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Jonathon H. Stillman

Sexual dimorphism in Tidarren spiders is extreme; the males are only 1% of the mass of females, and the males' copulatory organs (pedipalps) can be 20% of the male's body mass! The arachnid's large pedipalps may be essential for mating success, allowing the delivery of sufficient quantities of sperm and morphologically matching the genitalia of a female that is 100 times larger. Sexual selection, which resulted in these extreme traits, has been hypothesized to conflict with other concordant traits, such as aspects of locomotor performance essential to the mating process. In most spiders, mating behavior is initiated when males load their pedipalps with sperm and set off in search of receptive females. Upon mating, male spiders deliver sperm into the female's paired spermathecae by alternate insertion of each of the two pedipalps. However, Tidarren spider males have been observed to remove one of their pedipalps immediately prior to mating behavior, leaving it with only one. In a recent paper, Margarita Ramos and colleagues tested the hypothesis that the removal of one of the pedipalps increases aspects of locomotor performance critical to the male's mating success, namely speed and endurance.

The team raised 956 male spiders from hatching as second instars through three molts to adulthood. Before and after the spiders naturally lost their pedipalps, the team measured each spider's maximum speed with high-speed videography as they moved across a strand of silk spun by a virgin female. The spiders ran 44% faster following pedipalp removal, with their maximum speed significantly increasing from 2.7±0.2 to 3.8±0.3 cm s –1.

Next, the team estimated the spider's endurance as the time taken to reach exhaustion by chasing the spiders with a small brush on a piece of paper until they failed to respond to 10 consecutive touches of the brush. The spiders ran for 63% longer following pedipalp removal, with the time to reach exhaustion significantly increasing from 1050±55 to 1710±45 s following pedipalp removal. Speed was also 75% greater in spiders with one pedipalp during these trials. The differences in time to exhaustion and speed result in spiders with one pedipalp being able to move three times further than spiders with two pedipalps.

These results suggest that if increased locomotor performance is advantageous during mating, there would be a strong selective pressure for male spiders to discard one pedipalp before mating. To complete the analysis of the selective advantage of increased locomotor performance resulting from pedipalp removal, direct comparisons of fitness between males with one and two pedipalps would be necessary. As pedipalp removal occurs 100% of the time in Tidarren spiders, performing these comparisons will require weight to be added to male spiders after removal of a pedipalp – a delicate surgery considering that these animals are only ∼4 mm long. Comparative studies in other spiders may play an important role in elucidation of fitness consequences of pedipalp removal.