WARNING: ANSWERS & SPOILERSOkay... so that leaves the panda eyes, feeding mechanism and iris pigmentation...
The answers are here if you decide to give up...
Panda eyes - the area around the eyes, the skin to be exact is packed with heavy pigmented cells known as melanocytes. In stress, the corticotrophin releasing hormone from the hypothalamus will be released excessively to stimulate the anterior pituitary glands to release adrenocorticotrophin hormone (ACTH) which in turn stimulates the cortex of the adrenal glands to release steroids to counter stress. ACTH is a byproduct from a larger molecule known as pro-opio melanocortin (POMC). Other byproducts from POMC includes melanocyte stimulating hormones. Under excessive stimulation, not all POMC is broken down to ACTH and cleared by the kidneys, some remain as break down products of MSH to stimulate the melanocytes, and as described, it becomes more pigmented and hence you get the eyebags...