Adaptive Valuemale

What is Adaptive Value?

Adaptive value signifies the immediate, practical benefits an organism receives from a given trait.  Some traits that are adaptive in certain circumstances prove maladaptive in other situations.  The conspicuous gold coloration of the male Midas cichlid may be useful in species identification, and impressing females, but it puts the animal at a disadvantage when evading predators.  Ultimately, only those traits which are generally more beneficial than detrimental- even if only by a very slight margin- will survive to be passed on to the next generation.

Sequential Hermaphrodism

Individuals reproduce with differential success over course of lifetime.  Male type and female types can switch to maximize reproductive success at a given time. Sex change occurs when expected reproductive success of secondary sex exceeds that of primary sex. (Godwin et. al 2003)

blueheads

Robert Warner, Wrasse Champion

The work of Robert Warner on various tropical wrasses has been crucial to the understanding of why fishes change sex.  While the specific subjects of his study were wrasses, not cichlids, the factors that affect sex change in wrasses (individual size variation, sperm competition, etc.) also feature in the ecology of other sex-changing fishes.

Size Advantage Model

The largest females will tend to become males, as their large size allows them to dominate other males.  Social status thus preserved, they will be able to mate with all of the females present, thereby spreading their genes farther than they would be able to as a female. (Warner 1988)

Revised Model

Following Shapiro’s 1989 critique of the size advantage model, Warner revised it to take into account sperm competition and size-fecundity skew in explaining seemingly anomalous conditions in which the largest females in a group did not become male.

Size-Fecudity Skew

If the largest female’s individual fecundity is already greater than that of her neighbors, then there would be no advantage gained from changing sex.  The female most likely to change sex in the absence of a male, then, would be the largest female who stood to gain some advantage through the male reproductive style. (Warner 2002)

Sperm Competition

External fertilization opens the door to the chance for a single female to be simultaneously fertilized by multiple males.  The reproductive advantage of maleness is not absolute, as even the largest, most dominant, potentially protogynous male can have his thunder stolen, so to speak, by a smaller, quicker ‘sneaker.’  Protogyny by smaller females can, in certain situations conducive to sperm competition, be more reproductively advantageous than sex change by their larger harem-mates.  It sometimes occurs that a small male has a better chance of surreptitiously fertilizing a female ‘through’ the coupling of the local alpha than that large, dominant male has of fertilizing his mate directly.