Parent–Offspring Conflict over the Transition to Independencein Nicrophorus vespilloides: Parental Chemical Cues andOffspring BeggingDeborah Leigh & Per T. Smiseth

Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, Edinburgh, UK

Introduction

In species where parents provide their offspring with

food after hatching or birth, the offspring must at

some point during their development undergo a

transition from nutritional dependency to indepen-

dence. Parent–offspring conflict theory predicts that

the optimal timing for this transition will differ

between parents and offspring because of asymme-

tries in relatedness between parents and offspring

(Trivers 1974). To be more specific, theory predicts

that the optimal timing from the offspring’s perspec-

tive should occur at a later stage in the offspring’s

development than that from the parents’ perspective

(Trivers 1974). The realised timing for the transition

to independence reflects the way in which this con-

flict is resolved, which in turn depends on the

degree to which parents and offspring can bias the

timing towards their respective optima. For example,

offspring may extend the period of dependency by

begging more intensely or for longer (Kilner &

Drummond 2007), while parents may bring the

Correspondence

Per T. Smiseth, Institute of Evolutionary

Biology, School of Biological Sciences,

University of Edinburgh, West Mains Road,

Edinburgh EH9 3JT, UK.

E-mail: per.t.smiseth@ed.ac.uk

Received: December 8, 2011

Initial acceptance: January 8, 2012

Final acceptance: February 1, 2012

(M. Herberstein)

doi: 10.1111/j.1439-0310.2012.02032.x

Abstract

In species where parents provide their offspring with food, the offspring

must undergo a transition from nutritional dependency to independence.

Parent–offspring conflict theory predicts that the optimal timing for this

transition will differ between parents and offspring and that the realised

timing depends on each party’s ability to control the transition. The

burying beetle Nicrophorus vespilloides is an excellent species for studying

conflict over the transition to independence; the larvae beg for pre-

digested carrion from their parents until they cease begging around 72 h

after hatching. The cessation of begging is not associated with changes in

parental behaviour, suggesting that the transition is mostly under

offspring control. However, recent work has demonstrated that caring

parents express distinct chemical cues that stimulate larval begging, the

expression of which varies between breeding and non-breeding beetles,

suggesting that parents might exert control over the transition to indepen-

dence by altering these cues throughout development. If so, we predict

that begging larvae should behave differently towards parental chemical

cues from different stages of development and that larvae of different ages

should behave similarly towards parental chemical cues from the same

stage of development. We found no evidence for either prediction: beg-

ging larvae did not behave differently towards parental chemical cues

from different stages of development, and larvae of different ages still

behaved differently towards parental chemical cues from the same stage

of development. Our results provide no support for the hypothesis that

parents can control the transition to nutritional independence by altering

their chemical profiles.

Ethology

460 Ethology 118 (2012) 460–465 ª 2012 Blackwell Verlag GmbH

transition to independence forward by increasing the

latency of feeding in response to offspring begging

(Davis 1976), showing aggression towards the off-

spring (Leonard et al. 1991), or simply abandoning

the offspring (Reiter et al. 1978).

At present, nearly all knowledge on parent–

offspring conflict over the transition to independence

comes from observational studies on birds and mam-

mals (reviewed in Rehling & Trillmich 2007; Rehling

et al. 2012). A notable exception is cross-fostering

experiments on rodents, which have been used to

investigate how older pups respond when cared for

by foster parents nursing younger offspring and how

female parents respond when caring for foster

broods that are younger or older than their own

(Pfister et al. 1986; Rehling & Trillmich 2007). To

advance our understanding of this subject, it is now

essential to extend this experimental approach to

additional taxa, including insects, where parents pro-

vide their offspring with food after hatching. The

burying beetle Nicrophorus vespilloides is a particularly

interesting species in this respect as it is one of the

few insects where parents provide the larvae with

food after hatching (Pukowski 1933; Eggert et al.

