Showing posts with label behavior. Show all posts
Showing posts with label behavior. Show all posts

Saturday, March 10, 2012

Evidence For Co-evolution of Primates and Snakes in Humans?

The folllowing is from the EMax Health web site:
Premenstrual Women Seek Snakes
By Timothy Boyer

Numerous studies have demonstrated that the hormonal influences of the premenstrual phase of a woman’s cycle leads to a wide range of cognitive, mood and behavioral changes. Of the three, cognition is the least-studied research area. However, recent research involving images of snakes has revealed that the luteal phase of a woman’s menstrual cycle enhances cognition when it comes to evoking evolutionary imprinted fear known as “fear modules.”

It is a well-established fact that an inherent fear of snakes is present in both man and non-human primates. Numerous studies involving anxiety responses to images of snakes in both adults and children have shown that an intrinsic fear of snakes has roots that go beyond and are independent of learned behavior and personal experience.

Studies using laboratory primates that have never seen a snake show that captive-borne primates can display typical snake-induced primate behaviors such as cries of alarm , anxiety, distress and other behaviors typically seen in wild monkeys from snake-infested environments.

Some scientists propose that the innate fear of snakes has evolutionary origins where early mammals evolved fear modules—a complex of mental, neural and behavioral systems that assist mammals in defending themselves against threats such as snakes.

The fear modules are believed to have developed when the earliest mammals co-existed with reptilian species that posed a constant threat to the mammals’ daily survival. Because snakes are descended from earlier reptiles it is likely that snakes represent a prototypical stimulus for activating the fear module behavior in modern primates both human and non-human.

While mechanistically identifying behavior directly to a hypothesized fear module presents some practical barriers toward research, behaviors attributed to hormonal influences are a simpler matter. One of the most studied behaviors related to hormones are those associated with premenstrual syndrome, and more recently, peri-menopausal rage.

In a recent study published in the journal Scientific Reports, researchers from Japan wanted to establish a new technique for investigating mood and behavior changes linked to the menstrual cycle, but that was not focused on the typical PMS relationship between hormones and mood. Rather, they decided to test how hormones may affect fear modules by testing the response of women during various phases of their menstrual cycles on detecting hidden Images of snakes.

In the study, 60 healthy women of child-bearing age were repeatedly shown images of flowers alone and images of flowers containing a hidden snake throughout the phases of their menstrual cycle. The participants were measured as to how quickly they spotted the snake hidden in the flowers.

The results of the study showed that the fastest women were those who were in the ovulatory luteal phase of their menstrual cycle. The luteal phase follows immediately after the release of an egg from an ovary, preparing the egg for fertilization by a sperm.

The authors of the study believe that their results strengthens the hypothesis that mankind possesses a “fear reflex” in the form of the fear modules believed to have developed during our evolution from early primate species. The benefit of such a fear reflex would be to increase the likelihood of survival and the ability to pass on genes to progeny. Women, it would then seem, in the past and in some regions today could benefit from this type of hormone-influenced behavior by identifying and avoiding dangerous snakes.

"It could contribute to women's ability to increase their vigilance towards biologically relevant threatening stimuli around themselves during this period of possible pregnancy," the study says.

The authors of the study further state that, “…this is the first demonstration of the existence of within-individual variation of the activity of the fear module in women, as a predictable change in cognitive strength that appears likely to be due to the hormonal changes that occur in the menstrual cycle, particularly due to increased progesterone and estradiol levels.”

Whether or not there is a true fear module evolutionarily hard-wired into our brains, the hypothesis that it does exist and that hormones may have played an additive protective role is fascinating to say the least. Women and snakes have played a biblical role in theology and the arts, and wouldn’t it be something if this were an example of evolution having an influence on theology as well.


N. Masataka and M. Shibasaki. 2012.    “Premenstrual enhancement of snake detection in visual search in healthy women” Scientific Reports Volume: 2, Article number: 307;

Arne Öhman and Susan Mineka. "The Malicious Serpent: Snakes as a Prototypical Stimulus for an Evolved Module of Fear”

Thursday, January 19, 2012

Time to Death - Boas Monitor Prey's Heartbeat During Constriction

A threatening Boa constrictor. JCM
The 19th century literature on constriction by snakes often describes the prey as being crushed. And, for many years it was thought that constricting snakes killed their prey by preventing the prey from breathing. A coil from the snake's body was tightened each time the prey exhaled, gases returning the the prey's lungs would have to be exhaled, so when the prey exhaled the snake tightened its hold, making it impossible for the prey to inhale. There was another hypothesis that was overlooked. In 1912 Frank Wall proposed snakes induce asphyxia by essentially stopping blood flow to the heart.

 A new study by Scott Boback and colleagues suggests that constriction may be more sophisticated than previously thought. Killing prey by constriction is both energy expensive and potentially dangerous to the snake, constriction requires and significant increase in aerobic respiration and the prey may retaliate and injure the snake. The authors tested constricting boa constrictors to see if they adjusted their hold on prey. They developed a method of isolating a rat's heartbeat as a potential cue, by implanting a simulated heart in a dead rat that replicated the size, rate and stroke volume of a rodent heart. They then tested how the constriction effort varied as snakes constricted rats with: a simulated heartbeat throughout constriction; a simulated heartbeat for the first half of constriction and then shut off, and no heartbeat. The results suggest tightness and duration of a constricting snake’s coils are timed to perfection, matching the heartbeat and weakening state of the snake’s prey.

Snakes constricting dead prey with a simulated heart beat constricted for a much longer time than in previous studies (averages of 12 vs 23 minutes). The authors propose that longer constriction times may have been required prior to the evolution of endotherms (birds and mammals) because ectotherms have slower metabolisms and can survive for longer periods of time with reduced amounts of oxygen.

S. M. Boback, A. E. Hall, K. J. McCann, A. W. Hayes, J. S. Forrester and C. F. Zwemer. 2012. Snake modulates constriction in response to prey's heartbeat. Biology Letters doi: 10.1098/rsbl.2011.1105.