Announcing the release of an exciting new paper linking maternal body size and condition during lactation to calf growth rates in southern right whales, using drones!
The cost of lactation in baleen whales is not well understood so this new paper led by Fredrik Christiansen (Murdoch University, Australia) provides a valuable insight into the relationship between maternal energy loss and calf growth rate. The study followed 40 southern right whale mother and calf pairs over the 3 month breeding season. Unmanned aerial drones were used to take repeated photographs of the mother-calf pairs over the breeding season. The whales were measured from the photographs to determine body length and body volume estimates for both the mother and calf and examine how these changed over the breeding season as the mother provisioned for the calf.
The results showed that calves grew at a rate of 3.2 cm/day in length with a 0.081 m³/day increase in body volume while mothers decreased in body volume at a rate of 0.126 m³/day. On average, the lactating mothers lost a staggering 25% of their initial body volume over the 3 month breeding season. Mothers with a better body condition at the start of the breeding season invested more energy into their calves than mothers in poorer condition. More investment into the calf means that the calf will be stronger and faster and therefore less vulnerable to predation. They will also be less vulnerable to heat loss which is important during the subsequent migration to the colder feeding grounds. However, a high investment in her calf could result in a future loss in survival and fecundity for the mother. Therefore the mother has to balance her own survival and reproductive ability against the survival of her current calf. A mother with a poor body condition might not be able to invest as much into her calf in order to maintain her own survival.
So what does this all mean? We’re interested in this kind of data because it has implications for assessing the population consequences of disturbance (PCoD) and understanding how disturbance links to survival and reproductive rates. For example, if a mother is disturbed and displaced away from feeding grounds (or if there is a bad year for right whale prey species) prior to the breeding season then she may not be able to gather enough energy to invest into her calf during lactation, which could result in reduced calf survival. This is one of the questions that has been considered in expert elicitation e.g.: how many days of disturbance can be tolerated by a mother before it has an effect on calf survival? There are related and follow up questions that we are interested in, e.g. is there a relationship between the initial body condition of the mother and the level of maternal investment with calf survival to age 1? We’d much prefer to have empirical data to answer those kinds of questions, which makes this study so exciting as a springboard to explore these kinds of questions.
Check out Fredrik’s excellent paper – read it here!
The main image has been taken from Christiansen et al., 2018 and shows the measurements taken for this study to determine length and body volume.