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Manual self grooming involves inspection and grooming of the genitalia with the hands or orally. ... Prediction: self grooming is predicted to be more common among promiscuous ...

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Behavioral defenses against sexually transmitted diseases in primates

By Charles L. Nunn

Presented by Kristen Abbott


Hypothesis 1: females were not more likely to inspect genitalia in more promiscuous species. The trait showed extreme variations and conflicting interpretations. In the conservative interpretations, patterns were opposite to predictions. Genital inspections of females showed weak associations with promiscuity levels. Most STD transmission is from male to female so it would be deduced that if genital inspection was used to prevent STD’s females would inspect more than males but this was not the case. (table 1)

Hypothesis 2: grooming of the genital was not correlated with promiscuity in either males or females. In fact the only two significant results were in a direction opposite to what was predicted. Even when the study was restricted to species that is known to use the properties of saliva on wounds the results were not conclusive. (fig. 1)

Hypothesis 3: Only one to two species were reported to display postcopulatory urination, therefore this prediction was unsupported for both sexes.

Hypotheses 4: Monogamous species were not more likely to exhibit slow life histories even after controlling fro body mass and parental investment. When taking into factor the number of reproductive events a pattern emerged in a direction opposite to predictions.


Sexually transmitted diseases have been in the news a lot recently due to the alarming rate the’re spreading. Over 65 million people in the United states are thought to have an STD and this number is estimated to be to lower than the actual number. In Africa alone Aids and HIV has reached a frightening height where 24.5 million people have the Aids virus and 4 million new cases are reported each year. Due to the difficulty in controlling the spread of STD’s it is important to see if there are behavioral adaptations that primates have evolved in the defense of transmitting the diseases. The author of this paper has devised four different hypothesis as to behavioral defenses.


For hypothesis 1-3 a survey was sent to a broad spectrum of primatologists. For genital inspection information was gathered on the sexes separately, and if the cleaning was done with the hands or the mouth. Information was also collected on oral grooming of wounds so a link could be made for species that were shown to use the medicinal properties of saliva already. No time period was specified for postcopulatory urination but the data was divided between sex. Conflicting data was analyzed using conservative methods and then again using liberal methods. To discern promiscuity testes mass was evaluated. (after correcting for body mass size of testes reflects sperm competition and serves as a measure of promiscuity.) For hypothesis 4 primate species were classified as monotonous or non-monogamous based on previous data. The data had to be corrected for body mass and duration of parental care, and lifetime reproductive events. Statistically phylogenetic comparative methods were used. All probabilities are two tailed with a significance criterion of α = 0.05

Hypothesis 1

Primates that are more promiscuous will inspect their mates genital prior to copulation

This enables them to avoid mates that potentially have an STD

Females must have the ability to reject potential mates

Prediction : genital inspection by males and females is correlated with mating promiscuity

Hypothesis 2

After copulation self grooming may function to remove bodily fluids that contain sexually transmitted diseases

Manual self grooming involves inspection and grooming of the genitalia with the hands or orally.

Oral grooming may directly kill parities due to the antipathogen properties of saliva

Prediction: self grooming is predicted to be more common among promiscuous species

Hypothesis 3

Postcopulatory urination can reduce STD’s

More effective in males for anatomical reasons

Prediction: postcopulatory urination is more common in promiscuous species, and seen more frequently in males than females

Hypothesis 4

Monogamy reduces the risk of STD’s

Species at greater risk of STD’s should be monogamous

Animals with slow life history will be more susceptible to STD’s

Prediction: STD risk increases with longer life history therefore those primates that have long life histories would correlate with being more monogamous.


Analysis of the data failed to reveal support for any of the hypothesized behavioral defenses to STD’s. Monogamy was not consistent with life behaviors and neither genital inspection, grooming or postcouulatory urination were seen more often with increasing promiscuity. One reason could be that the statistical test was to conservative, but when run with assumptions of a medium effect size patterns were opposite to predictions. Though it should not account for all non-significant results the use of surveys leaves a large room for different interpretations and human error. The big red flag is that intraspecific variation needs more study. It is possible that within species there is a high range of STD susceptibility and reproductive behaviors. The other possibility is that mammals have not evolved behavioral defenses to STD’s. This is supported by the trouble human populations have in preventing them. Many of the behaviors hypothesized could actually be a hindrance to reproduction and therefore would not evolved, such as monogamy or any reduction is sexual contact. Males that are more likely to have STD’s are those that have the best genes and have mated with the most females. Further direct observational studies need to be conducted on specific primate clans.