How do lizard mothers control the timing of birth?

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. Despite this, our understanding of how live birth itself first evolved and how mothers control the timing of birth remains a major challenge.

Our study addressed this. It took advantage of a unique radiation of Australian lizards, the Egernia group (hereafter Egernia), in which females not only give birth to live young but have evolved extremely fine-scale control over exactly when they give birth. Specifically, females give birth to their young one at a time, retaining fully developed offspring in their reproductive tract for several days following the birth of each offspring (termed birthing asynchrony). There is substantial variation within and between Egernia species in the extent of birthing asynchrony. Females of some species give birth to all offspring at the same time while females of other species can wait for up to 10 days between each offspring. As birthing asynchrony must have evolved via the fine-scale modification of the mechanisms that govern live birth, this system provided a fantastic opportunity to understand how live birth may have evolved and how mothers control the timing of birth.

Liopholis whitii family including two offspring. These offspring will have been born asynchronously with several days between birth. This project has provided crucial insights into the mechanisms by which this asynchronous birth is achieved.

To make the most of this opportunity, we first applied ultrasound technology to pregnant female lizards to identify the exact process by which females give birth to one offspring at a time. We found that while there was considerable variation between individuals in how long there was between births within a litter, the order in which offspring are born is remarkably consistent across females. We then explored whether this consistency in the timing of birth was linked to differences in how a females’ uteri responded to the birth hormone Arginine Vasotocin (AVT). We tested whether there were differences between uteri (left vs. right – lizards have two uteri) and within uteri (upper vs. lower) in how much they responded to AVT. We also tested whether these differences were mirrored in the expression of a key receptor gene (AVPR1a) that is known to be involved in the contractile response to AVT. While we found considerable differences between females in their response to AVT – both in terms of their contractile response and the expression of the receptor gene – these differences did not align with the predicted order of birth suggesting that birthing asynchrony is simply the result of variation between uteri in contractile responses to birth hormones.

What does explain variation in the timing of birth? A number of potential alternative mechanisms exist. First, birthing asynchrony may be as much about the relaxation of the uteri as it is about contraction. This may involve alternative hormonal mechanisms to those outlined above. Second, perhaps the embryos themselves play a role. Indeed, we found that uteri that had embryos in them contracted more than those where embryos were absent. To try and disentangle some of this complexity we followed up the above targeted approach with a whole transcriptome approach aimed at identifying the genes that are associated with the process of birth. This approach provided support for both relaxation and embryonic control – with genes associated with muscle relaxation and end-stage embryonic development both being upregulated during birth. Our results also suggested key roles for genes associated with uterine remodelling and repair following birth. Data we have collected on additional mechanisms (e.g., nerve abundance, yolk/amniotic fluid hormone levels, embryonic tissues) will advance our understanding of these mechanisms in the near further.

Liopholis whitii mother and offspring.

In summary, by generating an understanding how birthing asynchrony is achieved this project has provided novel insights into the process of live birth. In doing so, it has made a significant contribution to key areas in reproductive biology, the implications of which have the potential to inform the fields of evolutionary biology, conservation biology and agricultural science. Furthermore, understanding the mechanisms that underpin female control over birth has strong links to issues associated with human pregnancy, such as pre-term labour. While our work does not have direct medical applications, it does have potential to reveal novel targets for research into causes and treatment of premature birth (e.g., medications used to suppress premature labour).