Effects of Male Age Structure

Age structure of males also can influence breeding dynamics of age-structured populations. In polygynous species, populations with greater numbers of older-age-class males have been shown to have shorter, earlier, and less socially disruptive breeding periods. Conversely, where fewer older-age-class males are present, breeding periods tend to be longer and females are frequently bred later in the season. Because later breeding may lead to later birth dates, and later birth dates to lower juvenile survival, the age structure of the male population may potentially influence both pregnancy rates and survival of juveniles, thus affecting population rate of increase. However, this cascade of effects has not been conclusively demonstrated in free-ranging populations. Few studies indicate that later-bred females have significantly later parturition dates, while much evidence indicates that female nutritional condition can override potential effects of male age structure and breeding date by allowing females to shorten the length of gestation. Further, ages of males tending harems may not be dominated by prime-aged males until male/female ratios are very high even in polygynous species, indicating that younger males may breed a significant proportion of females regardless of male age structure. Consequently, observed recruitment of juveniles has been shown to be independent of adult sex ratios and male age structure in several polygynous ungulate species. Thus, whereas theory and modeling frequently suggest that male age and adult sex ratios can potentially have a strong influence on population productivity, actual management has driven male age structure and male/female ratios well below thresholds theorized to affect population-level productivity, without any significant decreases in population productivity being documented in free-ranging populations.

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