Parental Investment in the Class Insecta

Abstract:

Parental investment theory in insects involves many complex factors.  Factors that limit parental fitness 
dictate parental investment.  Natural selection acts on parental investment to find optimal levels of 
investment and number of offspring.  There are numerous ecological factors that exhibit the evolution of 
parental care.

Paternal care is related to certainty of paternity.  Natural selection acts against paternal care when paternity 
is uncertain. Paternal care should not include nuptial gifts and spermatophores give to females to nourish 
the eggs.  The complexities of paternal care are shown in three species of Belostomatidae (The Giant Water 
Bug).  Maternal care is the most prevalent parental investment.  Females provide nourishment and tissue 
from their bodies to provision the eggs. Guarding of eggs and nymphs from predators is found in many 
different orders.  The Ovoviviparous leaf beetle (Gonioctena sibirica) stays with larvae until the last larval 
instar. Several species show a more advanced behavior of providing food resources as well as a guarding 
behavior.  In the family Membracidae, females use aggressive guarding  as well as an alarm pheromone 
that is emitted from an injured nymph.  This communication from offspring to parent is another type of 
pre-social behavior.  Groups of females from the genus Elasmucha work together to guard the clutches, 
indicating a pre-social behavior.

Biparental care in dung beetles and burying beetles demonstrates that the evolution of parental care in both 
males and females increases their reproductive success.

Social insects by definition demonstrate parental investment; which may including sibling investment due 
to nepotism or indiscriminate altruism.

Keywords: Parental Investment, Paternal Care, Maternal Care, Insects


Introduction:

	Reproductive effort is the portion of an organism's total available energy that is used in 
reproduction.  Reproductive effort is then divided into mating effort (the amount of energy used to acquire 
mates) and parental effort (the amount of investment in offspring made by the individual) (Thornhill & 
Alcock 1983).  Individuals are constantly trying to increase their fitness, by the relative contribution one 
makes to the next generation.  When parents increase the number of offspring that reproduce, the number 
of their genes passed on to the next generation will be greater. Trivers (1972) defined parental investment 
as "any investment by the parent in an individual offspring that increases the offspring's chance of 
surviving (and hence reproductive success) at the cost of the parent's ability to invest in other offspring."  
E.O. Wilson described some ecology factors that are responsible for the evolution of parental care.   K-
selection species tend to have longer life spans, grow larger, and reproduce at intervals.  Due to movement 
into unfavorable or harsh environments some offspring may require help through a vulnerable time in their 
development.  Additionally, offspring may help parents take advantage of certain food sources.  Predation 
may be very high and longer parental care increases probability of offspring survival to breeding age 
(Mathews & Mathews 1978).   Any of these factors may act alone or in combination in the evolution of 
parental care for any species.  Parental investment has evolved in many species of insects to varying 
degrees. 
	
Paternal Care:

     Uncertainty of Paternity

	Natural Selection acts against paternal care when paternity is uncertain.  If a male cares for 
unrelated offspring, he is increasing another's genetic success while decreasing his own.  Paternal care is 
more often seen in aquatic organisms, which have external fertilization.  A hypothesis has been developed 
to explain the association between type of parental care and the mode of fertilization, internal or external 
(Zeh and Smith 1985).  External fertilization allows for a high certainty of paternity, because the father is 
present during fertilization and may then guard against other males that may try to fertilize the eggs.  
Internal fertilization allows for less certainty, because the female may have received multiple copulations 
before oviposition.  This hypothesis is incomplete and was further developed by the authors.  The next 
hypothesis suggests that advantage to parental care is unequal or the two sexes because of the difference in 
their association to the offspring.  Internal fertilization adapts females to provide the parental care.  
External fertilization often occurs in male territories, adapting males to provide the parental care.  When 
males are highly certain of their paternity, as seen in the water bugs in the family Belostomatidae, 
uncertainty of paternity is not considered in the decision to provide paternal care (Kight and Kruse 1991).

     Controversial definition

According to Zeh and Smith (1985) the definition of paternal care consists of three components: prezygotic 
investment including indirect contributions to offspring, Biparental care, and exclusive care provided by 
the male.  The controversy lies in the first component, prezygotic investment that consist of nuptial gifts 
and spermatophores do not count as paternal care, but as mating effort, according to Thornhill and Alcock 
(1983).  They believe that males evolved these traits to secure mating, or to increase the probability that the 
female will use his sperm.  This controversy lies on how you wish to divide reproductive effort.  I will 
follow the position of Thornhill and Alcock, and exclude prezygotic investment as a viable paternal 
investment.   Biparental care which will be mentioned in its own section and exclusive paternal care will be 
included in my coverage of paternal care.

