Table 2 Possible influence of different methodological moderators on male reproductive senescence at the level of ejaculate traits

Proportion lifespan sampled*

A higher proportion of lifespan sampled will increase the probability of detecting reproductive senescence, as the onset of senescence usually occurs late in life^4,18,35,47.

Ejaculate collection method*

If males have control over ejaculation during ejaculate collection (e.g. natural mating or mating with dummy females), males might have the opportunity to strategically adjust ejaculate phenotypes⁹⁹. This could cause age-independent changes in ejaculate traits, reducing the detectability of senescence. Additionally, when males have control over ejaculation, studies might obtain a smaller proportion of the sperm reserves available to a male, which may not be representative of a male’s whole-ejaculate phenotype, compared to studies that use invasive methods to obtain ejaculates (e.g. dissection).

Population type*

Reproductive senescence rates can differ between males in captive versus wild populations^68,100. Additionally, some domesticated animals are often culled prior to reaching ages where senescence can be detected¹⁰¹. Other domesticated animals have undergone generations of artificial selection for unusual life histories (e.g. extremely short generation time in broiler chicken¹⁰². These factors could lead to patterns of senescence differing between domesticated and wild animals.

Cross-sectional versus longitudinal sampling*

A cross-sectional sampling of males makes reproductive senescence harder to detect, especially if low-quality males selectively disappear^55,56. Cross-sectional studies might thus underestimate male reproductive senescence, compared to the longitudinal sampling of the same males at different ages¹⁰³.

Manipulations*

Manipulated environments that are outside of what healthy organisms typically experience, such as environments with stressful conditions, can exacerbate reproductive senescence¹⁰⁴. Thus, males exposed to manipulations such as thermal stress, poor diet, or toxins could be more likely to show reproductive senescence than males not subjected to these stressors. Other manipulations, such as experimental inbreeding¹⁰⁵ or selection for deleterious mutations¹⁰⁶, may exacerbate reproductive senescence.

Mating history

High mating rates can exacerbate male reproductive senescence²³. In studies where male mating history is not controlled for, old males often have more matings than young males. These studies might thus show stronger evidence for senescence in ejaculate traits. On the other hand, low mating rates (e.g. virgins) might cause old males to accumulate sperm for longer durations, thus producing larger ejaculates than young males⁶⁶.

Post-meiotic sperm storage

Temporal changes in sperm traits can also occur due to post-meiotic storage of mature sperm in males before ejaculation and in females following mating⁵⁴. The duration of sexual rest in males can influence the amount of post-meiotic damage to sperm, such that for a given age, males with shorter sexual rest (e.g. high mating rate) will incur lower post-meiotic sperm damage⁵⁴. Further, deleterious effects of post-meiotic sperm storage may be exacerbated in old males if old males are less able to repair post-meiotic cellular damage in sperm⁵⁴.