Revisiting the components of Macroscelidea social systems: Evidence for variable social organization, including pair‐living, but not for a monogamous mating system

Abstract

Elephant‐shrews (Macroscelidea) have long been considered the only mammalian order to be completely monogamous, based on observations of their pair‐living social organization. We reviewed primary studies on the four components of social systems (social organization, mating system, social structure, and care system) in elephant‐shrews to evaluate whether they truly are monogamous. To identify gaps in our knowledge of their social system, we reviewed evidence for a pair‐living social organization, mate fidelity (mating system), pair bonds (social structure), and biparental care (care system). Field data were available for eight species and seven were often pair‐living. However, these seven species exhibited intra‐specific variation in social organization; two of these species were also solitary living, two species were also group‐living, and the remaining three species were both solitary and group‐living. The eighth species was exclusively solitary. We reconstructed the ancestral social organization of Macroscelidea using Bayesian phylogenetic mixed‐effects models and found that variable social organization, rather than exclusive pair‐living, was the most likely ancestral state, though there was high uncertainty. No socio‐ecological factors (body size, population density, and habitat) predicted a specific social organization. Observations of mating have been rare, such that no firm statements can be made. However, one unpublished study indicated high levels of extra‐pair paternity. Regarding social structure, there was no evidence of pair‐bonding, but there was evidence of mate guarding. Only maternal care has been observed, with females having very short nursing bouts. Evidence suggests that despite having often a pair‐living form of social organization, Macroscelidea should not be described as a monogamous order, as little or no evidence supports that designation, nor are they exclusively pair‐living (social organization) and we urge further field studies on Macroscelidea social systems.

Primate social organization evolved from a flexible pair-living ancestor

“…For example, many regarded all species that forage alone as solitary ( 1 , 3 ), even when they share the same territory and sleeping sites ( 7 ). In contrast, we categorised a male and a female sharing a home range and sleeping site for extended periods with each other—but not with other adults—as pairs even when they forage alone, as has been done for many other mammalian taxa like elephant shrews ( 34 ), some carnivores such as red foxes ( 35 ), and many bird species. This demonstrates how differences in classifying social systems can influence the interpretation of social evolution.…”

“…Intraspecific variation in primate social systems recently received more attention ( 8 , 22 – 24 ). Kappeler and Pozzi ( 1 ) already considered IVSO in understanding primate social organization, but their statistical analysis used IVSO as one possible category and not as a continuous variable [as did several other studies on different mammalian taxa ( 34 – 37 )].…”

Primate Social Organization Evolved from a Flexible Pair-Living Ancestor

“…Thus, it might seem surprising that we found pair-living to be the most likely ancestral social organization. However, pair-living has also been suggested to be the ancestral form in other mammalian orders when considering IVSO, including Artiodactyla (26), Eulipotyphla (27), and in Macroscelidea (28). Moreover, Kappeler & Pozzi 2019 suggested that pair-living is not a derived complex social organization in primates, but an ancestral form before the evolution of more complex social groups.…”

“…Thus, it might seem surprising that we found pair-living to be the most likely ancestral social organization. However, pair-living has also been suggested to be the ancestral form in other mammalian orders when considering IVSO, including Artiodactyla (26), Eulipotyphla (27), and in Macroscelidea (28).…”

“…Previous studies have treated IVSO as a distinct category of social organization (26, 28). However, given the wealth of data available for primate social organization, we were able to continuously measure IVSO at the population level as the proportion of social units deviating from the most frequently observed (‘primary’) social organization within a population.…”