Watch out for those sneaky males!

A courting midshipman fish with his mate and a small sneaker male
Picture from the website of Dr. Andrew Bass at Cornell University

As I stated in my last post many animal species are socially monogamous but genetic analyses show that some level "cheating" occurs resulting some percentage of the offspring having a different genetic father.  The vast majority of fish species have external fertilization so if a male can get close enough he can release sperm and fertilize the eggs. Some, usually young or sub-dominant, males called sneaker males use such opportunistic fertilization opportunities as their primary mating strategy!  These males try to look as unobtrusive and non-threatening as possible and will often take on female coloration so that the dominant male partner won't notice him and try to chase him away.  Dr. Andrew Bass at Cornell University has done extensive research on the midshipman fish Porichthys notatus.  In this species males develop into one of two types: one type of male builds a nice cozy nest and lures a female in with his lovely (to female midshipman fish ears) loud humming vocalization.  This guy is really works hard to woo the girl!  The sneaker male doesn't bother with such niceties, he just rushes into the nest of a pair and attempts to fertilize the eggs.  However, this strategy does come with some costs, sneaker males must release more sperm to get a smaller percentage of eggs fertilized successfully and thus have much larger testes than courting males.  In this case a subset of males have found that forgoing courtship and "stealing" fertilizations is a viable alternative mating strategy.

Is monogamy for the fishes?

Now I have to start this blog post by saying that I don't believe in using observations of animal behavior to condone human mis-behavior.  Part of what makes us human is the ability of an individual to consciously decide how he or she will act. I do think it's interesting however to contemplate how and why evolutionary pressures influence human behavior and societal norms.  One question many people have contemplated is "Are humans meant to be monogamous?"

In biology monogamy is defined as forming a pair bond with only one mate for the length of a breeding season.  Throughout the animal kingdom monogamy is rare and there is a differentiation to be made between "social monogamy" and "genetic monogamy."  Many species of birds have been observed to be socially monogamous.  They build a nest together, incubate their eggs together and feed their chicks together.  Therefore when scientists started using genetics to determine whether all of the chick were the offspring of the two parents they were surprised to find that in most species of socially monogamous birds over 10-25% of the chicks do not belong to the father!  Why would this be?  One explanation is that females don't want to put all of their eggs in one basket.  Females are choosy and look for the best mate based on characteristics such as bright coloration or a complex vocalization.  But what if the female is wrong and the mate she has chosen has a genetic mutation that will cause her offspring to live short lives or have trouble finding a mate?  For this reason it is advisable for a female bird to hedge her bets and mate with more than one male, just in case.  Biologist call this "cuckoldry".  It's important that the rate of cuckoldry not get too high or else it will no longer be to the advantage of the males to stick around to help feed and care for the young.

Parrotfish are broadcast spawners

What are the factors that lead to a monogamous mating system?  The intensity of parental care needed to raise the young is one important factor. This point is made clear by a comparison of different marine reef-fish species.  At one extreme are broadcast spawners who release their eggs and sperm into the water column where fertilization then occurs, sometimes many individuals spawn at the same time in a massive spawning event.  This is a highly promiscuous mating system in which there is no sexual selection and no paternal care. Most reef-fish species employ this strategy.   These fish species are basically playing a numbers game; they will release millions of eggs and/or (many species are hermaphrodites or change sex) sperm over the course of a lifetime; a percentage of these will be fertilized and become planktonic larvae.  Those larvae that survive will then eventually return to a reef to settle, mature and reproduce.  Along each step of this process most of the offspring will be lost by being eaten, starving, meeting up with adverse environmental conditions or failing to find a suitable habitat.

                                            

Spiny damselfish with young

Picture by John E. Randall from Fishbase

Damselfishes take a less cavalier attitude towards the care of their eggs.  Most damselfishes lay eggs on a hard surface on the ocean floor and defend those eggs from predators then once the fry hatch they leave the reef and planktonic larvae before returning to the reef to settle. As part of my PhD I studied a unique species of damselfish called Acanthochromis polyacanthus or the spiny damselfish.  What makes this fish so special is that unlike the vast majority of marine species the larvae of spiny damselfish fry do not disperse from their home reef.  Instead the males and females form a monogamous pair bond.  The female lays large eggs and when the fry hatch they stay in a group close to the parents.  This is a perilous situation because there are many predators on the reef who will eat the young fish.  In order for the young fish to survive both the male and the female must vigorously defend their brood by aggressively chasing away would be predators.  The brothers and sisters stay together in a tight aggregation until they are old enough to disperse away from the parents.  This is a labor intensive breeding strategy that involves the full time occupation of both parents but it means that the larvae are sure to wind up in a suitable environment.  These fish have fewer, more developed young that both parents spend a lot of effort raising so that a larger percentage will grow up to adulthood.  In order to see whether all of the offspring are brothers and a sisters we collected broods of spiny damselfish fry and looked at several genetic markers.  In every group that we collected we found no evidence that there were more than 2 parents for each brood.  Thus this is a rare case of both social and genetic monogamy.  When raising the young takes the intense effort of both parents monogamy may be more likely because it is in the best interest of both parents to ensure that all of the offspring they are defending are actually theirs. Thus sound like any other species we know?

