[Amelia (OP)]

How does the frogs sperm get into the frogspawn to fertilise the eggs so they turn into tadpoles?

[SquarishTriangle]

This is a fairly complex question to answer because there are many, many species of frogs and for almost anything you try to say about them, there is always a species that will break the rules. But generally, frog eggs are fertilised externally (outside the body). This is different again to other groups of amphibians. The female frog lays a cluster of eggs into water or a damp substrate, and the male deposits his sperm on top of the eggs. Microcopically, the surface of each egg cell has a specific 'zone' at which a single sperm can enter, and other areas where it is impenetrable. When a healthy, 'fit' sperm contacts the egg, it fuses with the egg's membrane. Some rather complex molecular and chemical signals start to happen at this point, which I won't get into (mainly because I don't fully understand them myself). Eventually the nucleus, containing the all important DNA of the male frog, is able to move into the egg where it can combine with the female's DNA, already in the egg and begin the process of forming an embryo.

Overall, the process is not too dissimilar from what happens internally in other species, including mammals, or externally in fish. Arguably, land animals that use internal fertilisation are the weird ones...

Most of the current knowledge is based on a few 'model species' (the ones scientists often like to study in the lab). But the more we learn about frogs, the more differences we are bound to find...and the less we are able to make a single, generalising statement about "what frogs do" because there are probably thousands of versions of frog reproduction out there.

[chris]

Wonderful answer!

[atrox]

As many frogs are externally fertilized the fact that the sperm finds the egg can be especially remarkable. For the male, to ensure to fertilize as many eggs as possible, it is essential to release its sperm very close to the females cloaca when she is laying her eggs, as frog sperm is typically not very mobile. Those frogs one would typically observe in mating in large numbers in ponds or other water bodies usually produce a large amount of sperm to ensure at least some to find the eggs. The egg cell is releasing certain proteins to attract the sperm cells and is then binding the sperm cells with the help of another protein that is conained in the egg jelly. The sperm has then to find its way trough the jelly to the egg cell to be actually able to fertilize it. It has been shown, that sperm cells that are motile but swim slowly and steadily are more successfull in reaching the egg in the end than fast but unsteadily swimming sperm cells. Those often get trapped in the jelly and never make its way trough to the actual egg cell.
SquarishTriangle descriped pretty nicely how it goes on from there for the sperm cells, that actually made its way.

all the best

[chris]

As many frogs are externally fertilized the fact that the sperm finds the egg can be especially remarkable. For the male, to ensure to fertilize as many eggs as possible, it is essential to release its sperm very close to the females cloaca when she is laying her eggs, as frog sperm is typically not very mobile. Those frogs one would typically observe in mating in large numbers in ponds or other water bodies usually produce a large amount of sperm to ensure at least some to find the eggs. The egg cell is releasing certain proteins to attract the sperm cells and is then binding the sperm cells with the help of another protein that is conained in the egg jelly. The sperm has then to find its way trough the jelly to the egg cell to be actually able to fertilize it. It has been shown, that sperm cells that are motile but swim slowly and steadily are more successfull in reaching the egg in the end than fast but unsteadily swimming sperm cells. Those often get trapped in the jelly and never make its way trough to the actual egg cell.

Thanks for this very interesting extra detail.

Presumably the male frog also stays very close to the female to ensure that it's actually his sperm that does the fertilising? He wants the highest concentration of sperm around those eggs to be his sperm.

[Amelia (OP)]

@atrox and @SquarishTriangle
Thank you both for your answers!   

[atrox]

Yes, exactly.
However, it has been found that in cases where frogs reproduce in one big mating bout all at once (e.g. Rana species) a females eggs are not seldom (up to more then 50% of egg clutches) fertilized by more than one male. In those mating systems where they release eggs and sperm in the open water (also in many fish) it is very difficult for the male to ensure exclusive paternity, especially if they are explosive breeders, that spawn in maybe only a few hours. It has been argued due to the fact that the sperm concentration of the water is very high during those hours it is likely, that sperm of other males can reach also other eggs produced by females nearby. But with behaviours like the amplexus in many amphibians the male can significantly increase its chances of fertilizing most of the females eggs.

