Sex-specific effects of a parasite on stress-induced freezing behavior in a natural beetle-nematode system

PLOS ONE RESEARCH ARTICLE Sex-specific effects of a parasite on stressinduced freezing behavior in a natural beetlenematode system Andrew K. Davis ID1*, Richard R. E. Ladd2, Farran Smith1, Anna Shattuck3 1 Odum School of Ecology, University of Georgia, Athens, GA, United States of America, 2 Biological Sciences, University of Georgia, Athens, GA, United States of America, 3 Tulane University, New Orleans, LA, United States of America a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 OPEN ACCESS Citation: Davis AK, Ladd RRE, Smith F, Shattuck A (2023) Sex-specific effects of a parasite on stressinduced freezing behavior in a natural beetlenematode system. PLoS ONE 18(3): e0281149. https://doi.org/10.1371/journal.pone.0281149 Editor: Heike Lutermann, University of Pretoria, SOUTH AFRICA Received: May 3, 2022 Accepted: January 13, 2023 Published: March 14, 2023 Copyright: © 2023 Davis et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. * Abstract Some animals react to predation threats or other stressors by adopting a freezing posture in an attempt to avoid detection, and the duration of this behavior usually corresponds with individual personality, such that timid individuals freeze longer. Despite decades of research on this or related behaviors (thanatosis), never has the impact of parasitism been considered. Parasites could prolong the duration, if hosts are less motivated to move (i.e. lethargic), or they could reduce it, if hosts are motivated to forage more to compensate for energy drain. We examined this behavior within a natural beetle-nematode system, where hosts (horned passalus beetles, Odontotaenius disjunctus) are parasitized by a nematode, Chondronema passali. We exposed beetles (n = 238) to four stressors in our lab, including noise, vibration, light and inversion, and recorded how long they adopt a frozen stance. Afterward, we determined nematode burdens, which can range from dozens to hundreds of worms. Beetles tended to freeze for 20 seconds on average, with some variation between stressors. We detected no effect of beetle mass on the duration of freezing, and this behavior did not differ in beetles collected during the breeding or non-breeding season. There was a surprising sex-based difference in the impact of nematodes; unparasitized females remained frozen twice as long as unparasitized males, but for beetles with heavy nematode burdens, the opposite was true. From this we infer that heavily parasitized females are more bold, while males with heavy burdens would be more timid. The explanation for this finding remains elusive, though we can rule out many possibilities based on prior work on this host-parasite system. Data Availability Statement: All data from the two experiments described in this paper are provided as supplemental material. Funding: A portion of this work was conducted as a part of the Population Biology of Infectious Diseases REU Site, a program funded by the National Science Foundation (DBI-1156707) and administered by the Odum School of Ecology at the University of Georgia. No additional external funding was received for this study. The funders had no role in study design, data collection and Introduction Across the animal kingdom, different species have evolved a wide variety of anti-predator strategies that fit their lifestyle, body design and behavior. When directly faced with a predator or perceived threat, some animals react by feigning death as an attempt to fool the predator [1–4]. In some scientific disciplines this is referred to as thanatosis [5,6] or tonic immobility [7,8]. There can be many forms of this behavior, such as curling the abdomen PLOS ONE | https://doi.org/10.1371/journal.pone.0281149 March 14, 2023 1 / 14 PLOS ONE analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist. Effects of parasites on freezing behavior [i.e. pillbugs, 9], playing dead [i.e., snakes, 10] or stiffening the appendages [crickets, 11]. An alternative strategy is to attempt to avoid detection altogether, and simply freeze, or remain motionless, so the predator cannot detect any vibration or sound of the prey. One paper describes this as simply “quiescence” [12]. In a recent study of beetle behavior, researchers concluded that individuals of the same species can even display different strategies (freeze or feign death) for different threats [13]. Collectively, these anti-predator behaviors have been well-studied, especially within the entomological literature, with much research devoted to understanding the range of triggers that induce the behavior(s) [14], the factors that influence their extent [6,15] and even the successfulness of it in actually promoting survival [1]. Distinctly lacking from this body of work are investigations into how parasites might influence this behavior. In fact, in a thorough review of the topic of thanatosis (of 91 studies), no mention was made of parasites [8]. Parasites, by definition, utilize their hosts resources to grow and/or reproduce, and in so doing usurp the host energy stores [16]. This energy drain could motivate hosts to increase foraging activity [17]. Indeed, starvation trials across multiple species have consistently shown that hunger is a strong motivator and reduces the duration and/or and likelihood that animals will even initiate freezing in response to a threat [18–20]. In other words, hungrier animals are more motivated to resume activity and/or foraging, despite the risk of predation. Alternatively, given that infections can often result in lethargy and loss of mobility [21], infected hosts could actually be more likely to remain in place longer when exposed to a threat. So, given the sheer ubiquity of parasites in the animal kingdom [22], and this obvious potential for them to influence host anti-predator behavior (positively or negatively), it is a wonder that this issue has not yet been addressed, despite decades of research on this topic [23–26]. In the eastern United States, there is a common beetle that is host to an abundance of naturally-occurring parasites, making it well-suited to study how parasites affect anti-predator behavior; the horned passalus beetle, Odontotaenius disjunctus (Illiger, 1800; Fig 1), lives in decaying hardwood logs in forests throughout the eastern seaboard [27]. The beetles excavate tunnels in the logs where they raise offspring (grubs), and live their 1–2 year lives, consuming the wood, thereby aiding in the mechanical breakdown of the logs. This species is known to have a high degree of parental care, as the adults protect the grubs, provide macerated wood pulp for them, and even build cocoons for the pupae [28]. This species is host to a wide varie (...truncated)