Offspring sex ratio in moose Alces alces in relation to paternal age: an experiment

Abstract

In most of Fennoscandia, moose Alces alces is intensively managed by the use of age and sex specific harvesting. This includes strong male biased harvesting, which over the last decade has resulted in a change in the sex and age composition of the populations. During the same period of time, a reduction in the proportion of male calves in the harvest has occurred all over Norway, probably reflecting a change in the secondary sex ratio. In order to examine for any causal link between these two patterns, we manipulated the structural composition of an isolated island population of moose in northern Norway and at the same time closely monitored the variation in the secondary sex ratio. In the first stage of the project, which was assumed to represent the conditions in a natural, unhunted moose population, the proportion of male calves increased significantly with the age of their fathers. Potentially, as a consequence of the high mean adult male age in the population, the secondary sex ratio was highly male biased. When we altered the male segment of the population, leaving only young (≤ 2½ years old) males as potential mates for the females, a significant increase in the proportion of female calves occurred in the population. Finally, in the last 3‐years stage of the project, we reduced the adult sex ratio in the population to about 23% males and kept the adult male age relatively high. This again led to an increasing secondary sex ratio. These results suggest that the age composition of males in the population may affect the secondary sex ratio. The general significance of these results was supported by a larger temporal decrease in the proportion of males that were shot in the areas of Norway with larger changes in the sex composition of the hunting quotas than in the regions in which a less extreme harvest strategy were practised.

Male phenotypic quality influences offspring sex ratio in a polygynous ungulate

“…We are thus the first to report a skew in sex ratio related to mate attractiveness for mammals and, apart from peacock (Petrie 1994), for species with no paternal feeding or territoriality. Two earlier studies in the wildlife management literature report a similar effect of male age (and hence size) on offspring sex ratio (Ozoga & Verme 1985;Saether et al 2004), but with no link to any evolutionary model for optimal sex ratio adjustment. If differential allocation occurs, mothers can either produce more male offspring and/or invest more in offspring once they are conceived.…”

“…Often, more males than females are harvested, leading to female-biased population sex ratios (Milner et al 2006). Our study provides a novel explanation for declining proportions of male offspring in heavily harvested populations (Mysterud et al 2000;Saether et al 2004), because harvesting, by generating a high proportion of young, small and unattractive mates (Mysterud et al 2002), affects the secondary sex ratio due to differential allocation effects in females. The secondary sex ratio can thus be affected by human-induced changes in the opportunities for female choice.…”

Hematology and Serum Chemistry Reference Ranges of Free-Ranging Moose (Alces Alces) in Norway

“…Moose from the island of Vega,Norway (65u409N,11u559E), between 1992 and 2000, were immobilized and radio-collared to examine the effects of sex-biased harvesting on reproduction and behavior (Saether et al, 2004) Upon completion of that study, a longterm study was initiated to understand the demography, behavior, and life history variation in a managed moose population (Solberg et al, 2007). Hunter harvesting, a high reproduction rate, and immigration has maintained an annual moose population of approximately 40-60 animals on the island (Solberg et al, 2010).…”

Spatio‐temporal relationship between calf body mass and population productivity in Fennoscandian moose Alces alces

“…Noyes et al 1996, Mysterud et al 2002, such distortions of the population structure can affect reproduction if the number and/or age of males become inadequate to ensure impregnation of all females and/or affects the timing of parturition and hence the size of calves during autumn (e.g. , Saether et al 2004). However, while such a mechanism can explain why we observed a negative effect on the calving rate, it is less clear why the variation in twinning rate is also related to adult sex ratio.…”

“…Large variation in predation pressure can for instance lead to large variation in recruitment rates among regions and over time; variations that are mostly unrelated to population nutritional status (Boertje et al 2007). In addition, annual variation in female age structure and adult sex ratio , Saether et al 2004, Nygre´n 2009) can affect population productivity. These effects are particularly relevant for harvested species because biased harvesting of sex and age groups can create large variation in population structure (e.g.…”