Spotted hyenas live in hierarchically organised groups (clans). An individual’s dominance over another determines priority access to resources such as food or mating partners, and thus reproductive success. However, the rank within the clan can be calculated using different metrics – and that matters. Using data from almost 500 hyenas collected over 28 years, scientists from the Leibniz Institute for Zoo and Wildlife Research (Leibniz-IZW) showed that different metrics best predict the animals’ reproductive success – depending on which reproductive trait they looked at. In the journal “Ecology and Evolution” the scientists argue that the calculation of social rank, and thus also the explanation of hierarchical mechanisms in the world of hyenas and other group-living animals need to be refined.

The correlation sounds simple and obvious: spotted hyenas that rank higher in the clan’s social hierarchy are more successful at reproducing. Although this correlation is not incorrect, it needs to be refined in two respects: Firstly, the rank in the hierarchy can be represented in different ways, and secondly, there are many different traits and variables for what constitutes “reproductive success”. Scientists from the Leibniz-IZW’s Ngorongoro and Serengeti Hyena Projects have now taken a closer look and found that this nuanced view of the mechanisms of social order is not mere nitpicking: They analysed 28 years’ worth of data on 481 hyenas from the 8 clans in the Ngorongoro Crater and compared how well the two ranking methods – ‘ordinal rank’ and ‘standardised rank’ – reflect six variables for reproductive success: For female hyenas, these were the survival rate of offspring, the interbirth interval and the age at first reproduction; for male hyenas these were the number of offspring per year, the rank of the mated female and, likewise, the age at first reproduction.

“Ordinal rank” better explains four reproductive traits, “standardised rank” one
“The two ranking metrics differ in which specific aspects of the hierarchy they focus on”, explains the study’s lead author, Ella White from the Leibniz-IZW. The ‘ordinal rank’ represents an absolute ranking of all individuals within the clan. The dominant animal is assigned position 1, whilst the animal at the very bottom of the hierarchy is assigned a number corresponding to the total number of individuals in the clan. “This rank therefore reflects the absolute position of an individual in the hierarchy and indicates how many animals are above it”, says White. The ‘standardised rank’, on the other hand, is normalised by the total size of the group, so that the highest-ranking animal is assigned rank 1 and the lowest-ranking animal -1. This reflects the ratio of higher- and lower-ranking animals in comparison with the individual in question. The group size is irrelevant in this context, but the proportion of group members that are subordinate to the focal animal are considered as important as those that rank higher than them.

The scientists calculated the rank of each individual using both methods, based on interactions between two individuals: the behaviour displayed during encounters allows them to determine which of the two animals is higher-ranking and which is lower-ranking. They then developed statistical models for each of the six reproductive traits that asses how well the value of the respective variable can be derived from the rank. The result: For the survival rate of the offspring, the age of the females at first reproduction, the annual number of offspring of the males, and the rank of the mated female, the ‘ordinal rank’ ­– the representation of the hyena’s absolute position in the clan hierarchy – is the best metric for prediction; for the interval between two births, however, it is the ‘standardised rank’ of the female. The age of males at first reproduction, however, does not appear to be correlated with rank; none of the models based on the two metrics was able to predict this value reliably.

Different ranking methods indicate differentiated biological mechanisms of power and reproduction
Female mammals require a great deal of energy during pregnancy, lactation and for maternal care. Priority access to food is therefore of paramount importance for them. Direct competition for limited resources is better reflected by ‘ordinal rank’, as the decisive factor is how many other group members are allowed to access food before oneself. “Having fewer direct competitors for food means being able to produce more milk for the offspring that is also richer in protein and fat”, explains the co-leader of the Ngorongoro hyena project, Dr Eve Davidian from the University of Montpellier. The same applies to the age of females at first reproduction, which is heavily dependent on nutrition and growth during their early years. With a high-ranking mother, offspring grow faster, survive more often and reproduce earlier.

When it comes to the ‘interbirth interval’ a different picture becomes apparent: “We assumed that here too, access to food resources and thus the absolute number of competitors for food would be the decisive factor, as females can recover more quickly from the physically demanding period of lactation if they have better access to food”, says Davidian. “However, in this case, ‘standardised rank’ proved to be considerably more meaningful to explain this trait than ‘ordinal rank’. This shows us that other mechanisms are more significant than direct competition for food.” The scientists attribute the link between higher social rank and shorter intervals between pregnancy and births to the complex social interactions within the communal den, where the cubs are nursed. “Lactation is a stressful time for all females, as encounters with other clan members in the den are frequent and disrupt maternal care. Exactly how stressful this period is, clearly depends on one’s relative position in the hierarchy: Lower-ranking females are interrupted more frequently whilst nursing and must submit, which is why their young grow more slowly and they can become pregnant again later,” explains Davidian. “What matters for the female here is not only how many females are higher up in the hierarchy, but also how many conspecifics she can dominate to reduce social stress – in other words, the relative position in the hierarchy.”

“These examples illustrate just how complex and diverse the mechanisms of dominance are among hyenas”, summarises Dr Oliver Höner, co-leader of the project at the Leibniz-IZW. “If we are to gain a deep understanding of these nuanced biological processes in the highly social spotted hyenas, seemingly minor details such as different methods of calculating social rank could already make a significant difference.” Research must take this into account and not adopt a ‘one-size-fits-all’ approach, the scientists conclude.