The researchers had shown for the first time that older males may be contributing good genes to their offspring, perhaps counteracting the decline in fertility they experience as they age, university officials said.
''One of the big paradoxes in evolutionary biology is why so many females mate with older males, when male fertility should decline with age,'' study lead author Sheri Johnson said.
''The big surprise here is that our work suggests the genetic quality of sperm is not being compromised with age and that older males may deliver good genes,'' Dr Johnson, of the zoology department, said.
Improvements in offspring fitness might compensate for declining fertility in older males, so females could still benefit from mating with older males and may even secure ''good genes'' benefits.
In a study of zebrafish, just published in the Royal Society journal Proceedings B, Dr Johnson, Otago anatomy department head Prof Neil Gemmell and other colleagues used both cross-sectional and longitudinal methods to investigate the impact of male age on mating success, fertility, and offspring fitness.
The latter includes embryonic development, hatching and survival.
Funded by a $300,000 Marsden Fast-start grant, the researchers identified a significant decline in sperm production and sperm swimming speed as the zebrafish aged.
But, unexpectedly, the scientists found older males continued to produce offspring with high hatching rates and very high survival rates.
Dr Johnson was ''excited'' the study results were finally available, but said some aspects of the overall situation remained ''a little bit perplexing''.
Prof Gemmell said the paper explored ''the conundrum of the known decline in fertility with age, yet the strong preference of females in many species to choose to mate with older males''.
A key factor, at least in zebrafish, was that older males produced offspring that showed superior early survival compared with those of their younger counterparts.
This was an ''unprecedented finding'', he said.
