Increased aggressive motivation towards formidable opponents: evidence of a novel form of mutual assessment

Abstract

Contests are largely driven by resource value, but their outcome also depends on asymmetries in fighting ability between contestants. Consequently, individuals benefit from assessing these asymmetries when deciding to engage opponents or retreat. Yet, there is much about these assessments that we do not know. First, it is often difficult to discriminate if individuals only assess their own fighting ability or if they compare it to that of their opponents by mutual assessment. Second, the extent to which assessment improves over the course of a contest, as predicted by theory, has remained largely unexplored. We addressed these questions by studying assessment during territorial contests between male Siamese fighting fish, Betta splendens. Findings show the consistent use of mutual assessment when deciding to engage opponents, with a progressive increase in assessment accuracy over sequential contest phases by reducing the use of dishonest signals. Importantly, contrary to theoretical expectations, we find evidence of a novel form of mutual assessment in which fight motivation increases (rather than decreases) when contestants assess their opponents as more formidable than themselves. Although contestants shifted to opponent-only assessment when adjusting display and attack, the collective evidence shows greater aggressive intent towards more threatening opponents. We argue that explanations for this form of assessment may be provided by considering territorial dynamics related to reproductive success and parental investment.

Extrinsic stressors modulate resource evaluations: insights from territoriality under artificial noise

“…The marble landed in the water surface from a 10 cm height and invoked a startle response in focal animals via visual and mechanosensory cues from the distinct splash ( Fig. 2a [36][37][38][39];). This probe was performed twice during contests, at 5 min after onset of contests and again at 5 min after focal fish resumed interaction.…”

“…After a 72-h acclimation to laboratory conditions, measures of size were recorded for all fish to be used as estimates of RHP (see review by Arnott and Elwood [ 7 ]). First, the wet weight of each fish was calculated by the difference in mass between a water-filled container with and without the fish (cf Kareklas et al [ 36 ]). Then fish were briefly removed from the container and placed with some water on a prepared waterproof surface marked by 1 cm squares.…”

“…From the recordings of contests, the time taken to recommence display following each of the two startle probes was used as an estimate of aggressive motivation, with shorter recovery times (in seconds) indicating greater aggressive motivation [36][37][38][39]. The combined duration of frontal and lateral display across the contest was also measured, both for focals and opponents.…”

“…For male Siamese fighting fish, Betta splendens, territories are a key resource because they provide space to build bubble-nests for their offspring, which plays a significant role in reproductive success [27][28][29][30]. The high fitness value of territory for males looking to build nests, and the even higher value for males protecting existing nests, underlies much of the B. splendens well-defined agonistic repertoire, including their display, attack and use of visual cues from opponents to assess their ability (RHP) and compare it to their own [31][32][33][34][35][36]. Importantly, motivational variation in the use of territory for nest building can be quantified by nest-size changes, which indicate the expansion or maintenance of buoyancy and holding capacity [33].…”

“…Also, contest motivation can be accurately quantified via the startle-probe approach, which indicates motivation independently of ongoing behaviour and outcome by quantifying recovery from a startling stimulus. This can be applied at various stages of the contest [ 37 ], and has been validated in fish [ 38 , 39 ] and specifically male B. splendens [ 36 ]. Consequently, the effects of external stressors on resource value can be identified in this species by measuring motivational changes in territorial defence and bubble-nest construction.…”