Parental Care in Burying Beetles

Abstract 

Burying beetles are one of the few examples of parental care in insects outside of the social insects.  These 
beetles (Nicrophorus) exhibit advanced parental care, by feeding and guarding their offspring on buried 
vertebrate carrion (Eggert & Muller, 1992)  Their behavior, called subsocial care, has been studied on a 
variety of fronts.  In this paper we will look at some of this care in the subsocial beetles.  First, we will 
look at male parental care.  This is highly uncommon in insects as whole and seen sparingly in most of the 
higher vertebrates.  Some studies have shown varying degrees of the male's helpfulness in raising a 
brood.  At times the male can even hinder the brood's chance of survival.  However there are times when 
without his help, the brood would most likely be taken over by another beetle pair.  Also, we will examine 
the competition that the beetles must face in order to secure their carcass, both before and after it is buried.  
Obstacles from scavengers taking a newly dead corpse to another female attempting to parasitize the brood 
are all very real dilemmas for these beetles.  One more problem with biparental care addressed in this paper 
is that of sexual advertisement.  At first glance this may seem to be a way for the beetles to meet on a 
carcass.  While that is true, it is also a point on which the two sexes do not agree.  Once a pair has secured 
a body, if the assessment of the male beetle shows it to be a large carcass, he will begin to signal for other 
females to joint them on the carcass.  The female, wanting the corpse all to herself, will proceed to 
interfere with his signaling.  The battle has begun to see which offspring will be raised on the choice 
corpse.  These burying beetles have quite an assortment of actions all of which contribute to the result of 
raising their offspring to pass on their genes to the next generation.

Introduction

Subsocial behaviour is of interest because I some respects it resembles the behaviour of true social insects, 
and analysis of simple parental responses may suggest ways in which certain respects of social behaviour 
could have been derived from less complex habits (Klemperer, 1982). 

Subsocial care increases the probability of juveniles reaching adulthood (Halffter et al., 1996) and is found 
in at least 10 families including all cases of nidification known for Nicrophorus (Tallamy & Wood, 1986).  
Parental care greatly increases offspring survival. In certain species of sub-social beetles, mortality can rise 
as much as 44% (Copris diversus survival rate went from 76% down to 32% in the absence of parental 
care (Halffter et al., 1996)).  

Subsocial behaviour is critical when food resources are ephemeral and must be protected against 
competitors and decomposers (Halffter et al., 1996).  Burying beetles must outcompete congeners, 
carrion-breeding flies and scavengers to exploit small vertebrate carcasses as a breeding resource (Trumbo 
et al., 1995).  Often available carcasses are not found by the beetles and therefore are taken by scavengers.  
This makes it extremely important that burying beetles find and bury their carcass quickly.  According to 
Ratcliffe, adult beetles can detect the odor of recent death from up to two miles away and can arrive at that 
spot within an hour.  In one study, over 90% of mouse carcasses not found by scavengers were 
discovered by burying beetles on the first available night (Wilson & Fudge, 1984). 

Adult beetles, after flying to the vicinity of a carcass, drop to the ground and crash through the litter to it.  
They go under the body, turn over onto their backs and experimentally lift the remains (Ratcliffe).  
Prominent behaviors immediately following discovery of a potential resource include palpating the carcass, 
lifting, circumambulating, and making forays on the surrounding soil (Trumbo et al., 1995).   The female 
or pair will then judge with the aforementioned behaviors, the suitability of the carcass.  That being 
decided, the beetle(s) will proceed to bury the carcass.  Following burial, the carcass is stripped of fur and 
molded into a ball.  This ball is formed by oral and anal secretions that both preserve the carcass and 
modify it into the proper shape for the larvae.  The female will then construct a short chamber above the 
carrion in which she lays her eggs (Ratcliffe).  When the larvae hatch, they will then crawl into the brood 
ball.  Here the larvae concentrate and are fed regurgitated carrion (Muller et al., 1990).  The third instar 
larvae disperse from the corpse and pupate in individual chambers in the surrounding soil (Bartlett & 
Ashworth,1988).   

Discussion

Male Parental Care

Male parental care and monogamy are uncommon in invertebrates, and with the exception of the eusocial 
insects, any form of parental care is rare (Scott, 1988).  There must be some advantage to the male or the 
brood as a whole for the male to remain with his prodigy.  There have been several postulated theories 
why a male would remain with his brood.  Two parents working together could perhaps provide more 
suitable care.  Or perhaps the brood needs the protection of two parents to keep away parasites and 
predators.  And then again, why monogamy? It would seem that a male's success would be increased with 
additional females producing offspring. 

