Primate Mothers

I am going to take a little trip back in time here to talk a bit about the research I did for my thesis for my Masters of Research. In 2011/2012 I undertook the MRes Primate Biology, Behaviour and Conservation degree with the University of Roehampton under the wonderful supervisor Dr. Stuart Semple. This fascinating course taught me so much about all areas of primatology and allowed me to spend 3 months carrying out my own research project on the Puerto Rican island of Cayo Santiago.

View from Cayo Santiago (2012)

Cayo Santiago is a 16ha island about 1km off the coast of Puerto Rico and is inhabitated only by 1000 rhesus macaques (Macaca mulata). Starting in March 2012 I carried out a study of the effects of the weaning process on the stress levels of the macaque mothers. Rhesus macaques live in large multi-male/multi-female groups. Rhesus macaques social groups are female–bonded and male dispersal. The females remain in their family group and their dominance hierarchies are based on their matrilineal kinship  (Melnick et al. 1984). Female rhesus macaques very rarely leave their natal groups (Fooden 2000). I hypothesised that weaning would be a stressful period for the mother as if, her newly weaned offspring dies before it can reproduce itself, her investment has been wasted.

I started my study at the beginning of the mating season, when the female macaques were beginning to wean their infants. I looked at levels of self-scratching, a well known behavioural indicator of stress in primates, normally (to determine a baseline frequency of scratching) compared to the frequency of self-scratching in the minute following a rejection of the infant when it tried to nurse. There were three distinct types of rejection: passive rejection involved the mother turning her back or gently blocking her nipple; active rejection where the infant was pushed away or restrained from reaching the mother and violent rejection in which the infant was attacked when attempting to nurse. I also looked at the type of reaction that the infant displayed when it was blocked from weaning. Some infant displayed completely hysterical tantrums that seemed very likely to draw negative attention from other group members, especially adult males.

Female relationships are very important to rhesus macaques.

 In addition to this I looked at whether the stress reaction to rejecting the infant was affected by the dominance rank of the female, how many female relatives she had in the group (and therefore, likely level of support), how many previous infants she had raised, whether the infant was male or female and how socially connected the female was within the group.

Adult female's older offspring trying to steal her new baby.

I found that there is a significant increase in self-scratching directly following the rejection of an infant and that this is effected by whether the infant is male or female. In rhesus macaques, the structure of their society means that dominant males have a huge monopoly on mating opportunities. If the female macaques want to ensure their genes pass to the next generation they have to invest a lot more in a male to give him the best chance of becoming big enough to become dominant. Females have a lot more to lose if a male infant dies between weaning and mating.

In capuchins, things are a bit different. On average, female capuchins give birth every 19.4 months (Janson et al. 2012). Birth season varies by area and is dependant on rainfall, food availability and photoperiod of the habitat (Carosi et al. 2005). In Laguna Blanca, I have only recorded once instance of mating (captured on a camera trap in July 2015) and most of the new babies I have seen have appeared in late December, although both F Group and N Group currently have babies that are less than a month old. As the groups become more habituated to researcher presence I aim to modify my masters research methodology in order to investigate whether weaning causes stress to the mother in the hooded capuchin.

 

Adult female grooming a juvenile.

Adult female carrying a juvenile as another young capuchin approaches her.