Disease transmission and personality

Thanks Jack and Jonathan for a push to get back in the saddle! Experiments have become manageable again (at least for a short period of time) and Zach and my parents have completed their summer visit. Time for SCIENCE!

Feral_CatOne topic that I find particularly interesting concerns how personality traits influence patterns of disease.  For example, people who are frequently stressed out are at a greater risk of becoming obese or acquiring heart disease. We often look at disease transmission at a population or community level, but I think individual level differences in behavior are a crucial and understudied area of research.

When diseases are transmitted from one individual to another, then social behaviors are an important factor in transmission dynamics. While it’s often difficult to study this phenomenon in humans, numerous social animals provide ideal study systems.

Natoli et al. 2005 is 0ne of the coolest studies that I’ve read on personality traits and disease. This research group examined 3 colonies of feral domestic cats in Europe and collected data on how male personality traits correlated with FIV (feline immunodeficiency virus) infection. According to the article, FIV infections in feral cats are usually lethal within approximately 5 years, at which point the cat’s immune system is too weak to fend off otherwise nonlethal infections.

FIV transmission occurs when one cat bites another, transmitting the virus from its saliva into the other cat’s bloodstream. Bites usually occur when two males are battling over territory or dominance rank or when a male bites a female during sex.

Feral_Cat_2In order to get a handle on the “personality” of the feral cats in each colony, the researchers logged hundreds of hours taking notes on the frequency with which each cat engaged in aggressive, submissive, affiliative, territorial, display or mating behaviors. They aggregated all of these measures and came up for a single score for each individual that indicated how “proactive” or “reactive” that individual was. Proactive cats more regularly marked their territory, were the most aggressive (often winning aggressive interactions) and often affiliated with other members of the colony. Reactive individuals rarely displayed aggressive behavior and were frequently submissive towards other members of the colony.

When the behavioral results were compared with information about which cats were infected with FIV, a clear patterns emerged. The males at the top of the hierarchy (i.e., the most proactive, dominant males) were the most likely to be infected with FIV. Their aggressive demeanor meant that they incurred bites more frequently than more submissive males, increasing the probability that aggressive males contracted the disease.

This finding brings up a logical question. If all of these aggressive cats are contracting terminal diseases, why are aggressive cats still around? Well, the first answer to that question lies in that fact the cats don’t succumb to the illness quickly. The disease has a long asymptomatic period during which the cats continue to strut around as if nothing is wrong.

Importantly, aggressive behavior also maintains access to the ladies. Paternity tests revealed that the proactive cats were fathering a lot more of the kittens than the reactive males. Because aggressive dads produce more offspring than submissive dads, we might not expect aggressive behavior to be diminishing anytime soon.

This was a particularly nice study because it examined multiple populations (lending support to the generality of the results), measured lots of behaviors and included a direct measure of evolutionary fitness. It’s rare to find so comprehensive a study.

When dealing with socially transmitted diseases, personality plays a large role in the number of times an individual may be exposed to an infectious agent. In the study described above, aggressive behavior was the key to transmission. In other systems the important relevant measures may be frequency of sexual behavior, energy put into hygiene, number of individuals with which one associates, etc. Personality plays a large role in determining the diseases to which one is most susceptible.

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