     Exclusive paternal care

	The most well known paternal care in insects was observed by Robert Smith in the species in the 
family Belostomatidae, the giant water bug, Belostoma flumineum (Smith 1976a) and Abedus herberti 
(Smith 1976b). In the beginning of the twentieth century scientist discovered that it was the males of many 
species of Belostomatidae that were carrying the eggs.  In his study, Torre Bueno reported that the males 
did not wish to care for the eggs and constantly tried to dislodge the eggs.  Smith believed that Bueno's 
theory went strongly against the theory of natural selection.  Natural selection implies that the mother is 
under intense pressure to select the best ovipositional site for her young.   Water bug eggs need to be 
covered by water, have water movement and frequently need to be exposed to atmospheric oxygen.  He 
tested egg pads attached to healthy males, unattached egg pads saturated in water, unattached egg pads left 
in air, and unattached egg pads in a partial water/air environment.  The results found the eggs on the back 
of a healthy male had a very high percent hatch.  Eggs left under water developed a fungus and did not 
hatch.  Eggs left in the air desiccated and were nonviable after 24 hours.  The eggs exposed to water and 
air did begin to hatch but were not successful in hatching all the way.  Males also exhibited another 
behavior, brushing their hind legs over the egg mass.  This behavior was originally was thought of as an 
attempt to remove the eggs; this was believed because cannibalism of eggs had been reported in many 
species of Belostomatidae.  Smith observed cannibalism of the eggs by the father in many cases.   
However, he believes that the main function of the leg stroking of the eggs is to supply the eggs with 
source of dissolved oxygen.  In adverse conditions, such as a loose egg mass (where all egg usually die) 
or when disturbed, the father does remove and cannibalize the egg mass.  The cannibalism of the eggs is 
thought to be a method of gauging the energetic cost of care when the probability of a successful hatch is 
low (Smith 1976a).  Males incur high costs when caring for eggs including, an increase in exposure to 
predation, a reduction in ability to hunt, a decreased ability  to disperse, and a probable reduction in the 
number of eggs that can be fertilized.  The father's fitness must be maximized by increasing the 
reproductive potential of the offspring even when it lowers the parent's reproductive success (Kight and 
Kruse 1991).  Males also can decide whether caring for a specific egg pad is maximizing their fitness.

	In Ichikawa's (1995) study of another genus of Belostomatidae, Lethocerus, a different male 
brooding behavior is evaluated.  In this species, the female lays the fertilized eggs on emergent vegetation.  
At night, the male crawls over the eggs and allows water to flow over the eggs.  Although there are equal 
numbers of males and females, because brooding of the eggs by the males takes longer than female to 
develop new eggs, males become unavailable and females must compete.  If females can not find a non 
brooding male they attack brooding males and destroy his egg mass.  Males successfully fight off females 
30 % of the time.  When the eggs are destroyed male and female move to another site and copulate.  The 
male broods the new batch of eggs.  In this study Ichikawa looks at why the males only water the eggs at 
night when evaporation is low, how long the males stay with the eggs, and how the eggs benefited from 
the male staying with them after he finished watering them.  As it turns out, females search for males at 
night and only destroy the brooding males' eggs when she detects him in the water.  Females can not 
detect males that are on vegetation near their eggs.  The longer that males stay with their eggs, the less 
likely a female will find and destroy their eggs.  Ichikawa also showed that the older the eggs are, the 
longer the male will stay with them.  This shows that the more investment that has been provided, the less 
the male wants to encounter a female that will destroy all his efforts and make him start over.  

	Exclusive paternal care as described above can be very successful.  It is more unusual in the insect 
world than maternal care.

Maternal Care:

     Nutrition provided from the mother

Maternal care is the most prevelant form of parental investment in insects.  By definition, the female of a 
species is the form that provides the egg.  The egg contains half of the genetic material but all the nutrition 
components needed by a developing zygote.  The nutrition comes from tissue developed by the female.  In 
contrast, sperm production has a relatively low energetic cost to the male.  However, following the 
definition of parental care, not all females exhibit investment by merely supplying the eggs.  The female 
must increase the probability of survival of an individual offspring at the expense of producing other 
offspring.  There are many examples of the production of fewer eggs of a larger size.  Larger eggs mean 
less time in a vulnerable state to predation or parasitism and possibly less time for the offspring to be non-
reproductive.  An extreme example of this is the case of the viviparous Tsetse fly.  The female Tsetse fly 
internally nourishes one offspring until it reaches the pupal stage.  In this strategy, the individual offspring 
has a high probability of reaching reproductive age.  In contrast, an egg mass containing hundreds of eggs 
rsults in only a ver smal number of viable offspring.