Ensuring paternity is often easier with internal fertilization.. but not as a general rule. In many internally fertilized species multiple paternities are still common due to females mating with multiple males, resulting in sperm competition, cryptic female choice or even males having evolveld certain active strategies like clearing out the spermatheca where the female stores sperm before ejaculating or sealing up the females genitalia to prevent other males from mating with her or simply guarding her for the same reason. Reproduction is a war! :-)

[chris]

One presumes that, in cases where the competition is this intense, the frogs compensate with prodigious sperm production to out-compete the other males? Or perhaps they go for super-fast sperm? Or both?

I interviewed Terje Laskemoen, from the Natural History Museum, University of Oslo, a few years ago on the equivalent sperm competition in songbirds. His insights are very interesting.

[Amelia (OP)]

So different types of frogs could fertilise the eggs so that is why they produce so many so that won't happen?

[SquarishTriangle]

Meanwhile, it is perhaps in the female's best interests for her eggs to NOT all be fertilised by the one male, in case that male turns out to be a bit of a dud. Or more specifically, so that she has genetically-diverse offspring and improves the chances that some of them possess traits that facilitate survival against a range of different future challenges.

[atrox]

Chris, I listened to the interview. Quite interesting!
Appearantly it seems to be different at least in some frogs. A study found, that the slower sperm are more likely to successfully fertilize the egg in one species ("Sperm competitiveness in frogs: slow and steady wins the race", Dziminsk et al. 2009). My first thought was, that it might have to do with the external fertilization tactic in the frogs studied. However, in fish they also found a correlation of higher fertilization success with higer sperm speed. So its a bit weird that it seems to be so different in frogs. In general frog sperm is much slower than fish sperm (25 μm/s compared to 140 μm/s) in species with external fertilization, but on the other hand it survives much longer after ejaculation (up to serveral hours compared to some seconds or at most minutes). One review ("Sperm motility of externally fertilizing fish and amphibians", Browne et al. 2015) suggests that there is a trade off between velocity and motility of sperm. So slower sperm is able to be motile for much longer. Regarding the eggs in anurans, that are surrounded by some (2-5) quite thick layers of jelly that the sperm cells need to pass before beeing able to contact the egg cell it might be advantageous to be motile for as long as possible to be able to do so (I think it was in the first reference where I read that higher motility increases fertilization success). However, this is just wilde speculation on my side, I am not a sperm expert.

@Amelia , not different types of frogs (as in different species) but different individuals, yes. Actually, this is an alternative mating strategy observed in some species. It has been observed that males of the Common frog might hop on freshly produced clutches and release sperm on them, securing them some fertilisation success even without having been able to outcompete other males dircetly (e.g. "Post-mating clutch piracy in an amphibian", Vieites et al., 2004). However, in producing more sperm than others you would increase your chances to fertilize most of the eggs.

[chris]

Meanwhile, it is perhaps in the female's best interests for her eggs to NOT all be fertilised by the one male, in case that male turns out to be a bit of a dud. Or more specifically, so that she has genetically-diverse offspring and improves the chances that some of them possess traits that facilitate survival against a range of different future challenges.

But surely the biggest, burliest male is the "best" parent because he's strong enough to have fought off the competition and protected his female during the mating period?

[SquarishTriangle]

But only the strongest under the current (and past) circumstances. If the future selection pressures change, say if water temperature increases or decreases by X degrees, then it would be beneficial to have a variety of offspring that could cope with slightly different temperatures ranges to ensure that some survive.

[atrox]

But surely the biggest, burliest male is the "best" parent because he's strong enough to have fought off the competition and protected his female during the mating period?

