By: Stefan Balestra, Adam Casey, Alessandro Puccio, and Walker Smith (Stonehill College, BIO323 Evolution, Spring 2018)
Spiders are considered by some as a source of fear, but many people find arachnids to be a cool group of arthropods. I mean, one of the most beloved superheroes is Spiderman, after all. Adding to the coolness factor associated with spiders, the spider discussed below is not only a jumping spider, but also a dancing spider with an interesting ritualized dance used to attract a mate. Additionally, the male individuals of the species exist in two morphs with different ritualized dances, competing in a spider dance battle of sorts to impress the ladies. Read on to learn more about the specific mode of sexual selection that contributes to the male morphs’ different looks and dance styles.
The 2016 research article by Busso and Davis Rabosky of the University of Zurich and University of Michigan, respectively, investigated disruptive sexual selection, an uncommon mode of sexual selection, acting on the species of jumping spider, Maevia inclemens. Sexual selection through mate choice, most frequently female choice, is a common feature for many species. M. inclemens exhibit male polymorphism, the occurrence of two or more clearly different morphs, or forms, referred to as alternative phenotypes. M. inclemens, has two male morphs, a tufted morph (T) and a gray morph (G), which exist at a 1:1 ratio in nature. The two morphs are anatomically similar during development but when reaching sexual maturity, the morphs differ in their color pattern, courtship initiation distance, and courtship dance. The authors term the differences between the morphs and their behavioral courtship styles alternative reproductive tactics (ARTs). The evolutionary development of ARTs within a species due to sexual selection is thought to be contradictory as sexual selection is expected to minimize additive genetic variation. Take a theoretical case of sexual selection where female birds choose a male mate based on the length of his tail feathers, where longer tail feathers are the preferred heritable trait. Female choice for males with the longest tail feathers will drive evolution, over multiple generations, to increase male tail feather length as long as tail length doesn’t become a significant detriment to the bird. Sexual selection is driving a phenotypic trait in one direction, thus depleting the genetic variation. The opposite seems to be occurring for these jumping spiders; sexual selection by females is increasing the genetic variation, resulting in two different morphs. To better understand this uncommon example of disruptive sexual selection, the authors sought to “address the role of selection, through female preference, as a disruptive force widening the phenotypic differences between male morphs.”
To conduct their research, the scientists collected 114 male and 100 female specimens from the woods of Ann Arbor, Michigan, and allowed them to mature in an artificial enclosed environment. After observing wild male courtship rituals in the field, the scientists chose to analyze female mate choice by exposing one sexually mature male to one sexually mature female and recording if she chose to mate or not. It was already known that one morph was not preferred over the other. Using one-choice, one-male trials allowed the scientists to record what male traits in each morph was preferred. After running 44 trials (26 were done with G morphs and 18 with T morphs), the scientists analyzed female preference for anatomical traits (weight, body length, and body condition) and behavioral traits (detection latency, detection distance, courtship latency, and number of courtship events) for each morph. To analyze traits of preference, ANOVA or ANCOVA statistical tests were performed for each trait, and then the intensity and variation in female preference for each trait was assessed using standardized regressions. Additionally, a chi-squared test was conducted to determine if the morph ratio in nature was significantly different then what was used for the study, and correlation tests were run between different traits to determine if any were related to each other.
It was determined that morph ratio in nature was not significantly different than morph ratio used in the experiment. Linear regression analysis revealed that assortative mating does not occur in M. inclemens, and that females of all sizes mated with males of all sizes. More simply put, the females were randomly mating with the two male morphs and not just based on male size in comparison to female size. Among male traits, it was found that weight, body width, and body condition were all directly correlated to each other based on the correlation tests. Based on the linear selection differentials from the standardized regressions, it was determined that females preferred males based on two anatomical traits, weight and body condition, and two behavioral courtship traits, courtship latency and number of courtship events. Interestingly, females preferred opposite values in weight/body condition, courtship latency, and number of courtship events for the two morphs. Females preferred lighter G morphs, with a correlated lower body condition, while also preferring heavier T males with a higher body condition. In terms of behavioral courtship traits, females preferred G males with shorter courtship latencies and more courtship events, while also preferring T morphs who waited longer to court and had fewer courtship events. Based on the results, M. inclemens is exhibiting disruptive intersexual selection, the first evidence of such in the Araneae family.
The conclusion that disruptive intersexual selection is occurring in this spider species was further supported by the fact that no intermediate male morphs were captured in the field, and there are no reports of any intermediate forms of male M. inclemens. Having no intermediate male forms shows that there is some force that has split and maintains two exclusively different morphs. For every evolutionary change, like generation of male polymorphism, there must be some benefit or advantage for the species to drive the evolutionary change. The authors hint at two possible advantages in maintaining the polymorphism in the male population. Firstly, having two male morphs serves to more rapidly increase offspring genetic diversity. A female who mates multiple times could mate with two different morphs, resulting in higher offspring genetic diversity, which often results in higher species fitness. Secondly, two morphs are beneficial against predators, lowering predatory risk. Usually, predators focus on only one, more-commonly encountered prey type. If one morph is easily recognized by predators, then the other is far less likely to be recognized and has a significantly lower predatory risk. The authors also mention that disruptive intersexual selection is a possible starting point for sympatric speciation, the process through which new species evolve from a single ancestral species while inhabiting the same geographic region. If the differences between morphs is maintained or continues to widen, it is possible that the random female mating between the two morphs will disappear. Nonrandom mating over time would lead the species to separate into two non-interbreeding species of jumping spider.
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Citation for article and picture: Busso, J.P., and A.R. Davis Rabosky. 2016. Disruptive selection on male reproductive polymorphism in jumping spider, Maevia inclemens. Animal Behavior 120: 1–10.
Link to access the article: https://www.sciencedirect.com/science/article/pii/S0003347216301543
(A video of the spiders’ courtship dances is in the Supplementary Materials.)
Podcast Music: Duncan. 2018. Bumper: Zachary Duncan. Stonehill College Digital Media Collaboration. https://elearn.stonehill.edu/bbcswebdav/pid-292136-dt-content-rid-1290942_1/orgs/201820-DMC/zacharyduncan_CM_85bpm.mp3