Couples who are faithful are more fertile than those who stray.

Couples who stay true to each other are more fertile than those who stray,
new research has found.
Scientists have discovered that those who remain faithful produce 25 per cent more offspring.
Monogamy is relatively rare among mammals, but the study, of owl monkeys in Argentina, has highlighted how important having just one mate can be.

Remaining faithful is the recipe for a large family, scientists have found

The wide-eyed nocturnal monkeys are considered a socially monogamous species, but scientists found when a new mate usurps one of the pair, there are fewer newborns than with pairs that stay together.
It's thought the birth rate difference could be down to the time it takes for new pairs to get to know one another.

Associate Professor Eduardo Fernandez-Duque of the University of Pennsylvania said monogamous relationships have played an important role in human evolution and helped increase the species.

 

He said: 'Call it love, call it friendship, call it marriage, there is something in our biology that leads to this enduring, emotional bond between two individuals that is widespread among human societies.
'Monogamy makes sense for these primates, because the male who sticks to a female is certain about the paternity of the young, and so he invests in their care.'
One theory is that a woman's fertility might be increased because having a steady partner 'reduces the burden of pregnancy and lactation'. 
Professor Fernandez-Duque and his team have monitored an owl monkey population in the Argentinian Chaco region observing behaviour, changes in pairings and rates of birth over 16 years.

The study, in owl monkeys (pictured), found that when a new mate arrives,. fewer children are born

The team observed 18 owl monkey groups totalling 154 animals.

The primates live in monogamous groups consisting of an adult male, an adult female and their offspring, with juveniles leaving the group around the age of three or four.

However in 2008 they noted a so-called 'floater' individual, which attacked the male in a pair and essentially replaced him as a mate and infant-care provider.
The study found this usurping of mates by both male and female 'floaters' is a common occurrence.

The replacements often involved dramatic fights, some of which ended fatally for the evicted individual.
But having a partner evicted harms the reproductive success of the remaining mate.
Owl monkeys with one partner produced a quarter more offspring per decade than those with two or more partners.
Professor Fernandez-Duque said: 'What we're showing is that if you manage to stay with the same partner you produce more infants than if you're forced to change partners.'
Although the drop in fertility is unclear it could be due to a delay in reproduction as female owl monkeys typically only conceive between March and May.

He suggested the delay could be down to the new pair taking time to assess one another before reproducing.