Parturition (birth) is the physiological process by which an offspring is expelled from its mothers’ body. During this process the uterine musculature switches from a quiescent state, during which pregnancy is maintained, to an active state, which produces regular contractions to expel the foetus. The exact trigger for this switch in activity is not entirely understood. However, it is likely coordinated by mechanical and endocrinological mechanisms controlled by signals from both the mother and the foetus . The key hormones involved in parturition are corticosteroids as well as changes in the ratio of circulating progesterone and estradiol. These factors increase uterine sensitivity to nonapeptide hormones (oxytocin- and vasotocin-like hormones), which then mediate uterine contraction and relaxation. Indeed, in a number of non-human animals, nonapeptides increase in concentration during pregnancy and bind to receptors to stimulate contractions. These changes likely initiate labour, as the physiological response of the uterus to the nonapeptides is directly correlated with the concentration of the receptors.
These processes may be conserved across vertebrates (Whittington et al. 2015), and typically result in mothers giving birth to offspring all at once. Egernia lizards differ from other live-bearing species because females carry litters of multiple young, all of which develop simultaneously, but then strategically release one offspring at a time. This includes offspring within the same uterus (oviduct in reptiles). This has important consequences because it alters the competitive environment within a litter which mediates sibling conflict and ultimately family dynamics (Egernia exhibit complex family life). How females achieve this birthing asynchrony is unknown. The most parsimonious explanation is that females have co-opted the existing mechanisms which mediate uterine contractions (or relaxation) to function around a single foetus at a time (rather than all foetuses at the same time). This would be unique within amniotic vertebrates and represent a major biological innovation. This behaviour may also be, in part or wholly, controlled by offspring rather than the mother. Indeed, research suggests that offspring can play a crucial role in determining when they are born. Understanding how birthing asynchrony is achieved, and the extent of mother vs. offspring control, will provide novel insights into how females control the timing of birth and the evolution of live birth more broadly.
Our project aims to understand how this fine scale control of the timing of birth is actually achieved (via either maternal or offspring control). To achieve this we use a range of different techniques, from contraction assays that measure the contractile response of different components of the uteri to circulating hormones known to trigger birth, to examination of uteri neuro-moprhology, to a fine scale molecular examination of the genes involved in this response. Combined we hope that this will provide novel insights into how females control the timing of birth the consequences of which have large scale implications for a range of disciplines from evolutionary biology, to conservation biology, to human health.
This work is funded by the Hermon Slade Foundation and is carried out in collaboration with Camilla Whittington (Sydney), Martin Whiting (Macquarie), James van Dyke (Latrobe), Laura Parsley (IMAS), Nathalie Feiner (Lund) and Tobias Uller (Lund).
If you want to know more:
While, G.M., Uller, T. and Wapstra, E. (2009) Offspring performance and the adaptive benefits of prolonged pregnancy: experimental tests in a viviparous lizard. Functional Ecology, 23: 818–825
While, G.M. and Wapstra, E. (2009) Effects of basking opportunity on birthing asynchrony in a viviparous lizard (Egernia whitii): consequences for offspring phenotype and fitness. Animal Behaviour, 77: 1465-1470.
While, G.M. and Wapstra, E. (2008) Are there benefits to being born asynchronously? An experimental test in a social lizard. Behavioral Ecology, 19: 208-216.
While, G.M., Jones, S.M. and Wapstra, E. (2007) Birthing asynchrony is not a consequence of asynchronous offspring development in a non-avian vertebrate, the Australian skink Egernia whitii. Functional Ecology, 21: 513-519