1998; Smiseth et al. 2003). Furthermore, this species

has proved a tractable study system for studying

family conflict over parental care as it is amenable to

various forms of experimental manipulation, includ-

ing traditional parental removal and cross-fostering

designs (Lock et al. 2004; Smiseth et al. 2003,

2007a,b), and novel designs based on the use of

dead parents as stimuli to trigger offspring begging

(Smiseth & Parker 2008; Smiseth et al. 2010). The

latter experimental design offers a unique advantage

in studies of offspring control over the transition to

independence to traditional cross-fostering designs as

it provides a simple yet powerful means for exclud-

ing potential confounding effects as a result of

behavioural interaction between parents and off-

spring.

Like all species of the genus Nicrophorus, N. vespillo-

ides breeds on carcasses of small vertebrates that pro-

vide the sole source of food for the developing

larvae (Scott 1998). The larvae obtain food by beg-

ging for pre-digested carrion from their parents and

by self-feeding directly from the carcass and are

therefore only partially dependent on their parents

(Smiseth et al. 2003). The transition to indepen-

dence is associated with a decrease in larval begging

in the parent’s presence with no apparent change in

the parent’s behaviour towards larvae, suggesting

that the transition to independence is largely under

offspring control (Smiseth et al. 2003). However, a

more recent study found that the peak in parental

food provisioning in asynchronous broods occurred

at the same time relative to the hatching of the first

larvae, regardless of variation in the peak demands

associated with different levels of asynchronous

hatching (Smiseth & Morgan 2009). Thus, this latter

study suggests that parents may exert some control

over the transition to independence through some,

as yet, unidentified non-behavioural mechanism.

Recent studies suggest that chemical cues provide

insect parents with a potential non-behavioural

mechanism for controlling the allocation of food to

their offspring. In both burrower bugs (Sehirus cinctus)

and common earwigs (Forficula auricularia), females

adjust food provisioning in response to cuticular

hydrocarbon (CHC) profiles or volatile chemical sig-

nals produced by larvae (Kolliker et al. 2006; Mas

et al. 2009). Recent studies on N. vespilloides show

that the CHC profile is flexible as adults express a dif-

ferent CHC profile while breeding and providing care

for larvae than they do otherwise (Muller et al. 2003;

Steiger et al. 2007, 2008, 2009; Haberer 2010). Fur-

thermore, a recent study shows that chemical cues

from caring parents trigger larval begging, providing

larvae with a potential mechanism for reducing dis-

crimination costs associated with begging. Given that

these beetles normally breed in darkness under-

ground, such chemical cues are thought to allow lar-

vae to detect whether the parents are nearby, thus

presenting a potential feeding opportunity, or

whether the parents are away performing other

parental tasks (Smiseth et al. 2010). Based on these

findings, we hypothesise that caring parents might

advance the transition to independence by altering

the expression of their cuticular chemical cues at crit-

ical periods during offspring development. If parents

can advance the transition to independence by alter-

ing their cuticular chemical cues, we predict that

begging larvae should behave differently towards

parental chemical cues from different stages of off-

spring development. Furthermore, if the changes in

begging behaviour are largely driven by these altera-

tions in the parents’ cuticular profiles, we predict that

larvae of different ages should behave in a similar

manner towards parental chemical cues from a given

stage of development.

Materials and Methods

The beetles used in the experiments were from three

outbred laboratory populations derived from Corstor-

phine Hill (Edinburgh), Sunbank Wood (Manches-

ter) and Kennel Vale (Cornwall). Prior to the

D. Leigh & P. T. Smiseth Parent–Offspring Conflict over the Transition to Independence

Ethology 118 (2012) 460–465 ª 2012 Blackwell Verlag GmbH 461

experiments, all beetles were housed individually in

clear plastic containers (12 · 8 and 2 cm high) filled

with moist soil at 20�C. Non-breeding beetles were

fed small pieces of organic beef twice a week. For

use in the experiments, we randomly selected pairs

of non-sibling males and females, which were placed

in plastic containers provided with 1 cm of moist soil

and a previously frozen mouse carcass (10–20 g)

supplied by Livefoods Direct Ltd, Sheffield, UK.

