Why Tarantulas Always Choose Miss Right

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Tarantulas are externally symmetrical but exhibit behavioral asymmetry, preferring females who are standing at their right side. Image by Lichtpuenktchen

A male tarantula prefers to court Ms. Right, that is the female on his right, according to a new study published in Journal of Zoology.

Bilateral Symmetry vs. Behavioral Asymmetry

It is common knowledge that many animals exhibit bilateral symmetry on the external parts of the body, showing uniformity on the left and right sides on the body. However, inside the body, numerous asymmetries are present, which may also dictate behavioral asymmetry. Such left-right asymmetries have been observed starting from the development of the embryo, according to Dr. Kawakami and colleagues’ study in 2005.

It is essential to understand behavioral asymmetries to further comprehend the evolution of brain asymmetries.

According to a study conducted by Dr. L. J. Rogers in 2000, exhibiting strong brain lateralization is favorable for animals because such animals are capable of processing information from multiple sources at the same time.

Other studies have shown evidence on left-right asymmetry of animals such as cuttlefish, honeybees, and flies. In some cases, correlation between behavioral and physical asymmetry is evident.

At present, available information on behavioral asymmetry of arthropods is very limited. Two studies have shown left-right asymmetries in the behavior of spiders.

The study conducted by Drs. Heuts and Lambrechts in 1999 showed that spiders from the field were found to have more lesions in the left leg than in the right, which may indicate that they prefer to move on a specific side when attacked or their predators prefer to attack on the left side.

In a more recent study conducted by and Drs. Ades and Ramires in 2000, spitting spiders were observed to use their left anterior leg to handle prey. However, no studies have shown spiders’ laterality during foraging and exploration.

How do spiders choose a Valentine? New research indicates that it might be as simple as who is standing where. Copyright image by Decoded Science, all rights reserved.

Behavioral Asymmetry in Spiders

To explore behavioral asymmetries, particularly in arachnids, scientists from Universite Rennes in France conducted a study with tarantulas, the results of which were published in 2017.

The primary objective of the study was to understand the physical, mental and behavioral patterns exhibited by tarantulas within 24 hours in laboratory conditions. The researchers also wished to assess the effects of light and odors on adult tarantulas’ exploratory activity and motor asymmetry using a T-maze.

The subject was initially placed on the start box, which led to two arms on opposite sides with stimuli boxes.

Male Tarantulas Choose the Right Female

Results showed that male tarantulas became most active at night and during early morning. They also show more activity when exposed to light or when they smell the presence of a female tarantula. When similar stimuli were presented on both ends of the T-maze, they tend to move towards their right side. For instance, when the tarantulas were exposed to light with the same intensity on both ends of the maze, they always chose to approach the right end. The same result was observed when they were presented with two females on each end of the maze; the male spiders always chose the female on the right.

Mr. Right or Ms. Right?

The male tarantulas were also given the choice between male and female tarantulas. When the female tarantula was placed on the right arm of the maze, they chose the right arm. When the female was on the left arm, they chose the left. This result showed that male tarantulas can efficiently detect the sex of their own species through chemical signals.

Right-legged Sensing

When the male tarantulas were exposed to another tarantula, they initially used their right first pair of legs and right palps (segmented appendages near the mouth) for contact. If the other spider was another male, the two separated right away; if the other spider was an adult female, the male tarantula initiated courting.

This reaction could imply that the chemoreceptors in the right palp function as a sex detector, though no significant difference was found in the chemosensitive setae in the left and right palps.

Tarantulas: Finding Ms. Right

Based on the findings of the study, male tarantulas exhibit motor asymmetry when exposed to light and odors. They often prefer to go right when exposed to similar stimuli such as light or a mating partner, that is, their Ms. Right.

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