Good welfare is attractive: Female zebrafish (Danio rerio) prefer males from complex, well-resourced conditions over males from conventional barren laboratory tanks (2025)

Lavery, J. M., Snaith, K., Pallarca, J. et al.

Abstract

Applied ethologists often find that sub-optimal housing (e.g. barren versus “enriched”, well-resourced conditions) impairs animals’ interactions with conspecifics. Furthermore, some housing effects on social/sexual interactions persist even in standardised test situations. For example, in mating tests on three mammalian and two insect species, males from sub-optimal housing have been shown to be less successful with females (e.g. less attractive to them) compared to males with better welfare. Here, we assessed whether similar effects occur in fish, using zebrafish (Danio rerio, Tübingen strain) as models. In a purpose-built maze, 16 groups of ready-to-spawn females were each given choices between two pairs of enclosed males that had been raised and housed differentially (in either conventional laboratory tanks, ‘Barren’, or large, well-resourced ones, ‘WR’). After this ‘Choice Phase’, they were allowed to spawn via free access to one type of male (half WR, half Barren). All trials were run (and videos analysed) blind to housing, to avoid unconscious experimenter biases. Results showed that in the Choice Phase, WR males were significantly preferred over Barren, attracting more proximity from more females; while in the Spawning Phase, WR males also attracted more courtship, and tended to elicit fewer escape attempts. Some of these benefits of being WR were only detectable, however, if male body size was statistically controlled for, because of an independent effect of male size (longer males being more attractive, despite WR males being unexpectedly smaller). Females thus used multiple housing-sensitive cues to select preferred mates: body length, plus unknown attributes of WR males (which could involve improved cognitive abilities, better physical health, greater stress resilience, and/or signs of greater libido: all topics for future study). This suggests many future avenues for fish research, potentially leading to improved welfare and reproductive success for laboratory-housed zebrafish (and even other species in aquaculture and conservation breeding facilities). Furthermore, these results (along with the studies inspiring this experiment) add to applied ethologists’ longstanding appreciation of animals’ sensitivity to conspecific signals of emotion: they indicate that animals can also detect longer-term welfare states, perhaps even finding poor welfare unattractive.