000. “Sex reversal of tilapia.” Pages 34–59 in B.A. Costa-Pierce and J.E. Rakocy, eds. Tilapia Aquaculture in the Americas, Vol. 2. The World Aquaculture Society, Baton Rouge, Louisiana, United States.