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Monday, 2 February 2015

Age-dependent behaviour in Black Grouse lekking. Kervinen et al 2015

Life-History Differences in Age-Dependent Expressions of Multiple Ornaments and Behaviours in a Lekking Bird

The American Naturalist 
  • Vol. 185, No. 1, January 2015 (pp 13-27)

PDF link

Authors
Matti Kervinen,1; Christophe Lebigre,2; Rauno V. Alatalo,1; Heli Siitari,1; and Carl D. Soulsbury,3
1. Department of Biological and Environmental Science, P.O. Box 35, University of Jyväskylä, 40014 Jyväskylä, Finland
2. Earth and Life Institute, Place de la Croix du Sud 4, Carnoy Building, B-1348 Louvain-la-Neuve, Belgium
3. School of Life Sciences, Riseholme Campus, University of Lincoln, Lincoln LN2 2LG, United Kingdom

Keywords: ageing, individual improvement, lek, life span, long-term data, senescence.

Abstract
Age is a major factor explaining variation in life-history traits among individuals with typical patterns of increasing trait values early in life, maximum trait expression, and senescence. However, age-dependent variation in the expressions of sexually selected traits has received less attention, although such variation underpins differences in male competitive abilities and female preference, which are central to sexual selection. In contrast to previous studies focusing on single traits, we used repeated measures of seven sexually selected morphological and behavioural traits in male black grouse (Tetrao tetrix) to quantify the effects of age and life span on their expressions and quantified this variation in relation to male reproductive effort. Trait expression increased with age, but long-lived males had a slower increase and delayed maxima in trait values compared with short-lived males. There was evidence of terminal investment (increasing trait values during the last breeding season) in some traits and senescence in all traits. These trait dynamics were largely explained by the timing of male peak lekking effort. This study shows that fully understanding the variation in sexually selected traits and fitness benefits associated with sexual selection requires accounting for the complex interaction among individual age, life span, and the timing of individuals’ investment in reproduction.

Paper Summary

Rationale
Age is a major factor explaining variation in life-history traits among individuals with typical patterns of: 
1. increasing trait values early in life, 
2. peak of maximum trait expression, 
3. senescence / ageing effects.

Background on Trait Selection
The black grouse (Tetrao tetrix) is a relatively short-living lekking galliform with intense male-male competition, and female choice is a major factor driving evolution of the male sexual traits. 
Females prefer males that are:
1. most present on the leks, 
2. fight frequently and successfully,
3. occupy central territories. 
Males with the greatest body mass loss during the lek have the highest fighting rate, the highest mating success, and the most central territories, indicating that strong physiological costs of male display are beneficial. 
Only a few dominant males monopolise nearly all copulations, with mating success linked to: 
1. lyre (i.e., tail) length and quality,
2. size of the testosterone-induced red eye combs,
3. blue chroma colouration of breast feathers. 
Therefore, the lekking black grouse is an outstanding natural system to study the age-dependent expressions of multiple morphological traits and behaviours in relation to sexual selection.

Data Validation
The records obtained were for morphological traits (body mass, lyre length, blue chroma, eye comb size) and lekking behaviour (lek attendance, fighting rate, territory distance from the lek centre) for 164 male black grouse with known year of hatching (2001–2008) and known life span of 1–6 years (totalling 423 records).

Results
1. Long-lived males (≥4 years) had lower age-dependent trait expressions and delayed maximum trait values.
2. Lek attendance and fighting rate increased significantly with age.
3. Males defended their territories closer to the lek centre as they got older.
4. No difference in the effects of male age and life span on the expressions of males’ morphological and behavioural traits.
5. Fighting rate was higher and distance from the lek centre was lower in males that had their peak effort at older ages.
6. Long-lived males were lighter, had shorter lyres, and had lower lek attendance as yearlings than short-lived males.
7. In black grouse, all morphological and behavioural traits reached a maximum expression and subsequently declined with age; therefore, senescence does occur in black grouse.

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