Going Out on a Limb: Appendage Length and Function Shape Leg Loss Without Evidence of Autotomy Modulation

ABSTRACT Autotomy, the voluntary shedding of an appendage, is a widespread antipredator defense that allows animals to escape attacks at the cost of losing a functional body part. Harvestmen are well known for their ability to autotomize legs, and lost appendages do not regenerate. Because leg pairs are functionally specialized and locomotion relies on a tripod gait, the costs of autotomy may depend on which legs are lost. Using a large sample of the harvestman Jussara sp. (Sclerosomatidae) collected across four non‐consecutive years, we tested whether individuals modulate autotomy according to their current leg condition. We used Monte Carlo randomizations to test two non‐mutually exclusive hypotheses: (1) the loss of one leg reduces the probability of losing its homologous counterpart, and (2) the loss of one ambulatory leg reduces the probability of losing another ambulatory leg on the same side of the body. To establish baseline probabilities, we characterized variation in leg loss among leg types, body sides, sexes, and years and measured leg length. Autotomy was common and differed markedly among leg types, with the elongated, primarily sensory legs II being the most frequently autotomized and the shorter ambulatory legs III the least. Although autotomy frequency varied among years, the relative frequencies among leg pairs remained consistent. Neither the frequency of complete leg‐pair loss nor that of same‐side ambulatory leg loss differed from the null expectations, providing no evidence for either autotomy‐modulation hypothesis. Instead, differences in leg loss among pairs were consistent with variation in appendage length, position, and sensory versus ambulatory function. The lack of evidence that autotomy is modulated by an individual's existing leg losses suggests that harvestmen are robust to the locomotor and sensory challenges imposed by autotomy and that a high benefit‐to‐cost ratio may weaken selection for such modulation.