Behavioral Variation in the Pygmy Halfbeak Dermogenys collettei: Comparing Shoals With Contrasting Ecologies

Abstract

Variation in biotic and abiotic factors among populations affects individual behaviors by transforming the social landscape and shaping mating systems. Consequently, describing behaviors in natural populations requires consideration of the biological and physical factors that different individuals face. Here, we examined variation in socio-sexual and locomotor behaviors in a small, livebearing, freshwater fish, the pygmy halfbeak Dermogenys collettei, across natural populations in Singapore. The pygmy halfbeak is a surface feeding fish that spends most of the time near the water surface, making it ideal for non-invasive behavioral observations. We compared behaviors between sexes among 26 shoals while simultaneously accounting for environmental variation. We demonstrated that sexual interactions and locomotor behaviors differed among shoals with varying levels of canopy cover and water flow. Specifically, in areas with greater canopy cover, sexual interactions decreased, whereas time spent in a stationary position increased. Sexual interactions were more numerous in still water, where fish spent less time swimming. Variation in the expression of socio-sexual and locomotor behaviors were not associated with differences in the amount of aquatic vegetation, water depth or halfbeak shoal size. Agonistic interactions were robust to environmental effects, showing little variation among environments. However, there were strong sex effects, with males performing more agonistic behaviors and spending less time in a stationary position compared to females, regardless of the environment. Moreover, sexual interactions, measured as actively performed by males and passively received by females, were on average more frequent in males than in females. Our findings help us explore the proximal causes of intraspecific behavioral variation and suggest that fundamental information on socio-sexual behaviors from wild populations can lead to a better understanding of how sexual selection operates when the strength of natural selection varies across environments.

Divergence in brain size and brain region volumes across wild guppy populations

“…It is noteworthy that guppies in lotic environments have a larger optic tectum and telencephalon, regions involved in visual processing, decision making/cognitive processes and spatial memory [69], which can be critical for foraging. In rivers and streams (lotic), water currents and rocks create a complex environment, with more heterogenicity and diversity [72][73][74], than the more static lentic environments [75,76]. Our results are therefore consistent with the general prediction more demanding habitats that require greater cognitive and sensory abilities for survival [77][78][79][80], will select for larger brains particularly, larger dorsal areas of the brain [21,49].…”

Immediate predation risk alters the relationship between potential and realised selection on male traits in the Trinidad guppyPoecilia reticulata

“…Differences in the operation of sexual selection in populations subject to different levels of predation on evolutionary timescales are relatively well understood [21]. However, immediate predation risk can vary over short time scales irrespective of the background predation intensity characteristic of the habitat [22,23] and it is possible for the same population to experience consecutive reproductive episodes under different levels of predation risk, with potentially different outcomes for the shape, strength and targets of sexual selection.…”

Does brain size affect mate choice? An experimental examination in pygmy halfbeaks

“…A dichotomous choice of males may not represent a sufficiently challenging cognitive task for females. In the wild, halfbeaks live in mixed-sex groups ranging from 6 to 128 individuals, which are loosely organized and occasionally shoaling ( Greven 2010 ; Ho et al 2015 ; Devigili et al 2021 ). This implies that females will consistently be choosing from a large mating pool—a far more difficult choice than that presented in a dichotomous choice chamber.…”

“…Pygmy halfbeaks are small, live-bearing, internally fertilizing fish that are native to freshwater areas in south-east Asia ( Meisner 2001 ; Greven 2010 ). Halfbeaks live in mixed-sex shoals ranging from 6 to 128 individuals, and males in shoals spend nearly one third of their time directing courtship behaviors toward females ( Devigili et al 2021 ). Halfbeaks are sexually dichromatic, with males displaying strong red coloration that females use to exert mating preferences ( Reuland et al 2019 ).…”