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Reproduction In Mammals
[A CASE STUDY OF UHUNMWONDE LOCAL GOVERNMENT AREA OF EDO STATE]
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Reproduction involves costs associated with mating (e.g., finding
mates), pro-ducing gametes and offspring (e.g., eggs), and for many
species parental care (Roff, 1992). Mechanisms underlying the
cost/benefit trade-offs involved in reproducing may be genetic or
social/environmental, or they may represent a genotype by environment
interaction (Reznick, Nunney, & Tessier, 2000). Social costs include
those incurred during intrasexual competition over mates and are
described later. Genetic trade-offs arise when the same gene or genes
affect two or more life history traits (Williams, 1957). In many
species, reproducing earlier in life is associated with a shorter life
span (Reznick, 1992). The same genes that promote early re-production
have the negative consequence of accelerating the onset of senescence
and reducing the life span.
Life span is also influenced by more
proximal reproductive costs, such as pro-ducing eggs, competing for
mates, and caring for offspring, which can compromise the physical
health and oftentimes the survival prospects of parents (Clutton- Brock,
1991; Steams, 1992). The underlying physiological mechanisms governing
these cost/benefit trade-offs are not fully understood, but include the
energetic demands of reproduction (e.g., parental care) and associated
hormonal changes (Sinervo & Svensson, 1998). For example, the
development of male secondary sexual char-acteristics needed to compete
with other males (e.g., antlers) or to attract females (e.g., a bright
plumage) requires an increase in testosterone levels which in turn can
compromise the immune system and survival prospects of unhealthy males
(Folstad & Karter, 1992; Saino & Møller, 1994; Saino, Møller,
& Bolzern, 1995). Similarly, in the female collard flycatcher
(Ficedula albicollis) large brood sizes are associated with a reduced
production of antibodies for a common parasite; the result is increased
infection rate and mortality rate (Nordling et al, 1998).
All other
things being equal, evolution should result in a life history pattern in
which females produce many, fast maturing offspring, that have an
increased probability of surviving to reproduce (Williams, 1966). The
fact that many species do not show this life history pattern indicates
that the associated trade-offs are costly. These trade-offs include
smaller and less competitive offspring that in turn suffer high
mortality rates (Steams, 1992). Across species of plant, insect, fish,
reptile, and mammal, offspring that are larger at time of hatching or
birth have increased survival rates due, in part, to decreased predation
risk and decreased risk of starvation (Roff, 1992). The trade-off is
that females of these species produce fewer offspring than do females of
related species that produce many smaller offspring. Thus fast
maturation and large numbers of offspring are associated with
low-quality offspring (i.e., high mortality risks and low
competitiveness). High—quality—larger and more competitive—offspring
come at a cost of fewer offspring produced during a reproductive life
span. Many factors will influence whether a species tends toward a
low-quality/high-quantity or high-quality/low-quantity reproductive
pattern, including age-specific mortality risks (e.g., through
predation), population stability or expansion, and intensity of
competition with conspecifics (Mac Arthur & Wilson, 1967; Steams,
1992; Roff, 1992).
Species that produce fewer and larger offspring
also tend to have slower rates of growth, higher levels of parental
care, and longer life spans in comparison to related species that
produce smaller but more offspring (Roff, 1992; Shine, 1978, 1989;
Steams, 1992). This life history pattern is more common in iteroparous
than in semelparous species and is associated with relatively low
juvenile mortality rates and a low probability of reproducing at an
early age (Roff, 1992). Low juvenile mortality is related to larger size
at hatching or birth as well as to parental protection and provisioning
(Clutton-Brock, 1991; Shine, 1978). As described later, a low
probability of reproducing at an early age can result from reproductive
competition with more mature individuals in the population. In this
situation, delayed maturation can improve reproductive prospects
through, for in-stance, an increase in body size. Large body size
enables females to give birth to larger and thus more competitive
offspring, and for males it facilitates male- male competition in
adulthood (Carranza, 1996; Steams, 1992). In some species, developmental
activity during the maturational period enables improvements in
survival- and reproduction-related behavioral/cognitive competencies.
Slow maturation and growth thus allows for the accumulation of more
reproductive potential, through physical development and developmental
activity, than is possible with faster maturing species. A sex
difference in potential rate of reproduction can create a sex difference
in relative emphasis on mating or on parenting. Most generally, the sex
with the higher potential rate of reproduction invests more in mating
effort than in parental effort, whereas the sex with the lower rate of
reproduction invests more in parental effort than in mating effort
(Clutton-Brock & Vincent, 1991). This pattern arises because members
of the sex with the higher potential rate of reproduction can rejoin
the mating pool more quickly than can members of the opposite sex. Under
these conditions, individuals of the sex with the faster rate of
reproduction will typically have a higher lifetime reproductive success
if they rejoin the mating pool and compete for mates than if they parent
(Parker & Simmons, 1996).
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ABSRACT - [ Total Page(s): 1 ]This study examined reproduction in mammals using man as a case study. In the study, some of the main features of reproduction in man are explored. After considering some of the various aspects of reproduction in man and the challenges faced, we move to consider in particular, pregnancy and childbirth. To this effect, a questionnaire was designed that addressed issues relating to pregnancy and childbirth. The primary audiences for the questionnaire were pregnant women/nursing mothers and medical ... Continue reading---
