How do lizard mothers control the timing of birth?

By 12/25/2020Current Projects
HSF 20068 | Amount: $84,023 | Project Leader: G While | Project Period:

A project undertaken at University of Tasmania, and supervised by Geoff While

Giving birth to live young represents a major biological innovation. It has facilitated the evolution of a wide range of traits that influence where organisms can live and how they behave. Birth itself is a physiological process by which an offspring is expelled from its mothers’ body. During this process, the uterine muscle switches from a quiescent state, where 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 hormonal mechanisms controlled by signals from both the mother and the offspring.

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, these nonapeptide hormones 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 hormones is directly correlated with the concentration of the receptors.

These processes may be conserved across vertebrates, and typically result in mothers giving birth to offspring all at once. Egernia group lizards differ from other live-bearing species because females carry litters of multiple young, all of which develop simultaneously, but are strategically released one offspring at a time (termed birthing asynchrony). This includes offspring within the same uterus. This has important consequences because it alters the competitive environment within a litter which mediates conflict between siblings and ultimately family dynamics (Egernia group lizards exhibit complex family life).

How females achieve this birthing asynchrony is unknown. The most likely explanation is that females have co-opted the existing mechanisms which mediate uterine contractions (or relaxation) to function around a single embryo at a time (rather than all embryo 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, has the potential to provide novel insights into how females control the timing of birth and how live birth evolved more broadly.

Our project aims to understand how this fine scale control of the timing of birth is achieved. To accomplish this, we will utilise a range of different techniques.

First, we will undertake contraction assays that will allow us to measure the contractile response of different components of the uteri to circulating hormones known to trigger birth. Second, we will examine variation between and within uteri in their neuro-morphology which will allow us to determine whether the uteri differ in their underlying structure. Third, we will undertake a fine scale molecular examination of the genes associated with differences in intra- and inter uterine response. Finally, we will determine the extent to which these responses are dependent on the presence and number of embryos housed within the uteri.

Combined, these data will provide novel insights into the mechanisms that allow females to control the timing of birth, the consequences of which have implications for a range of disciplines from evolutionary biology, to conservation biology, to human health.