Penguins are attracted to colours on their partner's bills that are invisible to our eyes

French scientists studied a colony of amorous penguins near Antarctica
Found King penguins are attracted to the colours in each other's beaks
'Beak spots' look orange to humans but the penguins also see UV light
Experts found birds with similar coloured beaks form committed pairs

By Sarah Griffiths for MailOnline

Published: 17 September 2015

The eyes may be the window to the soul, but for penguins colourful beaks are the most alluring feature on a partner. Scientists have discovered king penguins are attracted to the colours in each other's bills, which include hues that are invisible to humans. And the saying 'opposites attract' seemingly doesn't apply to the birds because committed pairs have been found to have similar beak colours to each other.

Beauty is in the eye of the beak-holder: Scientists have discovered that King penguins are attracted to the colours in each other's beaks, including hues invisible to humans. A stock image of a pair of penguins is shown

King penguins (Aptenodytes patagonicus) are the second largest penguin species, after the emperor. They stand 26 to 39 inches (70cm to one metre) tall and weight 24 to 35lbs (11 to 16kg). They are monogamous for around once year at a time and gather in huge breeding colonies in springtime.

Birds on the edge of the group flirt with each other, by rubbing against each other, for example, before pairing off. 'Committed pairs' then move further into the colony and mate, while 'temporary pairs' try to form new partnerships. While previous studies have suggested a penguin's 'beak spot' may be an important differentiator for choosing a mate, a French study has shed light on just how important the bright orange area on either side of the bird's beak is.

Bringing sexy beak: Birds on the edge of the group flirt with each other, by rubbing against each other (stock image) for example, before pairing off. Researchers found that penguins with similar-coloured beaks had more successful relationships than those in which the colours differed

Like many other birds, penguins can see ultraviolet light, so the area looks more than orange to them. A team led by Ismaël Keddar of the Centre d'Ecologie Fonctionnelle et Evolutive in Montpellier, observed king penguins living on the Kerguelen Islands, between Antarctica and South Africa. Around 2,000 penguins congregate there to breed.

The scientists followed 75 pairs of birds on the edge of a colony flirting with each other. They caught them, weighed them and measured the colours of their beaks and chest patches.

Plenty more fish in the sea: The scientists followed the birds' romantic trysts to find that some committed and laid eggs, while others went their separate ways in a bid to find a better match. A stock image of a larger colony on South Georgia Island is shown

They then released the birds, which were able to pair up again by recognising each other's voices. The scientists followed the birds' romantic trysts to find that some committed and laid eggs, while others went their separate ways in a bid to find a better match. Unravelling the secret to certain pairs' success using computer modelling, the researchers were able to see how beaks appear to other penguins.

And they found that the birds' beak colours correlated to the success of their relationships.

In particular, they found king penguins that formed committed pairs had similar beak colours to each other, but the colours on their chests and side of their heads didn't matter.The authors write in the study: 'The members of definitive pairs, but not members of temporary pairs, were more similar for the colour of their beak spots than members of randomly formed pairs...but only for the UV/violet and yellow/orange colors of the beak spots.'

'Colour of the beak spots of females involved in definitive pairs suggested a stronger stimulation of UV-/violet-sensitive cones, a stronger stimulation of medium wave-sensitive cones (i.e. more yellow coloured) and a weaker stimulation of long wave-sensitive cones (i.e. less orange coloured) in the retinas of other penguins than females involved in temporary pairs.'

It is thought mutual attraction is necessary between the birds because they both take turns incubating an egg and raising a chick. 'An alternative hypothesis, however, is that females choose males with highly coloured beak spots, and only females with the most highly coloured beak spots proceed into the breeding colony for egg laying,' the study says.

The scientists also noted: '...females involved in definitive pairs had longer flippers and thus were probably larger in structural body size, compared to females involved in temporary pairs.'

THE LOVE LIFES OF KING PENGUINS AND THEIR CHICKS

King penguins (Aptenodytes patagonicus) are the second largest penguin, after the emperor. They are between 26 to 39 inches (70-100cm) tall and weigh 24 to 35lbs (11 to 16kg). They live in the South Atlantic and breed on sub Antarctic islands.

Penguins are serially monogamous, choosing one mate a year. They stay faithful to them for a period of time before moving onto the next. Committed couples raise a chick together and later put it in a crèche (pictured)

King penguins can breed from three years of age, but most wait until six. They are serially monogamous, choosing a mate a year and stay faithful to them for a period of time. But fidelity between years is only 29 per cent.

The birds flirt with each other on the edges of a colony and choose a mate based on the colour of their beak spot, the new study says. They move further into a colony if they have formed a committed pair and mate. It takes 14 to 16 months from the laying of an egg to offspring fledging, so most pairs only breed successfully every two to three years. The female lays one pear-shaped egg which is incubated for 55 days with the parents taking turns.

Chicks with only a thin covering of down are sheltered by their parents in a pouch for 30 to 40 days, by which time the chick has grown large enough to stay warm and protect itself from some predators. It begins to explore its surroundings and stays with other young penguins in a crèche, guarded by a few adults. It's 14 to 16 months before a young chick is ready to hunt at sea.