Functions of Male Primate Coloration

Over at Beast Ape and The Bleeding Heart Baboons, Beast Ape posted an interesting blog about the functions of primate coloration as "badge of status" in males to indicate rank or status to other males or to convey information to females about the male's fitness or quality (think the peacock's train).

Female Orangutan Mating Strategies

There is a published paper by Knott et al. (2009) from next month's Proceedings of The Royal Society B about female Bornean orangutans and their reproductive strategies. The authors propose that female orangutan's mating strategies is a product of coevolution from male coercion and also selective resistance.

There are two morphs in male orangutans; flanged or unflanged. Both are secondary sexual characteristics (traits that distinguish the two sexes of a species but are not part of the genitalia). Flanged males are usually adult dominant males. Unflanged males are either juveniles (who haven't developed their secondary sexual characteristics) or non-dominant adult males. These adult unflanged males might develop a flange when they become a "dominant" male or might never develop one in their lifetime. High levels of forced copulation in orangutan is common, especially by unflanged males. Orangutans are polygamous and males generally do not spend anytime with the female after copulation. Orangutans are usually solitary (with the exception of mother and infant pair), due to scarcity of food sources.

The authors found that females, when near ovulation, mated cooperatively only with prime flanged males. When the conception risk for these females was low, they willingly associate and mate with unflanged males. These observations supported the hypothesis that, together with concealed ovulation, facultative association is a mechanism of female choice in a species where females can rarely avoid coercive mating attempts. Female resistance reduces copulation time and may provide an additional mechanism for mate selection. However, female mating strategies is important to avoid aggressive interactions from flanged males and also as infanticide avoidance.

Reference:
Knott, CD. Thompson, ME. Stumpf, RM. McIntyre, MH. 2009. Female reproductive strategies in orangutans, evidence for female choice and counterstrategies to infanticide in a species with frequent sexual coercion. Proceedings of The Royal Society B Retrieved October 9, 2009, from http://rspb.royalsocietypublishing.org/content/early/2009/10/06/rspb.2009.1552 [doi:10.1098/rspb.2009.1552]

Can I See Your Fingers Please?

That is what University of Liverpool's Emma Nelson probably would have said if she were to meet our hominan ancestors in person. Known to hold true in anthropoids (humans, apes and monkeys), the index (second digit) to ring (fourth digit) fingers ratio or 2D:4D is an indication of how much an individual were exposed to androgen (such as testosterone) in the womb. The more androgen you are exposed to, the longer the ring fingers are (and the shorter the index fingers are compared to the index fingers).

Photo of a human's left hand. From left to right: thumb, index, middle, ring and little finger. Photo modified from Wikipedia.

Nelson et al. believe that a high ratio (longer index finger, shorter ring finger) suggests monogamy (or pair-bonded) while a low ratio (shorter index finger, longer ring finger) suggests polygamy (or non pair-bonded). Simply put, individuals with high androgen level is likely to be non pair-bonded and the telltale sign is in the index and ring fingers. Also, some controversial studies had suggested that both men and women who receive high levels of androgen in the womb are more likely to be stronger, faster, and more sexually competitive.

Nelson and her researchers recently looked at the fossils of two Neandertals and one Australopithecus afarensis with complete index and ring fingers to determine their 2D:4D. They found that Neandertals had long ring fingers, suggesting that they were polygamous just like modern day primates that live in groups. A. afarensis on the other hand, had long index fingers. Nelson is puzzled by this discovery. "These were small creatures that probably lived in groups and were being eaten by predators. How do you keep from mating with different members of the group?", she said.

Indeed it does not make sense for A. afarensis to be monogamous if they live in groups. Notice that Nelson et al. only used one A. afarensis fossil to get the 2D:4D. Perhaps it is not their fault that only one A. afarensis specimen had complete index and ring fingers but such are the dilemma of using fossil specimens to generalize a whole species.The result might just be a statistical outlier. However, I can't speculate the result or the implications but anyone that are familiar with statistical data knows that a small sample size leads to a higher sampling error. Also what would a 2D:4D = 1 (same index and ring finger length) be?

Interesting enough, John Hawks at John Hawk's Weblog mentioned the correlation of 2D:4D with male homosexuality (Robinson and Manning, 2000). I would know about this. In fact, my 2D:4D is indeed low.  Robinson and Manning predicted right! Maybe ...

I do find both Nelson et al. and Robinson and Manning (2000) research interesting but I would like to stress that the results are mere predictors and correlations. Take it with a grain of salt. Don't go measuring 2D:4D of your future spouse, boyfriend or girlfriend and accuse them of not being monogamous or a homosexual.

Emma Nelson and her team presented their research at this year's Society for Vertebrate Paleontology meeting held in Bristol, United Kingdom. Read more about Emma Nelson's research.

References:

Reilly M. 2009. Human Ancestors Conflicted on Monogamy. Discovery News. Retrieved September 25, 2009, from http://dsc.discovery.com/news/2009/09/24/human-monogamy.html

Robinson SJ. Manning JT. 2000. The ratio of 2nd to 4th digit length and male homosexuality. Evolution and Human Behavior 21(5): 333-345. [doi:10.1016/S1090-5138(00)00052-0]