Vegetarians may have better sex lives, particularly those who regularly enjoy soy-based foods such as tofu, according to recent research.

University of California, Berkeley graduate Michael Wasserman observed a group of red colobus monkeys over a period of 11 months at the Kibale National Park in Uganda, to see how diet affected their behaviour.

Wasserman and his team found that the monkeys were more sexually active when their diet consisted mainly of leaves from the Millettia dura tree, which heightened the primates' levels of sex and stress hormones.

Millettia dura is a tropical tree that contains oestrogen-like compounds which are closely linked to soy.

Published in the journal Hormones and Behaviour, the study is the first of its kind to analyse the possible link between sex hormones (phytoestrogens) found in the plants which wild primates consume and how this affects their behaviour.

Researchers, including Wasserman, believe that humans would experience similar affects.

"By altering hormone levels and social behaviors important to reproduction and health, plants may have played a large role in the evolution of primate, including human, biology in ways that have been underappreciated," Wasserman was quoted as saying to website Sciencelive.

Wasserman and his team assessed fluctuating hormone levels in the monkeys by collecting and studying their faecal matter as well as measuring how often the animals mated, spent time grooming and how often they fought and chased one another (which would indicate their aggression levels).

According to the study, levels of the sex hormone (estradiol) and stress hormone (cortisol) were much higher after the animals ate Millettia dura leaves which affected their hormone levels resulting in them having more sex but spending less time grooming.

Animal rights charity PETA also believes fruits and veggies are the keys to virility.

"When it comes to making love, carnivores can be as sluggish as the blood trying to squeeze through their clogged arteries – but vegans have the stamina to keep the party going all night long," PETA UK associate director Mimi Bekhechi told the Daily Mail.