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This lecture is to introduce a series of concepts that relate tohow the animals came to be, how they find mates, the number ofmates that they have, how they produce their young and so forth.
The way I look at it, there are 4 major issues under this topicof Modes of Reproduction. The first one is courtship. Theway in which the animals identify each other as mates. The secondmajor issue is fertilization. Where and how the sperm andegg meet one another. The third one is embryonic development.Once the egg has been fertilized, where does it develop? Is itinside the female or is it out in the environment? And then thefourth one which I won't have very much time to talk about is parental-care.And that really refers to care for the young after they are born.So I'm going to go back and go through each of these 4 topics andexpand upon them in a little more detail here.
MODES OF REPRODUCTION:
COURTSHIP: There are several topics under this. One of these isthe behavior that the animals use to identify a prospective mate.The basic function of courtship is to determine that anotherindividual animal is a member of the same species and a member ofthe opposite sex, that's a necessary function.
Now human beings obviously go through all the same processes aswell but we don't think very much about the necessity foridentifying another individual as a member of the same species.It is in fact something that's important for animals becausethere may be a number of different and very similar lookinganimals living in the same habitat with fairly similar lifestylesand it's important that there be some behavior that is specificto an individual species that allows other members of thatspecies to recognize the individual as a member of the properspecies. Usually that behavior is also gender specific so thatmale and female can tell not only is that a member of the samespecies but it's a member of the opposite sex. That's the basicfunction.
SENSORY MODALITIES FOR COURTSHIP: It involves the exchange ofsome kinds of signals. Now those signals may involve any one ofthe 3 major vertebrate sensory modalities. In other words, theycan involve vision. They might involve various kinds ofdisplays. Many times when we see birds, we have drawn the factthat they have very brightly colored patches of feathers ondifferent parts of their body. Those brightly colored patches offeathers are usually involved in something to do with some kindof display so if a bird has a brightly colored patch of feathersin the wings, it might have a very precise stereotyped behaviorwhere it draws attention to that patch of feathers in its wings.There's a bird called the Green Winged Teal for example, it has alittle square green patch in it's wing and the bird, when it'scourting another Green Winged Teal, will pull that wing aroundand it'll spread the feathers out showing off it's bright greenpatch and then it points as it with it's beak and says,"see, look at me, I'm a Green Winged Teal".
There's another bird called the Blue Footed Booby and it has feetthat are a startling baby blue color. The courtship of that birdinvolves standing there in front of a mate and lifting it's feetup in the air, and with it's big long beak points at it's feet.The male and the female both do it and that's exchanging signalsvisually.
Signals can also be exchanged acoustically via the sensorymodality of hearing and so various kinds of sounds will beproduced. Frogs for example produce sounds and some bird song isalso involved in courtship. Again, these are going to be veryprecise, very virtually identical from one individual to the nextbecause of the fact that you have to be sure that you're givingexactly the right signal for that prospective mate so thatprospective mate understands the gender membership.
The third sensory modality that is used in signaling forcourtship is of course the sense of smell or olfaction.And olfactory signals are transmitted in the form of particularchemicals that are produced by one member of the species andintended to be received by another member of the species. And theterm of the use of those chemicals are pheromones. Pheromone is achemical produced by one member of the species which communicatesa specific information to another member of the same species. Inthe case of salamanders and in many species of mammals forexample, the primary signaling modality used in courtship ispheromones. The smells that are produced that are received by thenose, the olfactory epithelia of the other individual are used todetermine species membership. So that's the first major headingunder Courtship.
BREEDING SYSTEMS: The second major topic under Courtship involveshow many individuals are involved in this breeding system andwhat we can identify here is what we would call the breedingsex ratio. This is the ratio of the number of malesto the number of females that are involved in what we call thesex/pair bond. The pair bond is a special relationship between 2individuals of the same species. Two individuals of the oppositesex in the same species. It's a relationship in which those 2individuals recognize each other as being individual animals andthe function of the pair bond is to maintain that relationshipfor the period of time that is necessary for them to successfullyreproduce. And the breeding sex ratio is the number of males tothe number of females that are involved in these pair bonds. Therelationship that we're most familiar with in the human speciesgoes by or the name of the breeding system that we're familiarwith for human beings is known as monogamy. At least in theoryanyway.
Monogamy is a situation in which the breeding sex rationis one to one (1:1). That is, one male has a pair bond with onlyone female at any given time. The duration of the pair bond is aseparate issue but the idea is at any given moment in time, howmany males and how many females are involved in the pair bond?