It has been shown that a beetle pair together is no faster at burying a carcass than a female alone.  In a 
study by Scott (1990), under field conditions, pairs did not bury carcasses significantly faster than single 
females.  (Table 1, Scott, 1990) Scott also showed in that study, that reproductive success is reduced by 
the presence of the second parent in the brood chamber.  Total larval weight of the brood, not number of 
offspring, is the measure of success used. This measure is used because of a trade off between the number 
and the mean weight of the young (Scott, 1988) making total larval weight a more accurate gauge of 
success. If the adults did go on to produce a second or third brood after the failure of the first, the ensuing 
clutches were significantly smaller than the first.  The lower success rate of any given brood could be 
attributed to several factors.  For one, both parents consume part of carcass (Scott, 1988) and therefore 
making less available to the developing larvae.  If the female is determining whether the male should stay 
or not then this loss of corpse for herself and her brood may be a factor (Bartlett, 1988).  Also, both 
parents practice infanticide reduce the number of first-instar larvae for maximum survival rate (Bartlett, 
1987).   It could be that with two parents both culling a brood more larvae are killed than necessary.  
(Table 1, Scott, 1989)

We do need to keep in mind that there is no significant difference between a clutch raised by a male and a 
clutch raised by a female.  Adults of either sex make competent parents (Eggert & Sakaluk, 1995).  In fact, 
ethogram studies have shown that males perform the same behaviors as females and spend similar 
proportions of their time maintaining the carcass.  They will however spend less time at every stage of 
larval development regurgitating to offspring (Fetherston, unpublished data).   The difference therefore lies 
in the one parent versus two parent care of the brood.  The male is usually the one to leave the nest first 
because females always can be certain that they are directly related to their offspring, whereas males cannot 
(Tallamy, 1984). This makes it in the male's best interest to move on and start his next brood. In other 
words, a male could stay, with some effect on the survival of the current brood or he could desert with a 
variable chance to breed again (Maynard Smith, 1977).  Any advantage (for him( must be balanced against 
the energetic costs of the care and the loss of mating opportunities (Bartlett, 1988).  The female on the 
other hand increases her reproductive success by insuring her prodigy survive.  Because of these factors, 
most single parent broods will be cared for by females.

Sexual Advertisement

Male burying beetles are known to release pheromone when they are the first to arrive at a small carcass 
(Eggert & Muller, 1989).  They will do this only in the absence of a female in order to attract one to the 
newly found resource.  The sexual conflict arises when a pair together finds a large carcass.  A female's 
interests are best served by making sure that all offspring on a carcass are her own.  Any reproductive 
success gained by attracting a second female comes almost exclusively at the expense of the first female 
(Muller et al., 1990).  A male on the other hand can increase overall offspring raised on a carcass with a 
polygynous breeding association (Trumbo and Eggert, 1994).  This will occur only on a large carcass.  On 
a small carcass, male and female interests largely overlap and there is no significant increase in the number 
of offspring with an increase in the number of females (Eggert & Sakaluk, 1995). Monogamously paired 
males on large carcasses do in fact attempt to attract additional females by means of pheromone emission, 
whereas males on small carcasses do not (Eggert & Sakaluk, 1995).  (Figures 1 &2, Trumbo & Eggert, 
1994)

When a male and female are paired on a large carcass, the male will attempt to emit pheromone to attract 
other females.  A problem with the signaling is that the male may attract other male burying beetles.  
Pheromone-emitting males risk not only attracting conspecific males, but congeneric competitors as well 
(Muller & Eggert, 1987). There are some adaptations to ensure success for the males in their signaling 
venture.  By stopping pheromone emission when other species become active at sunset, N. defodiens 
probably avoids enhancing the attractiveness of a carcass to heterospecific burying beetles (Wilson et al., 
1984).  Pheromone-emitters, however, cannot avoid the response of conspecific males (Trumbo & Eggert, 
1994).  For this reason, a male will only choose to signal when he has weighed the costs of signaling 
against its possible benefits.

The female's interests are best served by a monogamous association.  While there will be an overall 
increase in the number of offspring produced in a polygynous association, the number of offspring per 
female will be reduced.  Females appear to attempt to physically deter pheromone transmission by 
mounting males, biting them, undercutting them, or displacing them from their perches where they emit 
pheromone (Trumbo & Eggert, 1994).  Males, in apparent response to such interference, often relinquish 
the pheromone emitting posture, secure another signaling perch, or cease pheromone emission altogether 
(Eggert & Sakaluk, 1995). The Eggert & Sakaluk study (1995) showed that this interference behavior by 
the female, reduced the numbers of females detecting the carcass thus preserving the monogamous 
relationship.  (Figure 3, Eggert & Sakaluk, 1995)

