Basics of Sexual Reproduction: External vs. Internal Fertilization
how does external fertilization work? what are 3 consequences of this?
- HUGE #s of gametes are broadcast into the environment 1. species recognition is necessary to prevent cross-species fertilization 2. large numbers are necessary to ensure that egg & sperm meet 3. gamete release must be synchronous
characteristics of reptile shelled egg
- must be permeable to respiratory gases - must be impermeable to water to prevent desiccation
what environment must external fertilization take place in? why?
- aquatic - prevent desiccation of gametes
reptile fertilization
- internal fertilization (land-dwelling, evolution of necessary accessory sex organs, embryo development)
describe the copulation of elephants & wolves
- lock & key (species specific genitalia) - females will clamp onto male genitalia until copulation is complete - elephants: males cannot pull out until the female releases & thus must follow her everywhere - wolves do the same thing, only the alpha female & alpha male mate, the female may go hunting with a male inside of her
amplexus
- mating process of frogs and toads - male grabs onto the back of female & squeezes out egg mass - male then releases sperm - this maximizes the probability of sperm joining *specific* eggs
describe indirect sperm transfer. who does this?
- mites, scorpions, & some salamanders deposit a spermatophore - females will straddle the spermatophore & scoop it up
amphibian fertilization
- must still produce in water (external fertilization) - exception: salamanders use spermatophore
how do externally fertilizing species synchronize their timing?
- seasonal breeding (depending on day length, temperature change, or weather change) - social stimulation (sexual behavior may cause others to become sexually active), ex: salmon travel super far
what is an anatomical requirement of internal fertilization?
- secondary (accessory) sex organs - vagina or cloaca & penis
what is copulation? what is its function?
- the physical joining of male & female secondary (accessory) sex organs - copulation allows for sperm to go directly into the female reproductive tract
how can you distinguish some species of nearly-identical insects?
- their genitalia (external sex organs) have species-specific shapes - "lock & key" - some male insects can scoop another male's sperm out of the reproductive tract of a female to replace it with their own sperm
internal fertilization by fish
- vertebrates evolved in an aquatic environment - many fish use external fertilization, but most fish have more specific mating behaviors - some sharks & rays have fins that hold males & females together to allow for direct transfer of sperm & internal fertilization - this allows for the formation of protective egg casings
define spermatophore
a moist, gelatinous packet filled with sperm
what is a cloaca?
a portal for digestive, reproductive, & urinary excretions
which is more common, external or internal fertilization?
external fertilization
how many gametes to male & female oysters produce in a year? why?
females: ~100 million eggs/year males: billions of sperm/year oysters are very sedentary
internal fertilization of snakes & lizards (reptiles) - cloaca & hemipenis
in some species, males also have a cloaca hemipenis: - fills with blood - extends from male cloaca - taken in by female - spiny on one end (to attach) - groove on surface for transfer of sperm - after sperm transfer, the hemipenis is taken back up into the male's cloaca * some birds also have grooved penises
describe the indirect transfer of spider & squid sperm
male spiders: secrete drop of sperm-containing web & use a special leg to pick it up & insert it into the female male squid: pick up spermatophore with a special tentacle & insert it into the female
what is an advantage of internal fertilization?
protection of early developmental stages of embryo
why is internal fertilization necessary for terrestrial animals?
to prevent gamete desiccation