Additionally to what SquarishTriangle said: it is not always the strongest and best suited male that win the price. In some species smaller and less competitive males might go for another strategy. Instead of directly competing with bigger (e.g. usually louder if display is based on vocalization) male they wait nearby in silence until a female got attracted by the lovely sound the big male produces. The smaller satelite male will then try to intercept the female and trick her to get into the amplexus. Once in this position the smaller male has an advantage over otherwise maybe more competitive males.

[chris]

Does that benefit the frog though? I wrote a piece a few years ago about squid that do a similar thing. Small males rearrange their tentacles in a more feminine manner and masquerade as female to slink past guarding larger males and snatch a quick mating with a female...

[atrox]

Which frog do you mean now? The female? Or the species?

In general, a quick reminder to be able to understand the "war called reproduction" (as always in biology, there is of course examples that prove me wrong here,certain restraints and environmental conditions transform this general pattern a lot.. but as a general trend it holds up.. ):
For a females reproduction the limiting factors are basically her own ressources and time. Egg cells are costly to produce and their production and "maturing" (in a very broad sense, also raising offspring in animals with extented care) often take some time in many species (the latter especially in mammals). For a female it is therefore important to choose the right male to have offspring that is best adapted to the environment she lives in, as she does not have many shots.
Male sperm cells are much less costly and can be produced in large amounts. For a male it would be best to try to mate with as many females as possible to spread its genes in a maximal efficient way. It does not even have to be especially choosy, when you are able to mate just with every female you meet. But males are usually limited by the access to females and that is were conflict arises and different mating strategies evolve.

In frogs you often find the common strategy of trying to monopolize a female. Male frogs will try to do that by getting to an amplexus with her and thus being able to release sperm very close to the freshly produced eggs. If a male goes for this strategy it is all or nothing. Either you are strong, loud, "good" enough to outcompete other strong males and convince the female to mate with you or you are not. In the later case your fertiliziation success is zero. Thus, this strategy can be quite risky if females are scarce and many males try to monopolize them. But if there are many "weaker" males, you have a high chance of monopolizing.

Some males, the so-called sneaker/satellite males, try to avoid direct competition with more competitive males where they very likely will not win. Instead they might be able to have fertilization success when they can intercept a female. They are "making the best of a bad situation". If the competition in monopolizing males is high, the chances for sneaker males might even by higher than for monopolizing males to get a female. It thus can be a evolutionary stable strategy, but usually only up to a certain percentage of sneaker males in a population that is much lower than the percentage of monopolizing males.
Additionally to intercepting females, in some species (I think Rana temporaria) it has been shown, that some males even approach freshly produces egg clutches and "mate" with them (they grab them and release sperm over them). Of course the chance is high, that most of the eggs are already fertilized. But they can still get some fertilization success.

For the female, the latter is an advantage, even if she did not choose this male, as it lowers her amount of non-ferlizied eggs und thus will have more offspring.
How much she will benefit or not from a sneaker male deceiving her strongly depends on current situation in the population. If the competition beteween males in this population is already quite high, her sons might even benefit from inheriting their fathers mating strategies (and by that also the mother has higher fitness). But at some point the whole system will tip again and producing monopolizing males will give her sons higher chances of reproducing successfully.

And there we are back to what Squarish Triangle wrote: having mixed offspring from different fathers might be an advantage for the female in a system were both strategies differ in how successfull they are. And that is just looking at it from this one side of mating strategies. Of course there can also be other changes in the environment, that might give an advantage to offspring with certain traits that was not as well fitted to the environment before. So if there is not one "superadapted" organism but situations can change it might be beneficial for the female to have offspring from several males. That is the reason why there has not been a selection against this multiple fatherhood in a females clutch.

I think I approached actually two questions there: Why is it ok for the female to mate with a sneaker male? and Why is multiple paternity not a problem when a female should aim for the best suited father for her offspring?

I hope it is anyways understandable.