Sunday, February 15, 2009

Sexual Selection, Good Genes and Condition-Dependent Handicaps

In regards to mate choice, ‘Choose, but choose wisely’ is the mantra of the discriminating female when seeking-out male suitors. By selecting the most fit, well-matched mates, females are able to reap the benefits of their own inclusive fecundity and be better positioned to obtain the dividends of more numerous, healthy offspring. Moreover, in looking to the future - beyond the perception and sight of Natural Selection - increased female-to-male genetic compatibility may contribute to the transmission of variability or adaptations that may ultimately factor into the fate of the population or the species as a whole. Mate choice matters; both in the here-and-now and in the long term.

On the average, the female gender is better positioned to choose mates because they are most commonly (but not always) the limiting gender in a population. Females take on the greater burdens of producing ova and caring for young; they also tend to be fewer in number within a given population. The power of mate choice belongs to the female; but on what grounds is her selection made, and what criteria are weighed and measured prior to committing to a costly reproductive venture? Certainly, any mate is better than no mate at all, but when the opportunity presents itself wouldn’t it be beneficial to capitalize on the availability of the most virile, successful or healthy male – how to choose?

Generally, selection of mates can be thought of as functioning along one of four lines; (1) through identifying Good Genes, (2) receiving Direct Benefit, (3) via Sensory Bias and (4) by Fisherian Runaway. These methods of mate selection may operate independently, collectively or in conjunction with other aspects of local ecology and Natural Selection – they’re not exclusive. The ability to identify “good genes” will be the focus here, with Direct Benefit, Sensory Bias and Fisherian Runaway reserved for later posts.

Identifying good genes…

Although human fertility clinics utilize modern molecular techniques to perform genetic assessments, “Good Genes” typically aren’t identified by non-human kinfolk in laboratory settings; rather evaluations are undertaken on the fly using an organism’s innate sensory capabilities.

In its simplest form, avoidance of certain physiological cues such as developmental deformities can aid a female in filtering-out unworthy genetic sets; unusual appearances, irregular gaits or abnormal vocalizations of male callers tend to stand-out to females and are almost always avoided. In other cases elaborate courtship rituals, fighting or induced ovulation tactics may be summoned into play as a means of determining the superlative mate through tests of endurance and strength. To maintain a lockstep with these challenges and to achieve female demands, males adapt to take on behaviors, morphologies and displays that maximize their chances of being selected – they sing, dance, bribe and mesmerize. The males strive to impress, because if they succeed, they’ll be selected as a mate and be provided the opportunity to pass on their genetic compliment. Of course, all of these additional behaviors and adaptations come at a price, even beyond that associated with the routine cost of maintenance, additional nutritional requirements and energy expenditures, since predators can also be attracted to flamboyant colors and patterns. As a result of this dynamic set of circumstances, male sexually selected characteristics may sometimes seem less like a benefit and more like a “handicap.”

Handicaps are those conditions that improve an animal’s chance at being selected as a reproductive mate while at the same time being detrimental to its survival. Essentially, handicaps exist as one of three types; Zahavian Handicaps, Condition-Dependent Handicaps and Revealing Handicaps.

Zahavian Handicaps (named for biologist Amotz Zahavi) describe the idea that only the strongest, most fit males could afford to take on the risk and cost associated with elaborate morphologies, colors or behaviors, and for this reason those males exhibiting such characteristics are preferred to females as reproductive partners. In short, if a peacock can lug around a massive, brilliantly colored tail and still somehow manage to survive in spite of its increased visibility to predators and cost of feather development, it must be extremely healthy! Indeed, a proponent of Zahavian Handicaps would make the argument that peahens find the boldness of the peacock to be quite sexy.

Revealing Handicaps are those sexually dimorphic characterizes which tend to be indicative of the presence of parasites or disease. The hypothesis was put forward by biologists W.D. Hamilton and Marlene Zuk in 1982 and details a scenario in which animals exhibit colorations based on the presence, or immunity against, specific parasites and diseases. These colorations can then be interpreted by mates prior to copulation as a means of determining the health of a potential partner.

[House Finch at my Feeder]
Conditional-Dependent Handicaps are those in which dimorphic characters are depicted as an overall indicator of an animal’s current health. For example, the coloration of a song bird may indicate its diet or foraging ability. Linking to current science news, research conducted by Assistant Professor Kevin McGraw of Arizona State University, indicates that dietary uptake of carotenoids by house finches (Carpodacus mexicanus) function in just such a way. McGraw explains (From ScienceDaily, here):

"We are proposing a positive fitness feedback loop for these 'self-loving molecules,' given how high carotenoid accumulation can improve one's state and one's interest in selecting carotenoid richness in mates and food. This provides a window into how major sexual selection models, such as sensory biases and assortative mating, may be explained by a common, nutritional and narcissistic currency."

In addition, the role of carotenoids in other areas of physiology is becoming apparent, including color vision, according to McGraw,

"Carotenoids play fascinating and multifaceted roles in the lives of animals. For years, we have known that, as antioxidants, they boost human health and, as colorants, make birds colorful and sexually attractive. Now, we are blending as well as expanding these paradigms and studying how consumption of carotenoids can improve or 'tune' their color vision, promote the health of offspring as they develop in the egg, and possibly improve male sperm quality."

More detail on identifying Good Genes in potential mates will be provided at a later time, and Direct Benefit, Sensory Bias and Fisherian Runaway will also be examined.

Arizona State University (2009, February 13). Carotenoids Are Cornerstone Of Bird's Vitality. ScienceDaily. Retrieved February 15, 2009, from

W. Hamilton, M Zuk (1982). Heritable true fitness and bright birds: a role for parasites? Science, 218 (4570), 384-387 DOI: 10.1126/science.7123238

Mark Kirkpatrick, Michael J. Ryan (1991). The evolution of mating preferences and the paradox of the lek Nature, 350 (6313), 33-38 DOI: 10.1038/350033a0

A. Pomiankowski (1987). Sexual Selection: The Handicap Principle Does Work -- Sometimes Proceedings of the Royal Society of London. Series B, Biological Sciences (1934-1990), 231 (1262), 123-145 DOI: 10.1098/rspb.1987.0038

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