‘What can I learn from you?’ is not a question we’re used to asking when staring down into the toilet bowl with our hand hovering over the flush. It’s not a question that Zebra ask themselves either, as far as we know, before trotting away from their kidney shaped droppings on the baking African savannah. However, biologists can be a little bit weird- and have asked the question for them.
Ecologists led by Dr Jessica Lea at the University of Manchester have turned their focus to the dung of the Cape Mountain Zebra to try and decipher what it can tell us about decreasing habitat quality, seasonal fluctuation and unbalanced population sex ratios.
Why this species? The Cape Mountain Zebra has been through a bit of a rough patch. It suffered a massive decline in population numbers reducing its population to approximately 80 individuals in the 1950s. From this population bottleneck it’s numbers have recovered to approximately 5000, which are currently managed in an active conservation project. This conservation project monitors the various populations and keeps them genetically interlinked (via transfer of individuals between populations) creating population stability through ‘metapopulation’ dynamic.
So, onto the next strange question: ‘What do the San Andreas fault, Trinity students in early May and Zebra droppings all have in common?’. They’re all stressed out! How do we know this? Well in the Zebra the sign of stress is glucocorticoids. These are the chemicals ecologists have been analysing in the faeces. Glucocorticoids are steroid hormones that play a role in regulating metabolic function, and are also involved in monitoring stress levels. An elevated concentration of glucocorticoids in droppings is a potential indicator of two physiological alterations in Cape Mountain Zebra individuals.
One – the animals are directing more energy into improving their metabolic efficiency to extract more nutrients from a poor-quality habitat. This in turn leads to less energy for reproductive output. In short, the more glucocorticoids, the poorer the habitat and the fewer the offspring. Two – the higher the glucocorticoids, the higher the stress. Now you see the link between the Zebra dung and the Trinity student in early May. Both showing clear signs of stress. What impact does stress have on the Zebra? High stressed animals are far less likely to breed successfully, which is a serious problem for a vulnerable metapopulation. Extinction after all is just a population game. If those birth rates fall below those death rates for a significant period of time – and it’s bye bye stripes!
What’s causing this rise in glucocorticoids? Poor quality habitat was already mentioned – and indeed according to the researchers’ statistical analysis, there is a strong negative correlation between habitat quality and glucocorticoid levels. The differing weather patterns between seasons is also strongly linked with increased glucocorticoid levels. The Zebra, based on their dung, are at their most stressed in Spring right before Toto brings the rains down to Africa in the summer.
But glucocorticoids aren’t the only clue in the poo. The Manchester scientists also assessed androgen levels in the faeces of stallions. Androgens are steroids that control the development and upkeep of male characteristics. An increased level of androgens can lead to increased aggression in male Zebra. It was found that androgen levels were significantly higher in populations with a high male to female ratio compared to populations with a more evenly spread sex ratio. They were also significantly higher in small populations when compared to larger ones. This of course makes sense. In a social system where groups of females are mated by a single male, an excess of males or deficit of females will intensify the amount of male-male competition for mating rights.
So higher levels of aggression will mean you’re a more successful competitor, right? Not necessarily. High levels of aggression have been shown to sometimes have a negative effect on male mating performance. The researchers also showed that, in this study, increased androgen levels were linked to a decrease in female fecundity. However, their long-term impact is less clear as they seem to have no effect on population growth rates. Is it an effective adaptation to increased male-male competition – or a strain on the success of the individual male as well as the success of the population?
‘So, what?’. Again, not a question we’d be usually asking of a pile of faeces. It is an important one nonetheless. The researcher’s statistical analysis churned out some very important revelations. Glucocorticoids and androgens are both negatively linked to female fecundity – and glucocorticoids levels are negatively correlated with population growth rates.
The hormones act as the physiological explanation. It makes sense that a stressed-out Zebra in a nutrient poor habitat will produce less progeny than a Zebra in a grass rich environment. The androgen augmentation of aggression clearly demonstrates how deviations from the demographic norm can have significant physiological effects on the animals. While its ramification for the future of the population is unclear, this androgen augmentation has been clearly linked with a decrease in female fecundity.
So back to that first question ‘What can I learn from you?’. Well, as the Manchester ecologists have shown, there’s more to these defecations than meets the eye. This non-invasive examination of changes in physiological processes has shed light on major pressures on the Cape Mountain Zebra metapopulation. It’s an easy way of demonstrating ‘oh there’s something wrong here’ – a statement that often eludes conservationists until it’s too late.
As Dr. Lea and associates indicate in the paper, it’s the first study of its kind that uses a ‘macro physiological’ approach to assess on ongoing conservation plan. In the scope of the Zebra, it could be used as a watchdog for habitat decline, spurring conservationists to investigate when significant deviations in the glucocorticoid levels are seen. Climate change, another hot topic, could also be a future aspect for Zebra dung research. Imagine the stress induced if the dry spring was picked up in a summer drought. Suddenly there’s a new piece of evidence about global warming’s negative effects on population growth rates of large charismatic terrestrial animals.
No large charismatic animals means no wildlife tourism, a sobering reality that would surely influence governments like South Africa’s to invest more in slowing the rate of global warming.
It doesn’t stop with the stripes. Faecal hormone analysis could be applied to many large mammals, assessing the impacts anthropogenic disturbance, unnatural population dynamics as well as the impact climate change might be having on them. It seems the detail is in the dung and the stage is being set for faecal analysis’ big splash into conservation science.
