Anyone who saw the recent episode of BBC Two’s Natural World “Meerkats: Secrets of An Animal Superstar” will know that the Kalahari Meerkat Project, led by Professor Tim Clutton-Brock at the University of Cambridge, is one of the world’s longest running animal behaviour studies.
Since its inception in 1993, this project has produced well over 100 peer-reviewed papers, including several in Nature and Science. Key to this success has been the ability of the project team to get a large marked population of wild meerkats used to the presence of human observers, a process known as ‘habituation’. This allows researchers to record detailed life history and behavioural data from known individual animals – in many cases throughout their entire lives.
Before moving to the Lancaster Environment Centre to take up a lectureship, I was lucky enough to work as a postdoc on this amazing project for five years. My research focused on understanding how the complex social system of meerkats influences the pattern of ageing that we observe in this species.
Cooperative breeding
Meerkats are ‘cooperative breeders’: social groups consist of a dominant pair, who together monopolise almost all of the reproduction in the group, and between two and 50 subordinates (of both sexes) who assist the dominants with raising their offspring – so-called ‘helpers’.
Reproduction is costly. The energy that animals invest in all stages of the breeding cycle is thought to be a driving force in the ageing process. By sharing these costs with helpers, dominant female meerkats may be able to reduce the rate at which they age. Exploring these processes would not be possible without the project’s incredible long-term database, containing the full life histories of many hundreds of individuals.
Perhaps the most unique part of this enormous data set is the body mass measurements. The animals are so well habituated that they are willing to step on to a digital balance in return for a few crumbs of hard-boiled egg or some water; in this way, each individual gets weighed anything up to three times per day.
Body mass data in pregnancy
In thinking about the different ways in which the presence of helpers may impact on a dominant female’s reproduction, I realised that the body mass data provide a fantastic opportunity to investigate one critical part of the breeding cycle that usually remains out of reach in studies of wild mammals: gestation. Could it be that the role of helpers somehow allowed dominant females to enjoy a less demanding pregnancy?
To test this idea, I looked at the rate of weight gain during 137 pregnancies in 29 dominant females. Gestational growth rate combines the growth of the foetuses, placentae, uterus, mammary tissue and the mother’s nutrient store, and so provides a useful index of maternal investment in the pregnancy.
Helpers take the strain
I tested the prediction that dominant females in larger groups – i.e. those with more helpers – may invest less in a given pregnancy because any associated reduction in the growth and survival of the offspring would be offset by the benefits gained from the postnatal care provided by helpers.
The results, published recently in Evolutionary Ecology*, suggest that this is indeed the case. The rate of gestational growth was significantly lower in dominant females with more helpers, even after controlling for litter size and other factors known to affect maternal body mass. This implies that mothers are able to strategically adjust their investment in the development of their offspring according to the conditions at the time – in this case, the amount of postnatal care by helpers (including pup feeding and even lactation) that is likely to be available.
On-going work should shed more light on this, and will no doubt provide further insights into the relationship between being social and getting old in these remarkable mammals.
*Sharp, S.P., English, S. & Clutton-Brock, T.H. (2013) Maternal investment during pregnancy in wild meerkats. Evolutionary Ecology 27:1033-1044
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