Don't judge a sperm by its cover.

Male fertility is primarily assessed using descriptive semen parameters, such as sperm concentration, motility and morphology. These conventional parameters provide an initial assessment of male reproductive health and may explain why a couple is struggling to conceive; however, they are not a direct measure of fertility potential. Growing evidence suggests that spermatozoa transmit far more than paternal DNA. Alongside the genome, spermatozoa deliver proteins, RNA and epigenetic information that are responsive to environmental and lifestyle exposures, and can influence embryo development, pregnancy outcomes and offspring health. This emerging concept challenges the view that male fertility is defined solely by successful conception. Here, we discuss evidence that spermatozoa act as carriers of environmental and lifestyle 'memories', transmitting molecular information shaped by paternal health to the next generation. We further consider the implications for assisted reproductive technologies, which select spermatozoa largely on their capacity to move while overlooking the molecular cargo they transmit. Recognising spermatozoa as vectors of environmental information reframes male fertility and highlights the need for biomarkers that assess molecular integrity alongside conventional clinical parameters.