     Guarding Behavior

	In the ovoviviparous leaf  beetle, Gonioctena sibirica, in the family Chrysomelidae, the female 
remains with the offspring until they reach their final larval instar (Kudo et al. 1995).  Females protect 
their offspring from predators by positioning themselves at the base of the leaf and exhibiting a defensive 
behavior when disturbed.  Defensive behaviors include rushing the intruder, swinging the body from side 
to side, and stamping the legs.  Kudo and coworkers tested the percentage offspring surviving under three 
conditions, with the female present, with female absent, and with a sticky barrier at the base of the leaf.  
Their results found a significantly higher percentage offspring surviving when the female or the sticky 
barrier was present than when the female or barrier was removed.  Kudo and Ishibashi (1996) look at 
another component of this leaf beetle, the effectiveness against parasitoids.  A similar experiment was 
designed to examine how well a female guarded against parasitoids.  Due to their position females were 
ineffectual against parasitoids and percentage of larvae parasitized was between 25 and 29 percent.  In 
summary, the evolution of this parental guarding is effective against one enemy but highly vulnerable to 
another.

	Guarding of eggs and larvae is found in many orders and families of insects.  For example, 
parental guarding was shown in Erixestus winnemana, a parasitic wasp of the Colorado potato beetle.  
After injecting her eggs into the egg of the beetle, the female stand guard until her offspring emerge so 
hyperparistoids and other parasites do not kill the larvae (Schroder et al. 1996).  Parental care is very rare 
in parasitic wasps but this guarding behavior is very significant in the offspring percent survival.

     Guarding and Provisioning

	The shield bug, Parastrachia japonensis, exhibits both maternal guarding and progressive 
provisioning.  The guarding behavior consists of carrying the egg mass away when confronted by a 
disturbance (Tsukamoto et al. 1995).  The female shields nymphs when disturbed; if the disturbance 
increases she would flee in an effort to draw the predator away.  Females also provision young with 
drupes, which a type of fruit.  It is necessary for the female to gather food because the drupe falls in a 
habitat that is unsuitable for young nymphs.  The study by Tsukamoto and coworkers tested how nymphs 
survived without females or drupes, without female but with drupes, without drupes and with the female.  
Their results showed that when drupes were provided, nymphs tended to remain longer and the female 
lived longer, than when the drupes were removed.  This extreme maternal guarding and provisioning 
through out the larval development is also found in two species of dung beetles, Copris incertus (Halffter 
et al. 1996) and Kheper nigroaeneus (Edwards and Aschenborn 1989).

	In the family Membracidae, the treehoppers, females of the species Umbonia crassicornis provide 
protection, provisions, and have a form of  communication with their offspring.  In a study by Wood 
(1975), mortality rates of the offspring were observed when females where removed at different times 
during their development.  The behavior of the female consisted of brooding the eggs, making feeding 
sites in the bark of the tree, keeping the nymphs together, and protection from predators.  Wood also 
discovered the use of an alarm pheromone, sent off by injured nymphs.  The female behavior consisted of 
aggressive movements toward the location of the injured nymph.  The ability of the nymphs to 
communicate to their parents is a form of subsocial behavior.

     Joint Guarding

	The parent bug Elasmucha grisea is a subsocial shield bug in which the females frequently jointly 
guard their offspring. In Finland, a study by Mappes and coworkers (1995) discovered when females 
guard their offspring together, there were significantly more eggs per female then when females guard 
singly. They also found that two females guard their offspring so effectively as to have twice as many 
offspring survive per female than a female guarding her offspring alone.  At four sites the observed 
frequency of joint females consisted of 0-30% of the females per tree.  Although joint guard is 
advantageous against most insect predators, the researcher speculated that the low frequency of joint 
females is probably due to birds being more likely to find these larger aggregations.
	
Biparental Care:

	Biparental care is very rare in insects, because male effort usually falls under mating effort instead 
of paternal effort.  Biparental care does occur in a number of species of both dung beetles and burying 
beetles.  

