'Is it hot in here or is it just you?' Clichd chat-up lines may serve some humans well, but other animals prefer more imaginative ways to captivate and attract potential suitors. Take, for example, the male sea lamprey, which 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. 'We thought it's just a structure that was used for some kind of mechanical stimulation that they needed to trigger the female to lay eggs', says Yu-Wen Chung-Davidson, from Michigan State University, USA, who has been studying lampreys for 10 years. However, she wasn't sure if this was the case, and so Chung-Davidson and her colleagues decided to investigate. They publish their findings in The Journal of Experimental Biology 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.' Explaining her next step, she says, 'I happened to have tissues from various life stages of these lampreys and so I compared them and it's very interesting. When they are in the immature state, the male and females look more or less the same. But when I looked in the mature males and females, they were very different. So there's very obvious sexual dimorphism in their morphology and this part of their body.'
When Chung-Davidson delved deeper, looking at
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