-gamy refers to "mates" and mono means "one".So the translation of the word monogamy is 1 mate. And thebreeding sex ratio is 1:1. That is a breeding sex ratio which isvery unusual. We look at all the species of vertebrate animalsand monogamy is a very unusual breeding sex ratio. It probablyonly involves under specialized breeding conditions where thecontribution of the male is very important to the successfulrearing of the young. That may mean that the male needs toprovide food for the young, and it may mean that the male needsto provide education for the young. It's a situation in which themale has a very important contribution to the mate to thesuccessful rearing of the young. in ecological conditions. It isfound in some of the more advanced species of birds. In the mostadvanced border/group of birds called the Pacerines (I'll betelling you about them towards the end of the quarter) which arethe song birds and sparrows, it's not uncommon for them toperform monogamy. But if you look at all the vertebrates, it'svery uncommon.
There are 2 other forms of breeding sex ratios where there reallyis a pair bond. One of which is collectively referred to as polygamy.Translation of polygamy is "many mates". We can justuse the simple N to stand for an integer larger than one and thisis a situation in which the breeding sex ratio is N:N. That isthere can be more than one female or more than one male and infact there are breeding systems in which both of those 2conditions occur.
There is the condition which is the one in which there is asingle male with 2 or more females and that's called polygyny.Poly meaning "many" and -gyny referring to"female". So the translation of polygyny is "manyfemales". We can identify this breeding sex ratio as beingl:N. Single male with 2 or more females. And this is probably themost common breeding system that we see among the vertebrateanimals. And the reason for that is that in many species, thefemales are capable of performing all the tasks involved inrearing the young all by themselves. So the male can in fact havemore than one female mate at a given time.
Now this condition should not be confused with one in which thereis no definable breeding sex ratio where there is no pair bondand that's a situation called promiscuity. Promiscuity isa breeding system which is also very common among vertebrateanimals in which the male and female go through courtship and donot establish a pair bond. They do not establish any kind ofenduring relationship between the individuals, the fertilizationtakes place and they go their separate ways never to see eachother again. There's no enduring pair bond at all. That's notwhat we're talking about when we're talking about polygyny. Inpolygyny, the male has an enduring relationship with 2 or morefemales.
Now polygyny is a system which is beneficial to both the male andthe female. In fact all these systems have to be selected wherethey are programmed into the genes of the animals. They aresubject to natural selection and they have to be advantageous tothe individuals that are involved in it. It has to work. So forus to understand why polygyny would evolve we have to understandwhy it would be beneficial for a female to form a pair bond witha male who already has another mate. It's the female's choice.She has the choice either to form a pair bond with this male whoalready has a mate and she knows he already has a mate or she hasthe other choice which is to go find an unmated male. Forpolygyny to evolve, it has to be to her advantage tochoose the male who already has another mate.
Now, it is extremely common among vertebrate animals and a coupleof examples will make it clear why that would be the case. Oneexample would be among large marine mammals like seals and sealions where the young must be born on land. The babies have tostay on the beach for the first weeks or months of their livesuntil they get enough fat on to where they are well insulated togo into the ocean. But the isolated beaches that are free frompredators are very limited. So a large male seal or sea lion canhaul out on the beach and exclude other males from his littlestretch of the beach which might be 20/30 yards of beach. Becausehe's bigger and stronger, he gets his territory of his choosing.And in these species where we have polygyny, frequently the malesare significantly larger than the females and the males must beable to stake out that stretch of beach and kick all the othermales out. Now the female doesn't have much choice. She has toget onto the beach in order to have her young and the wayphysiology of the animals works is shortly after she has heryoung, her estrous cycle begins and she's ready to breed and shewill breed with the male whose beach she's been using. It wouldbe advantageous for the female to breed with that beach malerather than go find some male who don't have a section of beachbecause the beach male has proven that he's got the genes to be abig strong dominant male that can go down through to theoffspring. If she mates with him, her male offspring will getthose genes and they too will then get to be big strong beachdominating males and will impregnate the females of the nextgeneration. So that's one example.
Another one is where they have a better territory. There are manyspecies of birds for example where polygyny exists where the maledefends the nesting territory and he allows only his mates tobuild their nests on that territory and maybe this is a situationwhere the nesting sites are restricted or maybe he defends boththe nesting and the feeding territory. His territory has lots offood on it and so the female can take this male who already hasone female feeding on his territory but he's got such a lushpiece of habitat that she too can get a good habitat and getenough food for herself and her young. She'll be in worse shapeif she goes to take the wimpy guy out on the edge who doesn'thave enough food for anybody. So the males fight amongthemselves, many times not physically but through their size andthe vigor of their displays, and the dominant males get the bestterritories and the females are better off because they get tofeed on that guy's territory.