	In Germany, a study by Sowig (1996) examined the biparental brood care behavior of the dung 
beetle, Onthophagus vacca.  When a male does not assist the female, the survival of  a single offspring is 
unchanged.  However, if a male provides assistance in brooding the young, the number of offspring is 
increased, which in turn increases both parents' reproductive successes.

	A study by Eggert and Sakaluk (1995) of the species Nicrophorus defodiens, a burying beetle, 
examined the attempt of males and females to increase their fitness while simultaneously caring for their 
offspring.  Both sexes of N. defodiens independently search for carrion, to feed their expected larvae.  
Once a suitable carcass is found, a sex pheromone to attract the opposite sex is emitted.  Immediately the 
pair will bury the carcass under ground.  The pair mates and prepares the carcass for consumption by 
larvae.  Both parents remain to feed regurgitated tissue to the young and protect the larvae from other 
predators or fungus that may grow on the carcass.  If the carcass was larger than was going to be used by 
the offspring of the mating, the male attempts to emit sex pheromone to attract another female.  This action 
would increase the male's fitness by increasing the amount of offspring.  This is a problem for the female 
because having another female in the brood chamber reduces her fitness.  "Females  often attempted to 
interfere with male pheromone emission.  The interference behavior observed most frequently was 
mounting of the male by the female . pushing the male, undercutting him or pinching his abdomen with 
her mandibles."- (Eggert and Sakaluk 1995)  In this study the researchers tethered the female so that she 
could not interfere with the male efforts to attract another female.  When the female was tethered the male 
spent significantly more time emitting pheromone than when the female was un-tethered (Eggert and 
Sakaluk 1995).  Males and females are both exhibiting parental investment to increase their own fitness; 
however, the attempts to increase their own fitness may interfere with their mate's fitness.

A medium sized burying beetle of the same genus, Nicrophorus tomentosus was observed to have groups 
of males and females work together in burying and using large carrion to raise their young.  Scott's (1996) 
study found advantages in large groups on large carcasses in fighting off other species of burying beetles 
and fly larvae.  This results in pre-social behavior that benefits the individual more  than when they attempt 
to act alone (Scott 1996).

Social Insects:

	Eusocial behavior is the highest level of cooperative behavior and is only found in the termites, 
ants, some bees and wasps (Evans 1984).  Evans also defines all eusocial insects as sharing the following 
attributes: Cooperative care of the brood, a division of labor, and an overlap of generations.  In social 
colonies the young are cared for by a special cast that may not include the parents of that offspring.  The 
caste exhibits guarding, and or provisioning depending on the species involved.  However, this 
investment may still be considered parental investment because the amount of energy used in rearing the 
offspring is done for the fitness of the colony that can be translated into the fitness of the individual.  
Evolution of parental or perhaps more exclusively, sibling care in social colony may be due to nepotism.  
Hymenoptera and Isoptera both undergo haplo-diploidy, resulting in haploid males (a result of asexual 
reproduction) and diploid females (sexual reproduction).  Because of haplo-diploidy, half of the genetic 
material of a female offspring is identical to the father, while the remaining half contains half of the 
mother's DNA.  This means that siblings' share three quarters of their genes but only half of their genes 
with their parent.  Historically, this argument explained why females would help rear their siblings.  
However,  "a lack of an efficient system of recognition, the high costs of recognition errors, a decrease in 
efficiency within groups consisting essentially of related individuals, and active conflicts of interest 
between care providers and receivers, may all work in concert to select against nepotism (Keller 1997)."  
There is a high probability that two individuals are (highly) related and so care of all individuals results in 
increased fitness of the individual providing care.  In fact, Keller (1997) hypothesizes that attempting to 
differentiate between related and unrelated (or highly to moderately related) individual results in decreased 
fitness due to recognition error.  Parental care or sibling care may not be directly related individual fitness 
per se, but it increases the fitness of many thousands of individuals of a very similar genetic constitution.

Conclusion:

In this review, many studies were used to illustrate the different forms of parental investment that occur in 
insects.  Several species of the giant water bug display different types of exclusive paternal care. Maternal 
investment ranged from nutrition provided by the mother, to guarding, to provisioning, to communication 
between mother and offspring, to joint guarding by females, and many combinations of these behaviors. 
More complex investment behaviors are also found in social insects and insects showing biparental 
investment.  Evolution of parental investment has been shown to appear in many different species and in 
many different ways.  Parental care ranges from simple actions to complex behaviors.  Regardless of how 
developed the investment seems, in each case it has maximized that species' fitness.

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