The Voyeur Biologist

The cheek lights on the deep-sea loosejaw fish may serve to locate prey in the dark, but they likely also function in mate selection, since the organs of males are much larger than those of females. When not in use, these lights can be rotated and pulled back into the fish’s head, sort of like the headlights on some cars. They also can be flashed on and off. So why retract them? Probably because they have a highly reflective layer that helps direct light outward. This shiny surface, in contrast to the fish’s velvety black body, might inadvertently reveal the animal’s whereabouts to predator or prey.

discoverynews:

Dolphins Team Up To Get the Girl
Male dolphins who formed an alliance of wingmen fathered more babies than those who worked the seas solo.

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[Image Description: Photograph of a female barred buttonquail, a species of quail with a large patch of black on the throat.]

Elaborate Bird Plumage Due to Testosterone?

In many bird species males have a more elaborate plumage than females. This elaborate plumage is often used to signal body condition, to intimidate rivals or to attract potential mates. In many cases plumage colouration also depends on the hormone testosterone.

[…]

In barred buttonquails that live in Southeast Asia, females are polygamous and pair with several males that incubate the eggs and raise the young. However, not only the behaviour, but also secondary sexual ornaments that depend on the male hormone testosterone are reversed between sexes

Colourful plumage and long feathers allow a male to express its quality and/or condition without further physical demonstration of its strength.

[…]

There is hardly anything known regarding function and regulation of plumage colouration in female birds: females mostly have a dull plumage with almost no variation between individuals. However, in a few bird species sex roles are reversed: here, the females aggressively defend territories and court males. The latter incubate the eggs and care for the young without any help from the females.

[…]

“It is really remarkable,” states Christina Muck, “that the sex role reversal in behaviours is accompanied by a reversed hormone dependency in the expression of secondary sexual characters.” Thus, female button quails succeed when they not only adopt male behavioural strategies but also use the underlying physiological mechanisms.

(source)

Initial phase blueheads rushing back to the reef after leaving a cloud of eggs and sperm during a spawning event in waters off Belize.

The bluehead (Thalassoma bifasciatum), a member of the wrasse family, like many other fish, has two primary groups of male fish - initial phase (“IP”) and terminal phase (“TP”), and displays a form of sequential hermaphrodism (similar to, for example, the parrotfish) in which individuals can begin life as either male or female, and females can later change to males. 

Initial phase fish, smaller than terminal phase males, primarily reproduce in large groups - large numbers swim upwards together from the bottom substrate, then dash upwards, and release gametes into the water at the peak of the dash. Terminal phase fish, however, mate in pairs with a single male and female each, and the largest TP males will form harems, mating with many female fish, one at a time.