Drivers of sexual traits: age and a whole lot more

Many male animals have multiple displays and behaviours to attract females; and often the larger or greater the better.

Understanding what has driven the evolution of these traits is an important evolutionary question.

A new study spearheaded by Matti Kervinen at the University of Jyvaskyla, Finland, working with Carl Soulsbury from the University of Lincoln, UK; Christophe Lebigre, Catholic University of Louvain, Belgium and Heli Siitari, University of Jyvaskyla, has revealed that these sexual traits are strongly age-dependent in black grouse.

The results have now been published in The American Naturalist journal.

The team explored how seven different morphological and behavioural traits were expressed across male’s lives in this spectacular lekking bird, the black grouse. Black grouse have a particularly interesting mating system called a lek, where large groups of male grouse gather in open areas and display to the females. Females come to these sites to choose a mate.

Using a long-term study of black grouse in central Finland, the researchers showed that these sexual traits are strongly age-dependent in black grouse. Expression increased with age to peak values at the peak of their reproductive effort (approximately 3-4 years old) before declining.

At the same time there were differences as to where this peak occurred depending on the male’s lifespan: long-lived males had lower trait expression at young ages and delayed upper limits in trait values compared to short-lived males.

Furthermore, males increased their investment into the expression of these traits as they reach the end of their life.

Finally, the team looked at how the expression of these traits related to the amount of effort put into reproducing.

These results reveal the combined importance of age, life span and individual scheduling of reproduction in driving trait expression. Accounting for these factors is therefore crucial to understanding how these traits have evolved and could explain the substantial variation observed in the sexually-selected traits in male black grouse and other species with weaker sexual selection.

Matti Kervinen, Christopher Lebigre, Rauno V. Alatalo, Heli Siitari, Carl D. Soulsbury ‘Life history differences in age-dependant expression of multiple ornaments and behaviours in a lekking bird’ The American Naturalist

Life Sciences lecturer to feature in New Scientist

New lecturer at the School of Life Sciences, Dr Carl Soulsbury, is to feature in the New Scientist’s December edition.

Dr Soulsbury’s research focuses on life history strategies and his work with black grouse will be used to illustrate the perils of sexual selection in nature, based on the winter meeting of the Association for the Study of Animal Behaviour held last December on “Why do animals mate with the wrong partner”.

It is just one of a handful of examples picked from the meeting to explain how female mate choice can be complex and what can be viewed as “wrong” mates may not always actually be wrong.

Dr Soulsbury came to the University of Lincoln following a post-doc in Finland working with black grouse, said: “Black grouse are of particular interest because they have a particularly interesting mating system called lek, where the male grouse gather in open areas and display to the females. It’s quite a tough selection process and in some years, 90% of females could choose the same single male.”

As well as continuing research on black grouse, Dr Soulsbury’s future work will involve magpies and how they alter egg quality and schedule egg production to benefit their own and offspring’s future success.

Commenting on the upcoming research, Dr Soulsbury said: “You need long-term data so my hope is that I will have enough time to put things in place to start the research next spring.”

He added: “My work links with both the existing people and the other new academics here and it’s a fantastic opportunity to be a part of a research-led university.”