FEATURED STUDENT RESEARCH PAPER Sex, shells, and weaponry: coercive reproductive tactics in the painted turtle, Chrysemys picta Patrick D. Moldowan 1,2,3,4 & Ronald J. Brooks 5 & Jacqueline D. Litzgus 1 Received: 22 July 2020 /Revised: 24 October 2020 /Accepted: 27 October 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract Males and females have divergent reproductive interests arising from their unequal investments in offspring. This sexual conflict drives an antagonistic arms race that influences sex-specific reproductive success. Alternative reproductive tactics are expected in long-lived species for which the reproductive strategy that maximizes mating success could differ across body sizes. The mating strategy of the painted turtle (Chrysemys picta) has been characterized as an elaborate and amiable male courtship display during which males use their elongate foreclaws to stroke females, coupled with female mate choice. Contrary to this long-held understanding, in situ field observations and experimental trials from our long-term study in Algonquin Provincial Park, Canada, demonstrate that males also exhibit an alternative, coercive mating strategy. Males are equipped with sexually size dimorphic tomiodonts, tooth-like cusps of the beak, as well as a weaponized anterior shell, with which they wound the head and neck of females. Behavioral trials during the breeding periods showed that male reproductive tactics shift from courtship (foreclaw display) to coercion (striking, biting, and forced submer- gence) across ontogeny, and male size predicts the occurrence and frequency of coercive behavior. We found phenotype-behavior matching whereby small males invest in putatively ornamental foreclaws used for courtship and large males invest in weaponry for coercion, challenging existing knowledge of this well-studied species. As a group with a long evolutionary history and varied mating systems, Testudines are a particularly interesting taxon in which to ask questions about mating system evolution. Significance statement Alternative reproductive tactics are hypothesized for long-lived species. We quantified a shift from apparent courtship to coercive tactics during the reproductive lifespan of a well-studied freshwater turtle. Male painted turtles ( Chrysemys picta) have sexual weapons that are used to promote female acquiescence. Using behavioral trials with turtles from a long-term study population, we demonstrate that males match their morphology (ornament/weapons) to reproductive behavior (courtship/coercion) as their reproductive tactics shift. Our findings hint at the behavioral complexity of aquatic turtles, a challenging and often-overlooked group in behavioral studies. Keywords Alternative reproductive tactics . Mating strategy . Sexual conflict . Sexual dimorphism . Sexual weapon . Titillation Introduction Sexual conflict and male-female antagonism Males and fe- males have divergent reproductive interests arising from the unequal, often female-biased, investment in offspring (Trivers 1972; Andersson 1994). The reproductive potential of a fe- male is limited by her ability to reduce costs associated with mating and to invest energy and time into offspring (Bateman 1948; Andersson 1994; Rowe 1994; Rowe et al. 1994; Andersson and Iwasa 1996). In contrast, males may invest relatively little into reproduction and incur relatively low mat- ing costs compared to females, yet males can achieve high reproductive success by mating with multiple females (Gavrilets et al. 2001; Chapman et al. 2003). This sexual Communicated by S. Joy Downes * Patrick D. Moldowan pmoldowan1@laurentian.ca 1 Department of Biology, Laurentian University, 935 Ramsey Lake Road, Sudbury, ON P3E 2C6, Canada 2 Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, ON M5S 3B2, Canada 3 School of the Environment, University of Toronto, 33 Willcocks Street, Toronto, ON M5S 3E8, Canada 4 Algonquin Wildlife Research Station, Whitney, ON K0J 2M0, Canada 5 Department of Integrative Biology, University of Guelph, Guelph, ON N1G 2W1, Canada https://doi.org/10.1007/s00265-020-02926-w / Published online: 10 November 2020 Behavioral Ecology and Sociobiology (2020) 74: 142