General Hardwicke’s Dabb Lizard (Saara hardwickii) has recently contributed significantly to understanding of how dietary intake and retention of key nutrients change according to the needs of an animal at a particular time and according to the availability of food of different composition.
The whole approach is that pioneered by Steve Simpson and David Raubenheimer first at Oxford and more recently in Sydney: what are animals attempting to achieve when they eat and how they cope with food that differs significantly in composition from that required for survival, growth and reproduction? For example, when animals need protein for growth but have available only a foodstuff high in carbohydrate but low in protein they overeat in order to obtain the protein they need.
David Raubenheimer has extended these elegant studies into looking at what drives food intake in animals in their natural environment, Orang-utans being one example. This time he has teamed up with Indian and Israeli scientists to see how General Hardwicke’s Dabb Lizard manages its diet and digestion during its reproductive cycle and the changing seasons in the Thar desert of northwestern India.
I should say at this stage that the authors of the paper do not call this lizard General Hardwicke’s Dabb Lizard. The common name I have used is that which the Zoological Society of London used consistently in its lists and cage labels in the now closed old Reptile House. The present authors use Indian Spiny-tailed Lizards, Saara hardwickii. Such lizards have gone by a variety of names: mastigures, dabbs, dabs, dhubs, spinytails, spiny-tailed agamas, spiny-tailed lizards or thorny-tailed lizards. I have taken that list from an excellent article on what he (and I) call mastigures by Darren Naish on his Tetrapod Zoology blog (HERE). I will not cover the same ground other than to point out the salient feature: they are mainly herbivorous. Articles on mastigures also state they will sometimes take insects. As we shall see there is much more to the story than that.
The Thar desert has extremes of temperature. In April to June it is very hot and arid; temperatures may reach 51°C. By contrast, the temperature can drop to 0°C in November to February. The monsoon brings sporadic rain during July and August. The few species of plant that live on the sandy plain therefore vary in absolute and relative abundance throughout the year.
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| From Joshi et al 2026 - see below |
The research, done between the months of April and October, involved intense observation of the lizards in the field over long periods, coupled with measurements and simple analyses. It does not get sidetracked into the over-molecular and over-modelling approaches of too much modern biological research. In essence they observed what lizards ate, estimated how much of what plant or animal was eaten, analysed the food material and calculated the intake of key components of the diet. To that end—and here is the hard work—they counted the number of bites taken from each plant species each day and also counted the number of insects consumed. Assuming bite size was pretty constant, they calculated how much of what was eaten each day.
The contribution of the four main plants species that the lizards eat varied with season but not according to how abundant each plant was at the time. In other words food preferences changed with season. One example is that one food plant species constituted about 55% of the available biomass in April and 40 of the diet. In October that plant had fallen to around 40% of available biomass but then constituted nearly 80% of the diet.
The amounts of carbon (C) and nitrogen (N) in the foodstuffs were used as proxies for carbohydrate and protein respectively. The total intakes of C and N varied with season as did the ingested C:N ratio. Collectively the analyses showed that in June the consumption of N was at its highest at the time increased demand for protein would be expected, i.e. the breeding season. The increase in N intake in June was accounted for by the consumption of insects (a surprise it seems for the authors who had expected that the lizards to be virtually completely herbivorous). In contrast to June, the lizards during October were eating and retaining more C as would be expected from an animal building up body fat reserves before hibernation. Incidentally, it must be those fat reserves which have made mastigures an important source of human food in parts of the world where they occur.
What I found particularly interesting is that the lizards showed not only differences in dietary preferences with changing season but also in differences in the retention of ingested nutrients. That knowledge was obtained by collecting faecal pellets dropped by the animals being observed. In June faecal matter contained less C and N than at other times of year while the shift in the faecal C:N ratio suggested N was being retained to a greater extent than carbohydrate. In October that was reversed: faecal N was at its highest while C was being retained.
The changes in faecal excretion are difficult to interpret in terms of physiological mechanisms since the faecal matter of reptiles and birds is not just faeces, i.e. material leaving the intestinal tract. It is mixed with material coming from the kidneys and therefore contains urates, the products of nitrogenous excretion. In studies like these we do not know if the nitrogen is that unabsorbed by the intestine or that absorbed but with any excess metabolised to supply energy with the waste product exported by the kidneys. I suspect that what the authors recorded were changes in the latter not the former. With carbon it seems more likely that the true faecal route is involved. If, for example, faecal carbon decreases it would seem either that carbohydrates are not being fully digested or that there is some physiological control on the amount absorbed by the intestine.
In herbivorous lizards as in herbivorous mammals it is generally thought, with some supporting evidence in some species, that intestinal flora (i.e. the grossly over-hyped microbiome) are responsible for breaking down the complex carbohydrates into simple molecules that can be absorbed. Any future research needs to investigate this aspect. The authors point out the possibility that the quantity of digestive enzymes released into the alimentary canal may also play a part in determining what is absorbed. Such questions of whether, for example, the intestine absorbs virtually all of the final products of digestion or whether there is, under particular circumstances like oversupply, control of the amount absorbed is of wide relevance, not least in terms of what is happening in human obesity.
It is also worth pointing out that these studies were done on adult lizards. Do the growing young depend on the relatively low nitrogen supply from plants or do they eat insects until they reach adult size? Similarly, do females which have to build protein-rich eggs, eat more insects during the breeding season than males?
As always with good research, more and more questions while answering others.
Joshi M, A Tatu A, Hawlena D, Raubenheimer D, Thaker M. 2026. Desert lizards modulate nutritional responses to match seasonal biological needs. Royal Society Open Science 13: 251690 doi.org/10.1098/rsos.251690
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