Abstract
The process of ageing is associated with negative effects of mutations acting late in life, which range from those affecting cells to those affecting the whole organism. In many animal taxa, the deterioration of the phenotype with age also affects traits such as males’ primary and secondary sexual characteristics. In three-spined sticklebacks (Gasterosteus aculeatus), males usually reproduce at one or two years of age. To see whether sexual selection has the potential to differ between young and old males, full-sib brothers of different age classes were compared, which were bred and raised under standardised laboratory conditions. During two simulated, successive breeding seasons males were allowed to build their nest in single tanks either in the first (“young males”) or in the second (“old males”) breeding season. A comparison of reproductively active brothers of the first and second breeding season showed that older males produce more but smaller sperm, which might be of lower quality. The fact that older males stored more sperm is size dependent as the results show that larger males possess a greater absolute testis mass, which is inextricably linked to sperm number. However, independent of body size, old males had a lower intensity of red/orange and UV breeding coloration as well as a reduced testis melanisation, which might have consequences in female mate choice and sperm competition.
Highlights
More than 300 different theories on the proximate and ultimate causes of ageing (Medvedev 1990) have been proposed over the past six decades
While absolute and relative testis mass were both significantly higher in older males, the results show that testis melanisation was significantly lower in young males meaning that the testes of old males were less pigmented (Figure 2a-b; Table 2)
By comparing reproductive traits of lab-reared full-sib brothers of different ages in their first reproductive season, the present study revealed that an advanced age can have considerable consequences on the expression of reproductive traits in three-spined sticklebacks, which might have an influence in inter- as well as intra-sexual contexts
Summary
More than 300 different theories on the proximate and ultimate causes of ageing (Medvedev 1990) have been proposed over the past six decades (for reviews see Hughes and Reynolds 2005; Rose et al 2008; Milewski 2010; Johnson and Gemmel 2012). If old males are of lower quality, females will discriminate against them (Beck and Promislow 2007). This exactly happened in the house mouse (Mus musculus domesticus); old males and their scent were less attractive to females (Garratt et al 2011). A number of studies investigated the influence of ageing on reproductive traits resulting in contradictory findings, which might partly be explained by the different breeding systems of the study animals (see Johnson and Gemmel 2012 for an overview)
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