Experimental beetles were between 12 and 60 d of

adult age. After pairing, all beetles were treated in

line with established protocols to create broods of a

standardised size and age (Smiseth et al. 2003,

2007a,b). We removed males 65 h after pairing, as

potential effects of males on larval and female

behaviour were beyond the aims of this study. At

the same time, we transferred the female and the

carcass to a fresh container, leaving the eggs to

hatch in the original container. The original contain-

ers were checked for newly hatched larvae four

times a day. Freshly emerged larvae were used to

establish broods of ten larvae of mixed maternity,

which were given to a female once her own eggs

had begun to hatch.

We designed two experiments to investigate

whether parents can advance the transition to inde-

pendence by altering the expression of their cuticular

chemical cues during larval development. In the first

experiment, we tested the prediction that begging

larvae behave differently towards parental stimuli

from different stages of offspring development. To

test this prediction, we presented 24-h old focal lar-

vae with an unfamiliar dead female that had cared

for broods that were 24, 48, 72 or 96 h old. We

tested for effects on the begging behaviour of 24-h

old larvae as larval begging peaks at this age (Smiseth

et al. 2003). We used dead females that had cared for

broods of 24-, 48-, 72- or 96-h old larvae as stimuli,

as this age range represents different stages in larval

development at either side of the transition to inde-

pendence. Larvae continue to beg for food from their

parents until roughly 72 h after hatching (Smiseth

et al. 2003); thus, 24 and 48 h after hatching repre-

sents stages where the larvae are dependent on their

parents (although there is a decrease in time spent

begging during this period), while 72 and 96 h after

hatching represents stages after nutritional indepen-

dence. In the second experiment, we tested the pre-

diction that larvae of different ages behave similarly

towards parental chemical cues from the same stage

of development. To test this prediction, we reversed

the treatments from the first experiment and pre-

sented 24-, 48-, 72- and 96-h old focal larvae with

an unfamiliar dead female that had cared for 24-h

old broods. The data from the second experiment

were also used to test whether larvae of different

ages altered their begging behaviour when exposed

to parental stimuli from different stages of offspring

development. To this end, we compared the behav-

iour of 48-, 72- and 96-h old larvae when presented

with an unfamiliar dead female that had cared for

24-h old broods and when presented with unfamiliar

females that had cared for broods that were of the

same age as the larvae.

We conducted behavioural observations by pre-

senting a brood of focal larvae with an unfamiliar,

dead female that had been caring for a brood of a

known age. Previous studies show that larvae

continue to beg for at least 2 h when presented with

a dead parent (Smiseth & Parker 2008; Smiseth

et al. 2010). We removed the female from her

brood 30 min before the observation and killed her

by placing her in a freezer ()23�C) for 20 min. The

female was thawed for 10 min at room temperature

and then pinned inside a clear plastic box lined with

moist tissue. The larvae were removed from their

container 10 min before the observation, counted

and weighed to the nearest 0.1 mg. The larvae were

placed next to the pinned female and allowed 1 min

to settle before the observation started. We recorded

larval begging using instantaneous scan sampling

every 1 min for 30 min. Begging was defined as lar-

vae touching the female with their legs (Smiseth &

Moore 2002). We calculated time spent begging as

B = (Rb ⁄ l)(100 ⁄ 30), where Rb represents the total

begging events, and l represents the mean number of

larvae present during each scan (Smiseth & Moore

2002). Because the data on begging are proportional,

they were transformed using arcsine square root

transformation. We observed larval begging from ten

broods of each treatment group. We used general lin-

ear models (GLM) in SPSS 17.0 (SPSS Inc., Chicago,

IL, USA) to analysis the data.

Results

We first tested the prediction that begging larvae

behave differently towards parental chemical cues

from different stages of offspring development by

presenting 24-h old larvae with an unfamiliar dead

female that had cared for broods that were 24, 48,

72 or 96 h old. The developmental stage of the

female’s own brood had no significant effect on

the time the focal larvae spent begging (GLM:

F3,36 = 0.79; p = 0.51; Fig. 1a). Thus, there was no

evidence that larvae changed their behaviour in

Parent–Offspring Conflict over the Transition to Independence D. Leigh & P. T. Smiseth

462 Ethology 118 (2012) 460–465 ª 2012 Blackwell Verlag GmbH

response to potential alterations in the female’s

chemical profile throughout larval development.

