May 3, 2004

Mothers offer first line of defense for offspring

McMaster Image

Cichlids (centre and right) defend themselves and their homes from an intruder (left). The cichlids are mobbing one of their main predators, Lepideolamprologus elongatus.

Hamilton, ON - It’s a mother’s job to protect her children, teaching them to look both ways before they cross the street and not to talk to strangers. In the waters of Lake Tanganyika in Africa there are no streets to cross, but there are plenty of other threats that a mother fish must protect her offspring from.

Sigal Balshine, assistant professor of psychology at McMaster University, along with her graduate students, has been studying tiny fish from Lake Tanganyika called cichlids, trying to understand why some parents aggressively defend their young from predators and other parents defend less vigorously.

The species of fish studied by Balshine are cooperative breeders. Like wolves or some primates, these fish live in groups made up of two breeding fish (male and female) and up to 14 helpers. In some groups, parents work hard, carefully tending the eggs, cleaning their territories, and fiercely defending their young. In other groups, parents do little, allowing the “helpers” to take on the lion’s share of the workload. Balshine and her students are examining the genes, hormones, brains and environmental surroundings of these fish to try to understand how different levels of parental care evolve.

“In the species we are studying, mothers typically work hardest to defend their brood," says Balshine, who uses scuba to study these fish underwater in Africa. Helpers and fathers also care for the young, but to a lesser extent. Balshine and her students have discovered that mothers (females) actually have the highest levels of testosterone, commonly thought of as a “male” hormone.

After carefully choosing a sheltering rock and arranging the sand underneath to make a safe spawning bed, mothers typically lay between 20 and 50 eggs. After three days of fanning eggs with their fins and turning the eggs with their mouths, the young fish will hatch.

And recently hatched fish are particularly vulnerable. Initially the baby fish don’t know how to hide from predators, when to swim, or when to stay still. They are completely dependent on their parents for protection. Within a week, the young are decimated by predators. Usually only one to four babies survive the first week of life. Says Balshine, “Mothers in particular appear to take big risks to protect their young.”