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Thermometer of evolution

Difference in temperature in the ears suggests the existence of a complex brain in marmosets

EDUARDO CESARBlack Tufted-ear Marmoset (Callithrix penicillata): more refined emotional responses in situations of stress EDUARDO CESAR

A thermometer similar to the one used by mothers to measure the temperature of their children has provided an indication that the brain of a small species of primate found in Brazil, Callithrix penicillata, the black tufted-ear marmoset, may be more developed than used to be thought. When submitted to a situation of intense psychological pressure, like being captured by a human being with the help of a net, some of these animals show a significantly lower temperature a few tenths of a degree Celsius less in the right ear than in the left. “This fact suggests that stress sets off a major activation of the neural structures in the right hemisphere of the marmosets brains”, says Carlos Tomaz, from the Primatology Center at the University of Brasilia (UnB), the principal author of the work with these little animals, which weigh between 250 grams and 450 grams and grow no more than 25 centimeters in length.

The recording of a lower temperature in the right tympanum indicates that there was a greater activation of the nervous structures located on this side of the brain. How does one deduce this? Because the active areas of the brain are slightly colder than the inactive ones. This happens because activity of the neurons attracts a greater flow of blood, which has a cooling effect on the activated areas. Accordingly, if a stressful event makes the temperature of the right ear of the marmosets fall a little below the temperature found in the left ear, the researchers can deduce that this hemisphere of the brain, the right one, is used more in this kind of situation. This conclusion leads to another, even more interesting. “Our work is physiological evidence that there is some specialization or asymmetry in the marmosets brains”, Tomaz explains. The results of the measurements made by the team from the UnB, which recorded the temperature of 24 marmosets (14 males and 10 females) held in captivity in the UnB?sprimate center, were published in the July issue of the Brazilian Journal of Medical and Biological Research.

The study also provides signs that there is an inversely proportional relation between the temperature of the marmosets right ear and the number of times that the animals had been captured by man in the course of their lives. The monkeys that had been caught between 5 and 9 times had an average temperature in the right ear of around 38° Celsius one degree less than the average for the marmosets that had been captured 4 times at the most. It is as if the experience of repeated captures had taught the marmosets to activate, in a more intensive manner, the portion of their brain involved in the response to stress, in this case the right hemisphere. As if they had learnt to activate this side of the brain in threatening events.

Strategy for survival
If one of the sides of an animal’s brain is used more than the other during the performance of a task or in response to a stimulus, scientists say that this animal shows hemispheric specialization, or brain lateralization. This trait denotes a certain operational complexity of the central nervous system, and it is usually associated with anthropoid primates, similar in form to men, like the chimpanzee besides, of course, the human being himself. “Brain lateralization makes the emotional responses more refined”, comments Tomaz. “This could be advantageous for the animals defensive behavior.”

According to a theory known as the hierarchization of brain functions, hemispheric specialization is of very valuable in a situation of great stress, such as the risk of being captured by a possible aggressive agent. Faced by danger, the brain of animals endowed with this capacity activates as a priority the structures involved in a strategy for survival. The other brain circuits, which are not directly connected with the response to stress, but rather to other stimuli, will only be activated later, at a more opportune moment. This is a way of avoiding competition between the various brain functions and structures, which would generate loss of time and indecision at a critical moment.

For being a representative of the so-called New World monkeys (a group of simians that arose 40 million years ago in Central and South America and, in evolutionary terms, are more distant from man and the anthropoid primates), marmosets are rarely looked on as natural candidates for showing cerebral specialization, a typical characteristic of a more sophisticated central nervous system. According to the still dominant view of these animals, the workings of their brain ought to be more primitive than those of primates closer to the human being.

That is to say, although it is divided anatomically into two hemispheres, the marmosets brain ought to operate in a more primary way, it ought to work as if it were a single entity, without displaying any hemispheric specialization. By showing, with the assistance of a thermometer, traces of complexity in the neuron response to stress in specimens of C. penicillata, the work from the UnB casts doubts on the more widespread perception concerning the workings of the brain of New World monkeys and heats up the debate about the origins of hemispheric specialization in animals. “Our data from this kind of marmoset may be important for understanding better the origin of cerebral asymmetry in primates”, says Tomaz.