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A discussion of compensatory growth and how it could play a role in captive rearing strategies

One aspect of plasticity that interests me is the idea of compensatory growth (CG). CG is when limitation in resources restricts something like a tadpole from growing at an early stage, but then later, a release in that restriction results in an accelerated growth rate above the average in a population. Observationally, this may just look like less fit tadpoles. Bigger is always better, right? 

 Maybe not.

Consider an extreme example, where a large spider has caught two types of prey on its web, a fruit fly, and a housefly. Now suppose the spider can only pick one prey (perhaps they're only loosely caught on the web and there's a short window of opportunity). Which will the large spider most likely go for? It will probably choose the larger housefly. 

Similarly, think of two tadpoles. This time they're the same species, but one is larger than the other. A giant ambushing Anax larva also lives in this pond and it's hungry. But it will give away its position when it attacks one of the two oblivious tadpoles.Which will it choose? Probably the larger tadpole!

 Now you can argue in both cases that a smaller prey item would offer less resistance and be less energetically demanding to consume than the larger prey. And you may have an argument there. But there is also a higher reward energetically for consuming larger prey, that's why you probably thought in both cases that the predator would choose the bigger prey (ignoring the possibility of things like gape limitation).

Generally speaking as it relates to fitness, it's usually better to be bigger. Being larger removes the possibility of many smaller predators to even consider you on their plate. You also have larger energy reserves and swimming power, and you can outcompete smaller conspecifics. But I think we need to recognize that there are more nuances to survival than just size. Species have morphological, behavioral, and physiological differences to help survive as well.

So it's not farfetched to believe that food restriction due to less foraging (because a predator's around or maybe there's just not enough food, period) could actually have fitness benefits. It keeps organisms from undergoing behavioral or physiological senescence. Like how working out keeps your muscles strong.

The real interesting question is, "Are individuals that are able to "use" compensatory growth as a strategy helping or hurting their survival and reproductive success (fitness)?"

We don't fully understand the ultimate costs of compensatory growth. Some posit cellular damage due to rapid replication of somatic cells required to accelerate growth, oxidative stress, immune response, competitive ability (Yearsley et al. 2004). But few studies have tracked compensatory growth in the juvenile stage and none that I'm aware of have tried to measure latent effects into adulthood, at least in amphibians.

I think CG is important to research from start to finish. In the gopher frog, for example, current conservation efforts aim at removing subsets of eggs from the wild, raising them in captivity, then releasing them as metamorphs. The hope is that by increasing survival of new metamorphs, overall adult survival per cohort will increase and help bolster populations. But they're finding that the majority are dying (Roznik and Johnson 2009) right after being released. Right now they're being reared at 50 tads/mesocosm. What would happen if we just reared massive tadpoles by removing competition (i.e. reduce to something like 25 tads/mesocosm, 10 tads/mesocosm? One?? How do long-term tradeoffs compare if you raise tadpoles with unlimited food versus restricting food at first, then giving them plenty to induce CG? Would the former or the latter have a higher survival rate after being released? Would there be latent effects from inducing CG? Does subjecting tadpoles to challenging environments (like inducing CG) prepare them for being an adult or is size the only important thing? Size at metamorphosis is one of the best predictors of survival of newly metamorphosed amphibians. But what happens when you remove the need to overcome adversity? In captivity, birds are known to imprint on humans and it cripples their ability to function in the wild (improper conspecific communication, etc.). Is a similar phenomenon occurring in amphibians? Does a tadpole lose it's ability to hunt effectively by being "hand-fed"? Is sensitivity to predation diminished by captive rearing in situations without predators? Maybe size AND experience matters. 

References:

J. M. Yearsley, I. K. a. I. J. G. (2004). "Delayed Costs of Growth and Compensatory Growth Rates."
 

Johnson, R. a. (2009). "Burrow Use and Survival of Newly Metamorphosed Gopher Frogs (Rana capito)."
    

Comments

  1. Very cool! Compensatory growth is really interesting, and something I have thought a lot about. I wonder about how organisms know that they are "behind" to initiate that compensatory process. What is the trigger? Is it an external stimuli or internal clock that incorporates size? Why does overgrowth usually occur? What stops an organism from staying on that path and continually growing at that rate? Maybe the pathways that initiate such growth are not easily initiated/shut off and are only slowly responsive to the stimulus. I wonder how we may be able to alter/prevent compensatory growth and see how that would affect fitness. Additionally, as you mentioned, what other costs are associated with that growth later in life? I think of things like fecundity, longevity, and overall organism quality and ability to get mates.

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  2. Great perspective on compensatory growth! There seems to be a lot to explore on this topic. I wonder how the energetic costs of greater onset growth effects other mechanisms in larval development. I presume that a significant portion of energy is relegated towards expedited growth that other biological mechanisms and developmental processes are inhibited. Can a larvae overcome these disadvantages? Are adults how underwent CG as larvae more likely to be predated on as well?

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