So polygyny exists where it is advantageous to the female whereshe benefits by making that choice.
Polygamy in a scientific sense which includes both polygyny(where there's a single male with more than one female) and theopposite condition called polyandry (where there areseveral males with only a single female). So polyandry andpolygyny are 2 different forms of polygamy. And they each havetheir own special definitions.
Now polyandry is much, much less common. It's probably the mostrare of all of the breeding systems that we can identify. And thereason for that is that usually the female invests a lot more inreproduction than the male does and it's an unusual situationwhere it is advantageous to several males to share a singlefemale. One example though which is very interesting is in wildhunting dogs. Wild dog packs have a single dominant female whoproduces all the young so several of the males will form pairbonds with her. They will mate with her when she is in heat andthey will all contribute to the group hunting by the pack toprovide food for the young that are born and some of the pups maybe theirs and some of the pups may not be theirs but all of theprogeny produced from that mating are supported by the grouphunting efforts so that's a situation to give an example ofpolyandry.
Those are the 2 major headings under courtship. What kinds ofbehaviors and signals are used and what sort of breeding systemsdo we have? When we have no identifiable pair bonds(promiscuity), is it a monogamous one (1:1) or is it a polygamousone? Under the pair bond business of course is also somethingwith the duration of the pair bond. In other words monogamy isnot necessarily for life. In some species of birds for examplethere might be a monogamous pair bond that lasts for 2 months andthen the pair splits up and they form another monogamous pairbond with another individual of the opposite sex for anothercouple of months. During the same summer there might be 2monogamous relationships one right after the other. Or it canlast for several years. Or it can last for the life of theindividual. Again, that's a fairly uncommon relationship becomingmore uncommon among human beings these days. But nevertheless, itis one that exists in other species. For example in swans, thereare studies of individual swans that have been followed over thecourse of several decades in England where the same male andfemale swans have flown off to the mating grounds and came backwith babies every year for over 20 years in a row and those bondsonly break up when one of the 2 doesn't make it back fromthe migration.
FERTILIZATION: The fusion of the male and female gamete. A coupleof things that we are interested in here is where is it that theeggs and the sperm meet? Is the fertilization external? The frameof reference here is the females' reproductive system. Isfertilization external to the female's reproductive system or isit internal to the female's reproductive system? Is it inside
the female's reproductive system?
In primitive conditions, that is the condition that presumablyexisted in the very first vertebrates and which continues toexist in many primitive species is external fertilization.That is it takes place in the environment and this is found onlyin aquatic species (animals that live in either the ocean orfresh water). External fertilization does not occur interrestrial vertebrates (ones that live on land). What happenshere is what you see in the majority of the species of fish. Themale and the female fish will go through some kind of courtship.They have brightly colored patches of skin that they use forsignaling, they form a pair bond, they may make a nest of sometype or they may not, and then the female sheds her eggs into thewater, the male swims right next to her and he sheds his sperminto the water. The sperm swims through the water to the eggs,penetrates the eggs and fertilizes them. And so that's theprimitive condition.
Internal fertilization takes place within thefemale's reproductive tract and that means that the sperm must betransferred into the female's reproductive tract in some way. Nowusually internal fertilization is achieved because the male hassome kind of a specialized organ that he inserts into thefemale's reproductive tract and transmits the sperm in that way.There are, among the vertebrate animals, a variety of differentways in which that is achieved.
The general terms of that kind of an organ is an intromittentorgan. So an intermittent organ is an appendage on themale that is used for transferring sperm into the female. Inmammals we call that a penis. There are a very wide variety ofdifferent kinds of intromittent organs. Sharks for example havestructures called claspers that are just short finger-likeextensions of the pelvic fins. Many reptiles have pairedintromittent organs that are called hemi-penis. These reptilesactually have 2 completely functional penises.
It is possible for internal fertilization to be achieved withoutan intromittent organ. That is where the male does not have aspecial appendage on his body for doing that. One example of thisis found in many species of salamanders that have internalfertilization but the male produces a little package of spermthat is on a little gelatinous pedestal and he deposits thatthing on the bottom of the pond. Then a female swims over it andthe sperm get transferred into her cloaca and that little packageof sperm is called a spermatophore.
