Biology Reference
In-Depth Information
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Fig. 7.20
In this experiment, female fruit flies
Drosophila melanogaster
were given
varying exposure to male accessory gland proteins (Acps) at mating, while keeping
constant other costly aspects of reproduction, such as egg production, non-mating
exposure to males and rate of mating. Females exposed to males which produced Acps
(dark blue line) died significantly sooner (median lifespan 21 days) than females exposed
to three other types of males (median 29 days), namely: males genetically engineered to
lack Acps (red line), and two control groups of males (open symbols and pale blue line)
which courted females at the normal rate but could not mate because their external
genitalia were ablated. From Chapman
et al
. (1995). Reprinted with permission from the
Nature Publishing Group.
have been investigated by producing males genetically engineered to lack them or
overexpress them. Among their functions are: incapacitating rival male sperm;
protecting a male's own sperm from enzymatic attack in the female reproductive tract;
increasing a female's egg laying rate; decreasing a female's propensity to remate. All
these will help to increase the male's reproductive success, but experiments have shown
that these male benefits can be at the expense of female fitness because they reduce a
female's longevity (Fig. 7.20). The deleterious side-effects to the female may arise
because the Acps enter the female haemolymph through the vaginal wall and then
perhaps interfere with essential enzymatic processes inside the female body cavity
(Chapman
et al
., 1995, 2003).
Strategic allocation of sperm
. Comparative studies of many taxa (primates, bats, other
mammals, birds, frogs, fish and various insects) have shown that testis size relative to body
size (a measure of investment in sperm) increases with the degree of female promiscuity
(a measure of sperm competition; Wedell
et al
., 2002). The potential for evolutionary
change is revealed by selection experiments with male dung flies; when exposed to
increased sperm competition, larger testes and ejaculates evolved within ten generations
(Hosken
et al
., 2001). These results show that investing resources in sperm production is
costly and evolves only when this brings a competitive benefit. Within species, too, it is clear
that males do not have limitless potential to copulate (Dewsbury, 1982). For example, male
