Experimental heatwaves reduce the effectiveness of ejaculates at occupying female reproductive tracts in a model insect.

Globally, heatwaves have become more common with hazardous consequences on biological processes. Research using a model insect ( Tribolium castaneum ) found that 5-day experimental heatwave conditions damaged several aspects of male reproductive biology, while females remained unaffected. However, females' reproductive fitness may still be impacted, as insects typically store sperm from multiple males in specialized organs for prolonged periods. Consequently, using males which produce sperm with green fluorescent protein (GFP)-tagged sperm nuclei, we visualized in vivo whether thermal stress affects the ejaculate occupancy across female storage sites under two scenarios; (i) increasing time since insemination and (ii) in the presence of defending competitor sperm. We reconfirmed that sperm from heatwave-exposed males sired fewer offspring with previously mated females and provided new scenarios for in vivo distributions of heat-stress-exposed males' sperm. Sperm from heatwave-exposed males occupied a smaller area and were at lower densities across the females' storage sites. Generally, sperm occupancy decreased with time since insemination, and sperm from the first male to mate dominated the long-term storage site. Reassuringly, although heated males' ejaculate was less successful in occupying female tracts, they were not lost from female storage at a faster rate and were no worse than control males in their offensive ability to enter storage sites occupied by competitor sperm. Future work should consider the potential site-specificity of factors influencing sperm storage where amenable.