Chester Zoo is celebrating a rhino baby boom after a five-year study of the animal's dung, researchers said.
Experts monitored the hormone levels of female rhinos at the zoo in a bid to discover the best time to introduce them to a potential partner.
After a decade without any rhino births, the zoo is celebrating its fourth in four years.
It is hoped the project may help boost numbers of the critically endangered species in breeding programmes across Europe. Less than 650 Eastern black rhinos remain in the wild.
Scientist Katie Edwards said daily testing on the rhinos' dung helped to measure hormones associated with reproduction.
"Tracking hormones gives us an insight into what is going on inside these animals," she said.
"It can help tell us things like whether or not an animal is a seasonal breeder, whether it has reached puberty, whether it's cycling on a regular basis or not and when the optimum time to introduce a male to a female is, as well as diagnose pregnancies and estimate when an animal will give birth."
The project was devised in 2007 by Chester Zoo's scientific manager Dr Sue Walker and Dr Susanne Shultz from the University of Manchester.
They hoped to understand the differences in reproduction rates of rhinos across European zoos.
Dr Walker said: "Before the endocrine project was established, it was sometimes difficult to see behaviourally when a female was receptive to a male, so introductions could be difficult.
"But, based on her hormones, we can now predict when the best time to introduce her is and that gives our keepers that extra piece of information to help them get the timing absolutely right, hopefully increasing the chances of a successful mating."
Dr Shultz said: "Although collecting rhino dung isn't the most glamorous work, this project is an excellent example of how academics can collaborate with conservation organisations to save endangered species.
"Getting a large number of zoos across Europe to contribute to science has been very exciting."