The fascinating sex lives of Bluegill
Alternative life history strategies is the fancy name for the fact that Bluegill "sex" is a pretty curious and fascinating thing. There is a lot more going in with Bluegill than you might know about even though it is happening under our watch. You are likely familiar with the fact that males make and guard nests - these are the parental males. The parental male guards the nest and the female deposits her eggs and then gets chased off by the male. That is interesting enough in the animal world where females are much more likely to care for their offspring than are males (eggs are expensive, sperm is cheap). Where it gets rather crazy is in the cuckolder males which begin their lives as "sneakers" and change to female mimics when they get older and larger. In neither stage do cuckolders guard a nest, they dump their sperm cells into the mix and are on their way to try it again. Cuckolders are a separate lineage from the parental males. As cuckolders age, they do not grow into a parental male, rather they remain female mimics until they die, almost certainly at a younger age than the parental male (more about differences in lifespan below).
Source: Partridge et al. 2016
There is rather a lot going on in the quite informative figure above. First, I think many would be surprised to know how old the parental males and females are before they spawn for their first time. Females are typically at least 4 years old when they first spawn but parental males are typically about 7 - 8 years old when they first spawn. By way of comparison, a "keeper bass" (one over 14 inches in Wisconsin) is likely to be 4 or 5 years old and has probably spawned once or twice by the time it reaches "keeper size". While it is rare, there are cases of 1 year old bass spawning. The same is true for trout which sometimes - but rarely - spawn before they even reach 1 year old. Many trout - and in Wisconsin, I would say most trout - are spawning in their second year when they are about a year and a half old. They have different life history strategies.
The second thing that stands out are the lifespan differences between the parentals and the cuckolders. Parentals are "live slow, die old" (K selected) and the cuckolders have more of a "life fast, die young" strategy (r selected). This is largely due to their allocation of energy. If you are a loyal reader, you may remember the post on bioenergetics and how growth is divided into somatic (body) and gonad (reproductive) growth. Essentially, cuckolders are putting more energy into growing testes and making sperm cells early in their lives and this limits their body size and their lifespan. Parentals on the other hand, do not put any energy into reproduction until they are relatively old which allows them grow to a larger size, faster. This is particularly true for parental males whom will not reproduce until at least their 6th or 7th year of life, more likely their 8th year of life. While that is not "Lake Sturgeon old", it is a pretty old for a fish, particularly a relatively small "panfish". In Wisconsin, a Largemouth or Smallmouth Bass that is 8 years old is almost certainly over 18 inches and likely over 20 inches (except for the slowest growing of individuals or in really cold lakes).
Why Size Matters
Size matters for Bluegills for two very important evolutionary reasons. First, being larger allows the parental males to attract more and "better" females, increasing their reproductive success. Sexual selection in Bluegills tends to favor larger and more colorful males - the fish we sometimes refer to as "bull Bluegill". And second, growing to a large size quickly means that the individual grows to a size that limits its risk of predation. More simply, there are not many predators - pike, musky, bass, Flathead Catfish, herons, etc. - that can eat a 7 inch or larger Bluegill. Fishes are strongly gape limited; they can only eat what the can fit in their mouths. And a 7, 8, or 9 inch Bluegill is one hell of a thing to try to get into your mouth in a single bite. And then think of that spiny dorsal fin being raised and presenting a larger - and more painful - challenge. Cuckolders on the other hand are unlikely to escape predation from most of their predators within their lives. Different life history strategies within the same species. Cuckolders live a more r-selected life and parentals have a more K-selected life history (r/K selection theory).
Bluegill are not the only species with a unique alternative mating strategy. In many species, sperm competition evolves so that the sperm of individuals in different groups have different qualities. There is not a lot of evidence for this occurring in Bluegills (but there may be some...). Cuckolders have much more concentrated sperm but because of their size, they produce much less sperm than do parental males (Leach and Montgomerie 2000). The number of sperm cells is strongly correlated with parental success so this is less than ideal for cuckolders but some reproductive success is better than none.
Parental and Cuckolder Mating Success
First, not all parental males are created equal - that is the nature of sexual selection. The largest, oldest, most experienced males tend to secure the best places to spawn. This is true of nearly all fishes but particularly territorial species. Bluegills are colony nesters and the best locations are near the center of the colony as they are better protected from nest predators and from cuckolders. Approximately 75% of the offspring a parental male guards are his (on average) but this ranges from lows into the teens and twenties and to 100% of the offspring belonging to the guarding male. This 75% estimate is quite low compared to other sunfishes (Lepomis spp.) where parental males are more successful.
Figure is from Neff 2001, genetic paternity analysis and breeding success in Bluegill Sunfish (Lepomis macrochirus).
The figure above maps the nests as circles and the numbers in the nests are the estimate of the percentage of offspring belonging to the guarding parental male. Circles without numbers are nests which were abandoned. Rocks and a stick are also evident in the figure. Interestingly, some male Bluegill abandon colonies and nest far from colonies. These parental males are shown be the genetic parent of all of their nest's offspring almost all the time but the trade-off is that females are attracted to male nesting colonies and satellite nesting males run the risk of not encountering females. The strategy is most likely more successful when cuckolder densities are higher.
It is quite obvious that for this unique mating system to work, there needs to be a "critical mass" of parental males or the cuckolder life history does not work. The dynamics of when each group is the most successful is as interesting as the mating strategies. Cuckolders success is frequency dependent, meaning that success of a group is dependent upon its frequency in a population. Mart Gross write in a paper in the journal Philosophical Transactions of the Royal Society B,
As the density of cuckolders in colonies of parental males increases, the average mating success of cuckolders initially peaks but then declines. The cuckolder density at which their success peaks is determined by ecological characteristics of each colony.
And it would make sense that cuckolders - either female mimics or "sneakers" - are more likely to be successful when they are relatively rare. And as a group, as the population of cuckolders increases, the success of the group increases but at some point, too many cuckolders means that parental males become more aware of cuckolders and increase their defenses against them.
Effects of Harvest
When we hear about a "stunted" Bluegill population, what we are really experiencing many times is a population with an abundance of cuckolders. Adult parental males are particularly susceptible to harvest because they are of a desirable size and while they are guarding a nest, they are particularly vulnerable due to their aggressiveness in nest guarding. We have probably all had days where we could have caught an ungodly number of nest guarding males. Of course, this overharvest is particularly problematic in Bluegills where parental males are long-lived.
Much of the reduction in Bluegill size over time is likely due to overharvest. With parental males being so protective of their nests, they are particularly susceptible to overharvest while nesting. So much so that some lakes have regulations limiting May and June Bluegill harvest to 5 fish. For more on Wisconsin's Great Panfish Experiment, give that post a read.
I think that is enough for one post...hope you enjoyed and learned something about the sex lives of Bluegill.