I'm not sure of the exact design you refer to, but feasibly, if the top is narrower than the base, it could act to decrease climbing behaviour and force the animal to interact more with the central areas of the tank??

The top of the tank is wider than the bottom (see attached). Several studies that I've read use the 'novel tank diving test' to investigate stress response in zebrafish and all use a trapezoid-shaped tank but none explain why.

So are you looking at zebrafish? (i.e. my comment about climbing may have been a bit misplaced!)

Having looked at the paper you attached, I'd agree that encouraging thigmotaxis (i.e. movement along surfaces) may be a factor, in common with the open field commonly used with rodents. It sounds like the narrower base is viewed as a 'safer' zone compared to the more open water above, so ethologically, the organism is assessing the risk associated with being in open water (increased predation etc) compared with staying in the more enclosed lower level of the tank, with increased exploration indicative of reduction in perceived risk (analogous to reduced 'anxiety').

Thanks Rosie. Yes, it could be that the narrower base is viewed as a safer zone but I don't understand why the base angles of the tank are different. I will email the authors (and let you know what they say).

Good point - I can't think of any reason why the angles would be different unless during analysis, they split the lower area into 2, the one with the more acute angle being 'safer'? Or just that it's easier to store the tanks if one side is a right angle! :p

Interested to hear what they say!

Dr. Kalueff, co-author of the novel tank diving test protocol, has explained that a trapezoidal tank is not essential for this test - any relatively tall, relatively thin tank will suffice to allow vertical behaviour but minimise lateral swimming to/from the camera. I'm glad that's clarified!

Interesting! I wonder whether a Porsolt Tank could be used - I guess it depends on the size of your model organism!

Most likely, depending on illumination and light perception from zebrafish, alongside with thigmotaxis it can mimetize water depth with shading/dimming effect, especially if lit from above...