Miller-Sims et al (2008) Molecular Ecology 17:5036-5048

The Tyranny of the Mean

In the real world variability is a fact of life.  Every individual is unique. Yet when doing biological research we often try to minimize these differences in order to compare one group of individuals to another. For example if you wanted to know whether a particular drug is effective in treating a disease you would compare a group of individuals who received a medical intervention to another group of individuals who received a placebo. However every individual's genetics, environment, and disease progression is likely to be different resulting natural variability in the way that they respond to the treatment. In order to have confidence in the results of the study you must control for as many factors as you possibly can, randomly assign individuals to groups and have as many individuals in your study as possible.  When reporting results we often take the average (mean) of measurements for each group. Statistics tell us what the probability is that random chance could account for the differences observed between the two groups.  Large sample sizes are the best way to increase your statistical power (chance of seeing an effect) however we must remember that these results come with a caveat.  The mean results seen will not be true for every individual.

Picture taken by Marie Suver

 A talk I recently saw by Eve Marder from Brandeis University highlighted how important individual variability can be for biological systems.  The Marder lab studies the Somatogastric Ganglia (STG) of the crab.  The STG is a bundle of neurons that control the muscles of the gut that grind food. The STG is what is called a central pattern generator, neurons in a central pattern generator produce rhythmic patterns of firing (action potentials) without input from the sensory system in order to drive repetitive motor patterns. What's great about this preparation is that it can be removed from the animal and the neurons still maintain their characteristic pattern of activity.   In my previous post I talked about the fact that action potentials are generated by ions flowing across the cell membrane through ion channels.  The STG consists of a small number of large, easily identified neurons (see photo) making it relatively easy to stick an electrode inside of the cell to measure changes in the voltage that occur when ions flow into or out of the cell.  This is called ion conductance.  Each type of neuron has a characteristic pattern of firing action potentials but the way that this pattern is generated is different from individual to individual.  What is  most surprising was that in some cases the mean measurement of a particular type of ion conductance across a population actually occurred in very few of the individuals tested.  Instead there were several different ways ionic conductances could be balanced to generate the rhythmic spiking activity of the neurons and the measurements tended to cluster around these values.  This result is an important reminder that we cannot always take a mean value at face value. In a previous post I talked about how frustrating it is when scientific studies are not as clear-cut as we had hoped.  Misinterpretation of variation is one factor that can contribute to an incomplete understanding of our results.  One thing is for sure: we need more data!

Braaaiins!!!!! Part II: It's electric boogie woogie woogie

So in part I I told you that ion channels on the cell membrane control which electrically charged particles (ions) can enter or leave the cell. Some ions such as sodium (Na2+) or potassium (K+) are positively charged while others such as chloride (Cl-) are negatively charged. As some of you may remember ions flow from areas of high concentration to areas with low concentration. Ion pumps use energy to maintain the cell in an unbalanced state; at rest the cell maintains a negative charge inside the cell of about -70 mV and the concentration of sodium ions is much higher outside the cell than inside.Communication between cells occurs at the synapse.

The cell that is doing the talking releases neurotransmitter which binds onto the receptors of ion channels of the cell that is receiving the message. Depending on the type of neurotransmitter that is released and the type of ion channel that binds the receptor this acts to make the inside of the cell more positive (depolarized) or more negative (hyperpolarized). When the membrane reaches a certain level of depolarization this causes voltage gated sodium channels to open and sodium rushes into the cell resulting in more sodium channels opening which results in the generation of an electrical signal called an action potential. Sodium channels close and other channels open to restore the inside of the cell to its original resting state. The firing of action potentials is an all or none phenomenon. Action potentials propagate to synapses where they cause voltage gated calcium to open to result in the release of neurotransmitter which then influences the electrical charge on the inside of the next cell.  The ability of the brain to process sensory information and generate motor output is dependent on the physical connections between neurons (ie who is talking to who). Memories are formed through the strengthening of existing connections or the formation of new ones. The brain is unendingly complex but its function comes down to the properties of ion channels which control the flow on ions across a membrane and the pattern of connectivity between neurons. New techniques have given neuroscientists more powerful tools to visualize and record from many neurons at the same time in order to get a better understanding of how brain regions are connected and develop models of information processing.

My favorite part of this story is that a pair of scientists Hodgkin and Huxley worked out how the flow of ions generates an action potential by studying the squid giant axon (no not the giant squid axon). This is an axon in the squid that rapidly generates an escape reflex and its large size allowed these scientist to stick electrodes in it an measure currents across the membrane.

Picture by Tom Kleindinst