Washington: While cliched pic-up lines may serve some humans well in wooing their partners, animals prefer more imaginative ways to captivate and attract potential suitors.
The male sea lamprey, for example, will coax ovulating females into its nest by releasing enticing pheromones.
Once comfortably in the nest, the male will then perform an interesting dance routine, rubbing the female`s belly with a small bump of tissue on his back.
Should the female be happy with what she sees and feels, the two will then spawn their gametes simultaneously.
This unusual courtship routine is well characterised but no one is quite sure what role this bump, called rope tissue, plays in the proceedings.
Yu-Wen Chung-Davidson, from Michigan State University and her colleagues claim that rope tissue is a heat-generating secondary sexual trait-the first of its kind ever identified.
To begin her investigation, Chung-Davidson simply looked at the rope tissue under the microscope, and what she saw surprised her.
"It looked opaque, and it looked like fat to me," she said.
When they are in the immature state, the male and females look more or less the same. But when she looked in the mature males and females, they were very different.
When Chung-Davidson delved deeper, looking at the slides of the rope tissue under a transmission electron microscope, she was again surprised.
The cells weren`t just normal white fat cells.
She said that white fats cells have a characteristic giant oil droplet whereas these cells clearly had several smaller droplets and were packed full of mitochondria (powerhouse organelles that produce energy).
In fact, these fats cells looked remarkably similar to another, rare type of fat - brown fat cells.
While the fatty acid profile looked remarkably similar to that of mammalian brown fat cells, the pattern of proteins varied a little.
All in all, however, the fat looked very similar to brown fat but it remained to be seen whether it had brown fat`s defining trait - the ability to produce heat.