Variation in growth of Damaraland mole-rats is explained by competition rather than by functional specialization for different tasks

Abstract

In some eusocial insect societies, adaptation to the division of labour results in multimodal size variation among workers. It has been suggested that variation in size and growth among non-breeders in naked and Damaraland mole-rats may similarly reflect functional divergence associated with different cooperative tasks. However, it is unclear whether individual growth rates are multimodally distributed (as would be expected if variation in growth is associated with specialization for different tasks) or whether variation in growth is unimodally distributed, and is related to differences in the social and physical environment (as would be predicted if there are individual differences in growth but no discrete differences in developmental pathways). Here, we show that growth trajectories of non-breeding Damaraland mole-rats vary widely, and that their distribution is unimodal, contrary to the suggestion that variation in growth is the result of differentiation into discrete castes. Though there is no evidence of discrete variation in growth, social factors appear to exert important effects on growth rates and age-specific size, which are both reduced in large social groups.

Contrasts in body size and growth suggest that high population density results in faster pace of life in Damaraland mole-rats (Fukomys damarensis)

“…Recent studies on wild and captive Damaraland mole-rats have shown that males require more time than females to reach maximum body mass (Young et al 2015;Zöttl et al 2016a). These results would account for the differences in growth rates between the sexes in our study, where it appears wild males continue gaining body mass at a greater rate than females even after obtaining reproductive tenure.…”

“…Individuals >20 g were implanted with a subcutaneous transponder tag (Dorset Group, Aalten, The Netherlands) to uniquely identify them. Captured mole-rats were housed up to 10 days at 20-25 °C in artificial tunnel systems made from polyvinyl chloride (PVC) pipes with a plastic box filled with paper towels replicating a nest and provided with sweet potato and gemsbok cucumber ad libitum (Zöttl et al 2016a). After the last individual was captured we waited up to 48 hours to ensure no other individuals were present then released the group into their burrow.…”

“…The ecological constraints mole-rats face may constrain offspring to remain in the group due to limited opportunities for both independent foraging and dispersal (Jarvis et al 1994). However, increased group size also results in enhanced competition, which D r a f t may reduce foraging efficiency (Selman and Goss-Custard 1988) and may lead to reduced growth rates (Young et al 2015;Zöttl et al 2016a). Additionally, intra-sexual competition increases with group size, resulting in reduced fecundity and juvenile survival especially in species where dispersal is difficult and philopatry is favoured (Lacey 2004;Young et al 2006).…”

Social complexity and kinship in animal societies

“…In some species where average kinship between group members is high and traits associated with organisational complexity are present (including naked mole rats [ Heterocephalus glaber, Rüppell 1842] and Kalahari meerkats [ Suricata suricatta, Schreber 1776]), there are also obvious morphological differences between breeding and non‐breeding females which resemble those between queens and workers in eusocial insects, although they are less pronounced (Bennett & Faulkes ; Clutton‐Brock ; Zöttl et al . ). While high levels of average kinship in social groups of these species mean that individuals direct costly forms of cooperative behaviour almost exclusively at close kin (Griffin & West ), individual differences in contributions to alloparental care within social groups are seldom closely associated with variation in relatedness between helpers and the individuals that they are assisting (Clutton‐Brock ).…”

Sex differences in timing and context of dispersal in Damaraland mole‐rats (Fukomys damarensis)

“…When males joined an established group, it was more likely that the reproductive individual of the same sex went missing than when females joined a group. This may explain shorter tenures of reproduction in males, larger body size of males and lower reproductive skew in lifetime reproductive success among males than among females (Young & Bennett, ; Zöttl et al ., ; Finn et al ., ). These results combined suggest that philopatric females may sometimes have access to unrelated males (Bennett & Faulkes, ; Young & Bennett, ), which is also consistent with the genetic structure of wild Damaraland mole‐rats (Burland et al ., ) and the closely related Ansell's mole‐rat ( Fukomys anselli , Šklíba et al ., ; Patzenhauerova et al ., ).…”