In biology, we talk a lot about trade-offs. This usually means that when something gets better in one aspect of an organism’s biology, something else suffers. Consider a tree species in a forest and its ability to survive a forest fire. Now imagine that this species almost always experiences one type of forest fire. If it almost always encounters the same type of fire, it probably doesn’t need to hold onto any of the traits that allow it to survive other types of fires. Those traits require resources that could go toward other things – getting taller, growing faster, making more seeds or leaves, things that will help it survive the fire it always encounters. Over time, the tree adapts to this fire and loses its resistance to other types of fires. But what happens when a new kind of fire comes along, one the tree hasn’t ever experienced? How well will it be able to resist that? Because the tree has adapted so that it survives the fire it has always experienced, it has lost part or all of its ability to resist new or very rare fires. In this scenario, there is a trade-off between wasting resources to survive an event that almost never happens and using those resources to better survive the one that happens all the time.
Almost every biological organism exhibits trade-offs at some level, from viruses and bacteria to humans and other mammals. Giant water bugs are no exception. There are at least two major trade-offs related to giant water bug brooding: egg size and brooding costs (check out my post on giant water bug parents for more information about brooding behaviors!). Let’s talk about egg size first.
Giant water bug eggs are, well, giant! For aquatic insects, they have particularly enormous eggs. In fact, one researcher, Dr. Bob Smith of the University of Arizona, has suggested that the size of the eggs was what led to the origin of brooding behaviors in the first place (Smith 1997 – full citation available at the end of this post). Smith suggests that the giant water bugs started off a lot smaller than they are now and probably laid their eggs in water like most of their close insect relatives. Giant water bugs are predators, and to become more efficient predators, they needed to get bigger. In order to produce a bigger adult bug, Smith suggests they either needed to add an additional instar or they needed to start from bigger eggs. True bugs, including the giant water bugs, almost all have 5 instars, so it seems that it is hard to for them to add one. So, that left making the eggs bigger. The eggs increased in size, allowing the bugs to become bigger as adults. Eventually, the eggs got so big that they were no longer able to survive underwater, probably because they couldn’t get enough oxygen. So, brooding evolved because the eggs got too big to survive without help.
In this scenario, there is a trade-off between making bigger eggs that require care, but result in bigger adults, and making smaller eggs that result in smaller adults, but do not require care. You know which side of this trade-off eventually won: brooding evolved to allow the eggs to get bigger, and the giant water bugs became the huge, fierce predators that they are today!
Another trade-off relates specifically to the brooding behaviors of giant water bugs . Brooding is likely bad for the father water bug, but the eggs do not survive if they are not cared for. A water bug father thus faces this trade-off: he can care for his eggs, but at a cost to himself, or he can abandon his eggs to protect himself, but at the cost of his offspring. Either the eggs are going suffer or the father is going to suffer. Usually, the father makes a sacrifice himself in favor of the survival of his offspring, though the occasional aborted egg clutch has been observed.
So just why IS brooding bad for dad? This is a question that several giant water bug researchers have addressed and there have been many suggestions. Three broad categories of brooding costs have been identified (spelled out in an excellent paper by Kraus et al 1989):
1. Brooding decreases a male’s mating opportunities. A male who is brooding cares for only one clutch of eggs at a time. This means that while he is caring for eggs, he does not mate with other females. If he did not have to care for his eggs, he could mate with many more females. Thus, brooding decreases a male’s opportunities to mate.
2. Brooding interferes with a male’s ability to move around. A brooding male experiences decreased mobility compared to non-brooding males. At the very least, he is stuck in one place while he broods. A back brooder has eggs glued to his wings, so he is unable to fly. An emergent brooder has eggs stuck to a immobile object, so he can’t take his eggs with him if he needs to move to another location. Back brooders might also suffers further costs including increased buoyancy (though Kraus et al provide evidence to the contrary), slower swimming speeds, reduced ability to find and capture food, and reduced ability to escape predation.
3. Brooding increases a bug’s exposure to predators. Giant water bugs are big and full of high quality protein. Brooding males are likely at a higher risk of predation than non-brooding males. Back brooders may spend more time at the surface and have an increased surface area while they are brooding. Emergent brooders spend more time out of the water while brooding. In fact, a brooding emergent brooder is right out in the open, visible to everything! Most emergent brooders will also try to defend their clutch from anything that might try to take it away, including predators and the occasional graduate students who need their eggs for their research.
So, brooding males sacrifice mating opportunities, mobility, and safety from predators to brood. One might then ask, why do they do it? One simple reason: giant water bug eggs do not survive if are not cared for! Biologists generally believe that the ultimate goal of all biological organisms is to pass their genes on to the next generation. If so, a male water bug will do whatever it takes to ensure that his offspring survive, that his genes are passed on. This tips the trade-off in favor of the eggs, to the detriment of the father.
Next time, I’ll share another field story, one about an amazing water bug father who fought to protect his eggs as I tried to collect them. It would make a good premise for a B-grade horror movie, and just in time for Halloween, so tune in!
Kraus, W.F., Gonzales, M.J., and Vehrencamp, S.L., 1989. Egg development and an evaluation of some of the costs and benefits for paternal care in the belostomatid, Abedus indentatus (Heteroptera: Belostomatidae). Journal of the Kansas Entomological Society 62, 548-562.
Smith, R.L., 1997. Evolution of paternal care in the giant water bugs (Heteroptera: Belostomatidae). In: Choe, J.C. and Crespi, B.J. (eds), The Evolution of Social Behavior in Insects and Arachnids, Cambridge Univ. Press, Cambridge, pp 116-149.
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