Another means of achieving internal fertilization without anyintromittent organs is seen in most species of birds. By the tailis the cloaca which is the opening or bent of the bird - commonopening of the digestive tract of the renal and the reproductivetract. The male presses his cloaca against the female's cloacaand the sperm get shot straight across from his reproductivetract into her reproductive tract and that mechanism is called acloacal kiss.
So there are a couple of examples of ways that internalfertilization is achieved using and not using intermittentorgans. Now, the primitive condition is external fertilizationbut internal fertilization has evolved in virtually every groupof vertebrate animals. There are thousand of species of fish andsharks, hundreds of species of reptiles for example that haveinternal fertilization as well. And many of the amphibian,salamanders for example.
EMBRYONIC DEVELOPMENT: Once again there are several differentthings that we want to understand if we're going to understandembryonic development and the first one is where does thisdevelopment occur? Again, embryonic development can take placeeither within the female's reproductive tract or outside of thefemale's reproductive tract.
If it's going to take place outside of the female's reproductivetract we don't use the term external development because thatwould be confusing, so it's called oviparous. In other words if aspecies has external embryonic development then we could say thatspecies is oviparous or we would say that species exhibitsoviparity. Ovi refers to "egg" and parous or parityrefers to "giving birth or bearing young". So what thisterm refers to is that at the moment that this female becomesphysically separated from her prospective young, what we seecoming out of her reproductive tract is an egg so this is anegg-bearing species or an oviparous species. Now that egg may ormay not have been fertilized. You could have a species which hasexternal fertilization and the female sheds her eggs (she isbeing oviparous). Or it could have a species like birds in whichfertilization is internal but the female lays an egg and you haveinternal fertilization. So egg laying does not tell us whetherfertilization is internal or external. You have to actually seethe actual fertilization to know that.
When you have an oviparous species, one that lays eggs, thenthere are a couple of other things that we might want to knowabout. For example we might want to know are the eggs laid in anest or not? The primitive condition among fish which areoviparous with external fertilization is that there is no nest.The eggs may float to the surface, they may sink to the bottom,they may be kind of sticky so that they get stuck on plants andthings like that but there are thousands of species of fish inwhich a nest is prepared. A nest is really just any kind ofspecially prepared area in the environment. In other words, it'snot just a bird's nest where the bird brings lots of sticks andneat soft things together but fish for example will collect upthe rocks and make a little rocky nest that they then deposittheir eggs in.
The fact that there is a nest does not necessarily tell uswhether there is any care for those eggs after they are laid. Butsome fish do. You can have what we would call brooding of eggs orwe can have what we would call incubation of eggs. In vertebratezoology, brooding refers to caring for the eggs without warmingthem. So fish that don't have any need or even any ability towarm the eggs can brood their eggs. They might do nothing morethan sort of swim in front of the eggs and direct them to wellventilated streams of water to make sure the water around theeggs doesn't have too low of an oxygen content. Incubation isfound almost exclusively in birds where the eggs are warmed sothis refers to caring for the eggs and keeping them warm. Wherebrooding just means just caring for the eggs and does not includekeeping them warm.
Obviously embryonic development can also occur inside the femaleand when that's what's going on, there are really 2 terms that wecan use for that. The simplest one to understand is the one thatwe're most familiar with and that is referred to as viviparous.The species exhibit viviparity and the translation is "livebearing". What it means is that the moment the femalebecomes separated from her offspring, what comes out of her isnot an egg, it's some kind of a little live animal. It looks likea little live animal. That's viviparous.
The other condition at the moment of birth looks exactly the samebut there is an important difference and the other term isovoviviparous. It translates as "egg live bearing".
The difference between the 2, between viviparous andovoviviparous, once again is not detectable at the moment ofbirth. The difference is prior to birth, if there was thetransfer of nutrients (proteins, carbohydrates and fats) from thefemale to the embryo, then that's a viviparous condition. Andthere will be no transfer of nutrients in the case of theovoviviparous species. So human beings and all mammals areexamples of viviparous species. In viviparous species, nutrientsare transferred from the female to the young directly from oneblood stream to the other blood stream by means of a specialfetal organ called the placenta. So one example of a mechanism bywhich nutrient transfer can take place is a placenta.
