Animal Life Cycles: Reproduction and Development Strategies

Posted on Nov 25, 2024 in Geology

Animal Life Cycles: Strategies and Adaptations

Pairing

Nonexistent in simple animal groups.

Insemination

Consists of releasing sperm into the water. Some release them directly, others in a package called a spermatophore. Some use copulatory organs to achieve fertilization, while others deposit sperm in the female’s genitals.

Embryonic Development

A complex process that begins with zygote formation and ends with the beginning of independent life after hatching or birth.

Postembryonic Development

  • Direct development: The new being resembles the adult, and development consists of reaching reproductive capability.
  • Indirect development: The new being, the larva, is different from the adult and undergoes metamorphosis (simple or complex changes) to reach adulthood.

Asexual Reproduction

Does not involve gametes; only one parent is involved, and there are no distinct reproductive organs or specialized cells.

Forms of Asexual Reproduction

  • Budding: The body of the progenitor forms projections (buds) that develop into new individuals.
  • Gemmulation: A new individual forms from gemmules, which are groups of cells surrounded by a protective capsule.
  • Fragmentation: The animal divides into two or more pieces, each capable of reconstructing the complete animal.
  • Polyembryony: A special type of fragmentation that occurs during the early embryonic development phase. The embryo divides into multiple fragments, each originating a new individual.

Sexual Reproduction: Fundamental Processes

Meiosis: A process to form haploid gametes that join to form the diploid zygote and restore the species’ chromosome number. The offspring receives genetic material from both progenitors, developing into an individual genetically different from the parents but with the characteristics of its species.

Dioecious Species

Have separate sexes; males produce sperm, and females produce eggs. Sexual dimorphism (differences in size, color, and structure) is often present between the sexes.

Monoecious Species (Hermaphrodites)

Produce both types of gametes, either in different gonads or in a single organ called an ovotestis.

Advantage: They can produce twice as many offspring, and all individuals produce eggs, unlike dioecious species where only females produce eggs.

Animal Reproductive Systems

Consist of two types of organs:

  • Primary organs: Gonads (testes and ovaries), the unique sexual organs in some animal groups.
  • Accessory organs: Assist the gonads in gamete formation and release, as well as in embryo reception and protection. In males: penis, spermatic ducts, and accessory glands. In females: oviducts, uterus, accessory glands, and vagina.

Invertebrate Reproductive Systems

Vertebrate Reproductive Systems

Female Reproductive System

Independent of the urinary system, formed by:

  • Ovaries: Ovarian follicles are formed by follicular cells surrounding an oocyte.
  • Fallopian tubes: Transport oocytes from the ovaries to the uterus.
  • Uterus: In oviparous animals, the uterus retains eggs with shells until the mother lays them. In placental mammals, the uterus walls (endometrium) are modified during reproduction to form the placenta, which nourishes the developing embryo.
  • Vagina: A muscular tube connecting the uterus to the outside, receives the male penis, and serves as the birth canal.
  • Vulva: A longitudinal slit formed by the labia majora and minora and the clitoris. It has glands that secrete lubricating substances for the vagina and clitoris during copulation. The vaginal opening is partially closed by the hymen until it breaks.

Male Reproductive System

Closely related to the urinary system, comprising:

  • Two Testicles: With seminiferous tubules where sperm are produced.
  • Scrotum: An external sac that hangs from the abdominal cavity.
  • Epididymis: A coiled duct where sperm mature.
  • Vas Deferens: A duct that exits the scrotum, enters the abdominal cavity, and transports sperm during ejaculation.
  • Ejaculatory Duct: The passage for sperm during ejaculation.
  • Accessory Glands: Seminal vesicles, prostate, and bulbourethral glands.
  • Penis: An erectile copulatory organ that ejaculates semen and is protected by the foreskin.

Gametogenesis

Transformations during gamete formation occur in the testes (spermatogenesis) and ovaries (oogenesis).

Spermatogenesis

Occurs in the seminiferous tubules of the testes and aims to form haploid sperm from diploid cells. It starts at puberty with proliferation and growth stages, where spermatogonia divide continuously. The maturation phase involves meiosis, and during spermiogenesis, the sperm develops a head and flagellum, transforming into a motile cell that migrates to the epididymis. Spermatogenesis is regulated by the hormones testosterone and follicle-stimulating hormone (FSH).

Oogenesis

Occurs in the ovaries to produce haploid eggs from diploid cells. Phases include:

  • Proliferation: Occurs in the fetal ovaries, where oogonia (germ cells) multiply.
  • Growth: Oogonia increase in size, accumulate yolk, and become primary oocytes (diploid).
  • Maturation: Primary oocytes undergo meiosis, giving rise to one egg and three polar bodies (all haploid).

Menstrual Cycle

Divided into phases based on ovarian and uterine changes:

  • Follicular Phase: FSH levels increase, stimulating the maturation of a group of primary follicles in the ovaries. Eventually, one mature follicle (sometimes two) secretes estrogens. Estrogens promote oocyte growth, stimulate endometrial development, and trigger a surge in FSH and LH secretion before ovulation. LH stimulates follicle maturation, resumption of meiosis in the primary oocyte (forming a secondary oocyte and a polar body), and ovulation (rupture of the mature follicle and release of the egg).
  • Luteal Phase: LH transforms the ruptured follicle into the corpus luteum, which secretes estrogens and progesterone. If blastocyst implantation does not occur, the corpus luteum degenerates about a week after ovulation.

Insemination

The process by which male and female gametes are brought together.

  • External Insemination (Aquatic Animals): Sperm are released into the water to find the egg. In some cases (Porifera), sperm are released into the water. In others (fish and amphibians), females release eggs into the water, and males deposit sperm on them.
  • Internal Insemination: The male deposits sperm inside the female’s body for internal fertilization. Mechanisms vary: arachnids use pedipalps, birds use cloacal contact, and insects, gastropods, and mammals have developed a penis for sperm deposition into the female genital tract (copulation).

Fertilization

The union of egg and sperm, resulting in a diploid zygote. The egg provides the nutrient-rich cytoplasm, mitochondria, ribosomes, and mRNA, while the sperm contributes the haploid nucleus and a centriole for centrosome formation and the mitotic spindle. Karyogamy is the fusion of the two gamete nuclei.

Parthenogenesis

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