Vets have finally discovered why two love-struck caimans have been unable to produce babies despite 50 years of frequent mating - because the male's penis is too small.

Hektor and Luiza have been unable to successfully breed despite being an item for nearly incredible half a century at Poznan Zoo in the city of Poznan, in western Poland's Greater Poland Voivodeship region.

The alligator-like crocodilians have been together since they first arrived at the zoo when they were both small way back in 1973.

Caimans Luiza and Hektor in their enclosure at Poznan Zoo, Poland. The pair have been mating for nearly 50 years

The pair of caiman mated often during their time together but never managed to produce any offspring

Poznan Zoo workers kept the caiman under observation and performed tests to provide the proper treatment

Keepers had long been puzzled why they had failed to produce offspring despite their clear enthusiasm for each other during the annual breeding season.

Hektor has been spotted on top of Luiza every year and his frantic efforts often turn their pond into foam, according to keepers - yet Luiza has never laid fertilised eggs.

Malgorzata Chodyla, from Poznan Zoo, said: 'Eggs were produced several times, but nothing came of it.

'Our staff had no idea what was the reason for the failure. The truth only came to light during the visit of specialist doctors from Germany.

The cause of the lack of offspring was only discovered when specialist vets visited the zoo and discovered Hektor's small penis

Poznan Zoo workers in Poland look after the caiman in their enclosure with specialist lights and heat lamps

Vets discovered Hektor's (pictured above in his enclosure) penis was too small to fertilise Luiza's eggs

'Hektor needed a lot of research and consultation from a gastrologist because he did not have any appetite.

'It turned out he was fine. However, the reason for his infertility came to light. The doctors said it was not surprising that Hektor's efforts were futile, because his willy is too small.

'Hektor is always very loud and hundreds of visitors come to see him moan and groan.'

It looks as if Hektor and Luiza will have to console themselves with each other's company as they are unsuitable candidates for IVF treatment.

Ms Chodyla added: 'Apart from the fact that Hektor and Luiza are already too old, this is an expensive procedure that is used only for endangered species.'

Hektor and Luiza will have to console themselves with each other's company as they are unsuitable candidates for IVF treatment

Keepers spotted Hektor on top of Luiza every year and his frantic efforts often turned their pond into foam, but Luiza has never laid fertilised eggs

For reptiles, the size of the penis is significant when it comes to breeding.

Hektor and Luiza's long love affair is unusual as caimans do not choose partners for life in the wild.

Footage provided by the zoo also shows the caiman's feeding time.

Caimans, which inhabit the marshes, swamps and mangrove rivers of Central and South America, belong to the same family - Alligatoridea - as alligators.