Another mechanism for the transfer of nutrients that we see inmany viviparous species is referred to as uterine milk. Milk is aspecial nutrient solution produced by female mammals by theirmammary glands and it's involved parental care after the birth.Uterine milk is just nutrients that are produced by the cellsthat line the uterus. The cells actually rupture and dump theirnutrients into the fluid in the uterus and it causes the uterinefluid to look sort of cloudy gray. What happens then is that theembryo drinks the uterine fluid and the fluid goes down it'sthroat into the stomach, into the intestine and those nutrientsthat were shed into the uterine fluid are then absorbed acrossthe digestive tract of the developing young and those nutrientsthen fuel further development of the embryo. So nutrient transferis found in viviparous and it can take place either directly fromone blood stream to the other by means of placenta or it canoccur by means of the production of uterine milk by the cellslining the uterus.
An ovoviviparous, does not have an entry and transfer. Obviouslythe embryo is going to have to have nutrients with which to fuelits development, it's going to have to grow, it's going to haveto have some metabolism in order to made ATP to fuel that growthand so forth and those nutrients that are used for thedevelopment of an ovoviviparous species were present in the eggwhen it was shed from the ovary. Present in the form of what wecall yolk. So nutrient transfer as defined for a viviparousspecies means nutrient transferred after ovulation, after the eggis shed from the ovary. In ovoviviparous species, they willproduce an egg that has enough yolk in it for all of embryonicdevelopment to take place so there is no need for any nutrienttransfer after ovulation. Obviously those nutrients didultimately come from the mother but the transfer occurred priorto ovulation/fertilization.
Notice that in both the viviparous and ovoviviparous speciesthere will be the necessity for providing the young with oxygenand the elimination of carbon dioxide and the elimination ofmetabolic waste products that are produced by the embryo so therewill be transfer of some things between the female and thedeveloping embryo in both of these means but the distinctionbetween the 2 is the transfer of nutrients, not the transfer ofrespiratory gases which has to occur in both types.
Examples: Ovoviviparous species - there are a number of differentspecies of fish, salamanders, a group of amphibians that are longworm like relatives of salamanders and frogs that exhibitovoviviparity, many species of reptiles.
Since the oviparous condition is the primitive one, it's the onethat probably exists in the beginning of the fish and amphibians,and first reptiles. All of the them have species that areviviparous, this is probably an evolutionary intermediatecondition. Oviparous species - when the egg is laid it has tohave enough nutrients in it to fuel development. So if the femalekeeps that egg inside of her reproductive tract then she'sovoviviparous. The egg has enough nutrients to fuel developmentbut the egg stays inside the female and she carries it around andprotects it from predators and doesn't need a nest. But it hasenough nutrients in it so that it can develop in there. Not alleggs have shells so it doesn't necessarily mean the embryohatches inside the uterus but it will complete it's developmentinside of the female. What comes out is a little baby animal.
Oviparous (egg laying) species could have either internal orexternal fertilization. Fish and birds are both oviparous. Mostfish have external fertilization and all birds have internalfertilization. The same thing is not true here. This is onlygoing to result in internal fertilization. Both viviparity andovoviviparity are found only in species that have internalfertilization. You don't have a situation where the female laysthe egg, gets fertilized and then she sucks it back up insideher.
Condition of the young at birth: Here we're talking aboutviviparous or ovoviviparous species or oviparous species. Theconditions that we're talking about are exemplified by birth thatyou probably are familiar with. The terms are precocial andaltricial.
Precocial young are like baby chicks and ducks. They are veryindependent. They have well developed locomotory systems, theycan run around, and they probably have good insulation (inbirds). They are pretty independent. Reptiles at birth areprecocial. Baby horses at birth are precocial. They can get upand walk within hours of birth. It is the more primitive systemof the 2.
The other condition is altricial where the young is fairlyhelpless at birth. Human babies, baby sparrows, swallows,mockingbirds are some examples. Their sensory systems may not befunctioning very well and their eyes may not be open. They maynot have much in the way of insulation and they may not be ableto get around very well like kittens and baby mice. So thealtricial condition is one in which the young is born basicallyhelpless.
Both of these are examples of direct development. Situation inwhich the young look basically like the adults when they areborn. The other condition which is much less common, very rareamong vertebrate animals, is one that is described by the wordmetamorphosis. A frog is an example of a kind of vertebrateanimal that exhibits metamorphosis (meaning changed shape).
PARENTAL CARE: Some species have no parental care, babies are ontheir own the moment they hatch out of the egg. Some viviparousspecies have no parental care such as snakes which are bornthrough viviparity. You can have bringing food to the young,showing the young what food looks like, protecting them frompredators, warning them of predators but not protecting them andthen you can have very different forms of parental care likesending them off to college and party.