Brood Guarding

It seems males are a hindrance to the female and her brood however they are crucial in another important 
area of success.  This is that the beetles appear to assist their mates in driving off intrasexual competitors 
(Scott, 1989) and heterospecific burying beetle competitors (Trumbo, 1990).  Males sometimes stay with 
broods to defend the corpse from other males which might destroy their brood and attempt to father 
another upon it (Bartlett, 1988).  Two parents dramatically reduce the probability that conspecifics will 
usurp the resource, replace either the male or female, kill the newly hatched brood, and produce a 
replacement clutch (Scott, 1989).   On small carcasses, there was a strong tendency for the presence of a 
pair to reduce the probability of replacement  by an intruding male and a similar tendency for large 
intruding females to be successfully repelled by a pair (Scott, 1989).  Results of the study by Scott (1989) 
show that when the intruders replace the residences of the brood chamber, infanticide is always the result 
and the female will either be driven off or reinseminated.  (Table 3, Scott, 1990)

Males do not seem to be any more helpful in protecting the brood against insect predators than a female 
alone.  In fact, a female parent alone was sufficient to protect the brood against a staphylinid predator 
(Scott, 1989).   Males can also assist in defense against parasites and fungi which will attack the brood 
ball.  Oral and anal secretions are used to keep these parasites at bay and thereby keep the brood ball intact 
for the larvae.

 Inter/Intraspecific competition

Since carcasses are unpredictably distributed and a rare and ephemeral resource, competition leads to 
aggressive intra- and interspecific interactions on them (Muller et al., 1990).  These interactions are 
frequent and often violent.  The carcasses available are few and the need for them great.  The need for 
carcasses is not limited to burying beetles but includes scavengers, carrion flies, and many others.  
Generally the outcome of confrontations on corpses is determined by size of the individuals.  This is true 
for both inter- and intra- specific disputes.  (Figure 1 & Table 3, Muller & Eggert, 1990)

Interspecific 

Brood parasitism implies that adults of a host species show some kind of parental care (Muller et al., 
1990).  Females fight as soon as they meet on a corpse (Bartlett, 1988).  The superior (larger) female will 
drive away other females attempting to share her carcass.  An aggressive fight will insue the moment they 
meet in which the inferior female is almost always seriously injured.  Nevertheless, for a loser the cost of 
staying and risking more fights could be outweighed by the benefit of having some offspring of her own 
on the carcass (Muller et al., 1990).  Because of this, competitively inferior females do not always leave 
the carcass immediately but may stay near and lay eggs of their own (Muller et al., 1990).  With carcasses 
being a scarce resource, some offspring on another female's carcass may be better than the prospect of 
trying to find a carcass of her own.  For the superior female, there is a risk that adult beetles will raise 
larvae other than their own (Muller & Eggert, 1990). Muller & Eggert (1990) showed that a female does 
not recognize her larva as her offspring, but merely as larva at the correct stage of development.  The 
female's response to larvae encountered was strongly time dependent: most females killed and ate larvae 
that could not have hatched from their own eggs (Muller & Eggert, 1990).  This makes parasitism 
beneficial for the inferior female.  An inferior female that lays her eggs at the same time as the dominant 
female has a strong chance of the larvae being accepted and cared for by the dominant female.

Intraspecific

Successful takeovers resulted in the expulsion of the prior resident(s), killing of any offspring present on 
the carcass, and oviposition of a new clutch by the intruder (Trumbo, 1990).  Females aided by males 
were more likely than single females to avoid takeovers (Trumbo, 1990).   In fact the Trumbo study 
(1990) identified preventing intraspecific takeovers as the primary contribution of Nicrophorus defodiens 
males.  (Figure 2, Trumbo, 1990)

Some species have evolved to occupy a different niche to avoid takeovers.  N. defodiens is highly 
vulnerable to takeovers even after carcasses are buried (Trumbo, 1990).  It is able to coexist with its larger 
competitors because of its success on nights when temperatures fall rapidly after sunset (Wilson et al., 
1984).  Also, this smaller burying beetle is active prior to sunset while its competitors are nocturnal 
(Trumbo, 1990). Additionally, N. defodiens is able to reproduce on a much smaller carcass than its 
competitors.  You would assume this would provide a larger number of prospective carcasses, however 
findings by Trumbo (1990) show that pairs produced twice as many larvae per breeding attempt on small 
rather than large carcasses.  This is because the smaller carcasses are not at all attractive to larger 
Nicrophorus species allowing the small N. defodiens to successfully raise a brood on the small corpse. N. 
defodiens was even able to produce more larvae per gram of resource than the larger N. orbicollis 
(Trumbo, 1990).

Conclusion

Burying beetles give us a subsocial example of parental care in the insect world.  Few others venture along 
the path that these small creatures have forged for themselves on the forest floor.  They truly show us a 
new realm of exploration into the lives of the insects with their advanced parental care.  So few examples 
in nature show us the male playing a central role in the raising of his offspring.  The time dedicated by 
both parents to raising a brood is also remarkable.  They must give a large portion of their short lives to 
creating a brood ball, defending it, and keeping it from harm while at the same time caring for their 
offspring which reside in the ball.  These beetles are certainly worthy of the time and effort people have 
put in to studying them.

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