High-ranking mothers provide their sons with a privileged upbringing and this increases their son's success after leaving home. This was now demonstrated for the first time in a social mammal, the spotted hyena, by a research team lead by scientists from the Leibniz Institute for Zoo and Wildlife Research (IZW).
The results are presented in this week's issue of the scientific journal Nature Communications.
In many social mammals, there is a strict linear dominance hierarchy among the group members. In these societies, high-ranking mothers have preferential access to resources and can invest more in their offspring than lower-ranking mothers. As a result, their offspring often survive better than those of lower-ranking mothers. Whether high maternal investment also influences the long-term reproductive success of sons was unknown until now. To investigate this question, the scientists measured growth, behaviour and reproductive success of sons from mothers of different rank in a population of spotted hyenas in Tanzania.
"Sons of high-ranking mothers grew faster, joined groups with more females likely to choose them as sires, sired offspring at a younger age, and had a higher reproductive success than sons of lower-ranking mothers," says Dr Oliver Höner from the IZW. High-ranking mothers thus invested more in their sons than lower-ranking mothers and this had a significant effect on the long-term reproductive success of their sons.
The study is the first to demonstrate the influence of maternal social status and investment on offspring that leave the group in which they were born when they are young. The study also shows that the maternal influence on offspring is more important and far-reaching than previously suspected. Previous studies reported that mothers can influence the body size and fighting ability of their sons. In spotted hyenas, however, male reproductive success does not depend on body size or fighting ability. "The males rather need to conform to the mate-choice preferences of the females and develop friendly relationships with them because in spotted hyena society females are dominant and decide which male sires their cubs," explains Dr Höner. Thus, the fact that mothers influence which group their sons select and at what age they start to reproduce indicates that mothers also influence the behaviour of sons.
„Until now it was unknown whether in social mammals mothers gain long-term benefits from investing in sons. This arises from the practical difficulty of measuring both the maternal investment and the reproductive success of sons -- in mammals, male offspring usually leave the group in which they were born when they turn adult, making it difficult for observers to monitor them throughout their lives," explains Dr Bettina Wachter from the IZW. Dr Marion East from the IZW adds: "A unique aspect of this study is that the life histories of a large number of dispersing male hyenas were documented and the results advance our understanding of maternal effects in social mammals." For their study, the scientists monitored all hyenas from the eight clans in the Ngorongoro Crater in Tanzania -- currently more than 500 individuals -- during a period of 14 years, established the dominance hierarchies among the clan members, collected genetic samples from more than 800 hyenas and determined the fathers of 650 offspring using molecular genetic methods. "Only with such a large-scale and long-term study were we able to monitor the life histories of a sufficiently large number of sons and match them with their reproductive success in the new group," says Dr Wachter.
This study was conducted in collaboration with scientists from the University of Bielefeld and the University of Sheffield in the UK and was supported by the Deutsche Forschungsgemeinschaft.
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