Delaying the weaning of captive rhesus macaques (Macaca mulatta) destined for use in neuroscience research : an assessment of the costs and benefits

Abstract

This thesis aims to assess the costs and benefits of delaying permanent maternal separation (“weaning”) of purpose-bred captive rhesus macaques (Macaca mulatta) and to determine if UK guidelines on weaning age should be refined to improve animal welfare. Current UK guidelines advise weaning should occur as late as possible and not before 10-14 months. Whilst weaning earlier than this has negative and long-lasting consequences on behaviour and health; it is unclear if later weaning provides welfare benefits. Perceived costs of increased aggression and decreased productivity are used to justify earlier weaning, despite there being limited data to support this. Delaying weaning is a simple and inexpensive refinement that can potentially improve welfare as well as the validity and reliability of scientific data derived from animals destined for research. As such, this investigation is imperative. Taking advantage of variable weaning ages at the Medical Research Council’s Centre for Macaques, health record data was collated from 2004 – 2022 on injuries, alopecia scores, weights, Campylobacter incidence and inter-birth-intervals (IBI). The full retrospective dataset contained observations for 800 macaques (436 females). Additionally, in 2019-2022, 136 monkeys from 4 birth cohorts (59 females) underwent novel object tests close to their 3rd birthday to assess neophobia. Finally, in 2020- 2022, video data from breeding groups were collected to assess levels of aggression directed towards older juveniles. The data show that weaning is associated with increased Campylobacter infection, and increased alopecia prevalence and severity. However, a later weaning age was found to mitigate these acute effects. The data do not support the perceived costs used to justify early weaning: later weaning was not correlated with decreased productivity and whilst older juveniles in breeding groups were at greater risk of injury than weaned counterparts, overall rates were low. In conclusion, these data support the welfare benefits of delayed weaning.