Nov. 22 (UPI) -- Most frogs lay dozens of eggs and leave. The tadpoles-to-be are on their own from the start. In a new study, scientists examined why some frog moms stick around to help raise their young.
At least two species, including Madagascar's climbing mantella, a poison frog species, are much more nurturing and attentive than the average frog mom. Both the climbing mantella and Ecuador's little devil frog lay just a few eggs in pools of water that collect in cupped leaves. The moms sit next to their offspring and feed the growing tadpoles' unfertilized eggs until they're big enough to venture off on their own.
New research, published this week in the journal Current Biology, suggests the extra motherly attention helps the young frogs develop potent chemical defenses.
When scientists tested the unfertilized eggs being fed to the newborn climbing mantellas, they found surprisingly high levels of the poison.
"This egg provisioning strategy is a way for these frogs to chemically defend their offspring sooner," study co-author Lauren O'Connell, an assistant professor of biology at Stanford University, said in a news release.
O'Connell and her colleagues also compared the brains of the two motherly frog species to each other and to the brains of other mammals.
Despite the similar mothering styles of the climbing mantella and little devil frog, the two species aren't closely related. They're separated by thousands of miles and 140 million years of evolution.
How did two distantly related frogs evolve maternal instincts?
"Maternal behavior has evolved only once in mammals," O'Connell said. "We wanted to know, are there different ways to build the maternal brain? Or do they all use the same mechanisms and molecules? These frogs allow us to investigate these questions."
The study began with researchers sitting very still and remaining very quiet in dense rain forest vegetation, waiting for mom. The scientists watched as the hungry little devil tadpoles signaled for mom to deposit another egg.
"The tadpoles do this very short but intensive vibration right next to the mom," O'Connell said.
Researchers also collected brain samples from the mother frogs, as well as unfertilized eggs and tadpole skin from the nesting pools. Back in the lab, scientists confirmed traces of alkaloid toxins in the eggs and on the tadpole skin.
"Our finding is consistent with the idea that maternal provisioning evolved as a way to transfer nutrients but also various other goodies to your offspring," said study author Eva Fischer, a postdoctoral fellow in O'Connell's lab. "We mammals transfer a lot of things that are important for immune function via breast milk. These frogs are transferring toxins through their eggs."
Analysis of the brain samples revealed enhanced neural activity in two regions that also host amplified activity in nursing mammals, birds and fish.
"Both within frogs and also across vertebrates more generally, there seem to be shared brain regions that are being used to build the maternal brain," Fischer said.
However, when scientists tested the frog brains for sensitivity to oxytocin, a neurotransmitter known to promote nurturing behavior in mammals, they found only the brain of the climbing mantella responded.
"This study tells us that there is more than one way to promote maternal behavior in the brain because these frog species are using different sets of neurons to do it," O'Connell said. "Their behaviors are similar, but under the hood, the neural mechanisms are very different."