No task specialization among helpers in Damaraland mole-rats

Abstract

The specialization of individuals in specific behavioural tasks is often attributed either to irreversible differences in development, which generate functionally divergent cooperative phenotypes, or to age-related changes in the relative frequency with which individuals perform different cooperative activities; both of which are common in many insect caste systems. However, contrasts in cooperative behaviour can take other forms and, to date, few studies of cooperative behaviour in vertebrates have explored the effects of age, adult phenotype and early development on individual differences in cooperative behaviour in sufficient detail to discriminate between these alternatives. Here, we used multinomial models to quantify the extent of behavioural specialization within nonreproductive Damaraland mole-rats, Fukomys damarensis, at different ages. We showed that, although there were large differences between individuals in their contribution to cooperative activities, there was no evidence of individual specialization in cooperative activities that resembled the differences found in insect societies with distinct castes where individual contributions to different activities are negatively related to each other. Instead, individual differences in helping behaviour appeared to be the result of age-related changes in the extent to which individuals committed to all forms of helping. A similar pattern is observed in cooperatively breeding meerkats, Suricata suricatta, and there is no unequivocal evidence of caste differentiation in any cooperative vertebrate. The multinomial models we employed offer a powerful heuristic tool to explore task specialization and developmental divergence across social taxa and provide an analytical approach that may be useful in exploring the distribution of different forms of helping behaviour in other cooperative species.

The relationship between individual phenotype and the division of labour in naked mole-rats: it’s complicated

“…Moreover, a relationship between age and behaviour was only evident when frequent-workers were removed, for which younger individuals compensated by increasing their work rate ( Mooney et al, 2015 ). On the other hand, in the eusocial Damaraland mole-rat ( Fukomys damarensis ), age does seem to play a key role in cooperative behaviour ( Zöttl et al, 2016 ; Thorley et al, 2018 ). Both studies of Damaraland mole-rats found helping behaviours increased until the age of one, after which point there was either a plateau ( Zöttl et al, 2016 ) or reduction ( Thorley et al, 2018 ) in helping behaviour.…”

“…On the other hand, in the eusocial Damaraland mole-rat ( Fukomys damarensis ), age does seem to play a key role in cooperative behaviour ( Zöttl et al, 2016 ; Thorley et al, 2018 ). Both studies of Damaraland mole-rats found helping behaviours increased until the age of one, after which point there was either a plateau ( Zöttl et al, 2016 ) or reduction ( Thorley et al, 2018 ) in helping behaviour. The similarity of helping behaviour in Damaraland and naked mole-rats is interesting, and implies convergent evolution of these patterns given that sociality is thought to have evolved separately in the two species ( Faulkes & Bennett, 2013 ).…”

“…Task specialisation has been observed in some social insect species ( Charbonneau & Dornhaus, 2015 ) and, although it could be an important benefit to sociality in mammals, it has rarely been recorded ( Stander, 1992 ; Gazda et al, 2005 ; Hurtado, Fénéron & Gouat, 2013 ; Gazda, 2016 ). Mooney et al (2015) found that the amount of time spent on different tasks was stable in naked mole-rats, although Thorley et al (2018) found no evidence for task specialisation in the eusocial Damaraland mole-rat. We could not test worker specialisation due to the lack of pup tending observed.…”

Surprisingly long survival of premature conclusions about naked mole‐rat biology

“…after the removal of individuals), the existence of discrete castes among non‐breeders is doubtful (Lacey & Sherman, 1991, 1997; Mooney et al ., 2015). Extensive research demonstrates that the eusocial F. damarensis does not differ from other cooperative vertebrates in terms of task specialization (Zöttl et al ., 2016; Thorley et al ., 2018). Thorley et al .…”

“…Thorley et al . (2018, p. 9) even argue that ‘unequivocal evidence of caste differentiation in any cooperative vertebrate’ is doubtful and that we should be cautious about interpreting these differences among categories of animals as ‘castes’. Despite the inherent difficulty of observing animals underground in the wild, we also need to be wary of behaviour observed only in extremely artificial laboratory conditions.…”