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The Costs of Carnivory: Response to Marcus Clauss

Posted by pbio on 07 May 2009 at 22:24 GMT

Author: Chris Carbone
Position: Senior Research Fellow
Institution: Institute of Zoology, Zoological Society of London
Additional Authors: Amber Teacher, J. Marcus Rowcliffe
Submitted Date: June 18, 2008
Published Date: June 20, 2008
This comment was originally posted as a “Reader Response” on the publication date indicated above. All Reader Responses are now available as comments.

Clauss raises both qualitative and quantitative concerns regarding our model analysis (Carbone et al. 2007).

The quantitative concerns result from an unfortunate typo in our original manuscript where we give the wrong value for maximum intake (it should read 1,343 kJ/h and not 3,132 kJ/h). Our maximum mass estimate was based on the correct value. While we regret this error, it is unfortunate that he went ahead and published this part of the response after having been informed of this.

Regarding his qualitative concerns, we used the 0.75 scaling of costs because we wanted to account for the costs of hunting with increasing size assuming all other factors were held constant – that is without the energy minimizing behaviours which we present evidence that carnivores adopt in nature. The point of doing this is that it is clear there must be a limit to how much a mammal can adjust its costs/intake away from the overall scaling patterns observed (see Figure 1 in Carbone et al 2007; and upper and lower bounds in Figure 1C). Our view is that intake rates are limited, especially for larger species within each prey class and these species therefore adjust costs to balance energy budgets. If we were to assume a 0.6 scaling of costs, this would be equivalent to assuming that carnivores can deviate indefinitely away from the normal metabolic rate scaling for other mammals. To illustrate this point, if we take standard calculations for the scaling of mammal metabolism but changed the 0.75 exponent to 0.6 as implied by Clauss, the resulting metabolism of an animal the size of an elephant, would be 29% of that of real elephants in captivity. Flipping this around the metabolism of real elephants would be equivalent to an animal of 32 tons derived from this lower scaling exponent. Clearly this would not be a realistic approach to predicting maximum carnivore mass.
We do agree with Clauss about the importance of prey abundance as a factor contributing to hunting costs and intake rates, which is why we state in the paper that “extremely high prey biomass would be required to both minimize energy expenditure and maintain high rates of energy intake”. Clauss refers to marine mammals in his correspondence, however, while marine mammals may indeed experience very different patterns of prey availability to terrestrial mammals, they also experience fundamentally different energetic constraints in terms of mode of locomotion, thermoregulation and methods of obtaining prey. All of these differences must play a role in the differences we observe in size distributions. Similarly, while Clauss cites evidence that dinosaur prey availability was relatively high, we also have evidence that dinosaur metabolism differed from that of mammals in a way that would support higher body mass (ref 43 cited in paper).

In summary, Clauss has sensibly highlighted the role of ecological context in defining animal size distributions, but fails to provide a convincing argument against the role of intrinsic energetic constraints.

Carbone C, Teacher A, Rowcliffe JM (2007) The costs of carnivory. Plos Biology 5(2): 363-368.

No competing interests declared.