We next tested the prediction that larvae of differ-

ent ages behave similarly towards parental chemical

cues from the same stage of development by pre-

senting 24-, 48-, 72- and 96-h old focal larvae with

an unfamiliar dead female that had cared for 24-h

old broods. Larval age had a highly significant effect

on the amount of time spent begging (GLM: F3,36 =

7.50, p < 0.001). Larvae spent the greatest amount

of time begging when they were 24 h old, while

older larvae spent less time begging (Fig. 1b).

Finally, we tested whether different-aged larvae

altered their begging behaviour when exposed to

chemical cues from females that had cared for broods

at different stages of development. To this end, we

presented 48-, 72- and 96-h old focal larvae with an

unfamiliar dead female that either had cared for 24-h

old broods or, as a control, with an unfamiliar dead

female that had cared for broods of the same age as

the focal larvae. The age of larvae had no significant

effect on time spent begging (GLM: F2,54 = 11.1,

p = 0.083), suggesting that larval begging changed

little with age once the larvae had reached the age of

48 h. Likewise, there was no significant difference in

the time spent begging when larvae were presented

with females that had cared for 24-h old broods or

for broods that were of the same age as the larvae

(GLM: F1,54 = 0.29, p = 0.65). Finally, there was no

significant effect of the interaction between the lar-

vae’s own age and the age of the dead female’s origi-

nal brood (GLM: F2,54 = 0.30, p = 0.74).

Discussion

Here, we report the results from an experiment

examining parent–offspring conflict over the timing

of the offspring’s transition to nutritional indepen-

dence in an insect where parents provide their off-

spring with food after hatching. We specifically

tested whether insect parents can advance the tran-

sition to nutritional independence by altering their

cuticular chemical cues throughout offspring devel-

opment. In contrast to what we predicted, we found

that begging larvae did not behave differently when

presented with parental chemical cues from different

stages of offspring development and that larvae of

different ages still behaved differently when pre-

sented with parental chemical cues from the same

stage of development. Based on these findings, we

conclude that there is no support for the hypothesis

that N. vespilloides parents can advance the transition

to independence by adjusting their chemical cues

throughout offspring development.

Our results support a previous study suggesting

that the transition to nutritional independence in

N. vespilloides is largely under offspring control

(Smiseth et al. 2003). This conclusion was based on

behavioural evidence showing that transition to

independence is associated with a decrease in larval

begging in the parent’s presence with no apparent

change in parental behaviour towards larvae (Smis-

eth et al. 2003). However, this conclusion should not

be taken as definite as the study did not consider all

potential mechanisms by which parents might

advance the transition to independence. Although

our study excludes the possibility that parents can

6

10

0

(a)

4

24 48 72 96

4

5

0

(b)

3

24 48 72 96

Larval age (h after hatching)

Tim

e sp

ent b

eggi

ng (%

)Ti

me

spen

t beg

ging

(%)

Age of brood cared for by stimulusfemale (h after hatching)

8

2

2

1

12

Fig. 1: Percentage of time spent begging by Nicrophorus vespilloides

larvae presented with an unfamiliar dead female (mean � 1 SE). (a)

Begging by 24-h old larvae presented with a female that had been car-

ing for either 24-, 48-, 72- or 96-h old broods. (b) Begging by 24-, 48-,

72- or 96-h old larvae presented with a female that had been caring

for 24-h old broods. The sample size in all treatment groups is ten

broods.

D. Leigh & P. T. Smiseth Parent–Offspring Conflict over the Transition to Independence

Ethology 118 (2012) 460–465 ª 2012 Blackwell Verlag GmbH 463

advance the transition to nutritional independence

by altering their cuticular chemical cues, it is still pos-

sible that parents may do so through an unidentified

mechanism. Indeed, it should be noted that a recent

study on asynchronous hatching in N. vespilloides sug-

gests that parents have some degree of control over

this transition, as the peak in parental food provision-

ing towards the brood occurred at the same time rela-

tive to the hatching of the first larvae, regardless of

considerable variation in the timing of the peak in

food demand between synchronous, asynchronous

and highly asynchronous broods (Smiseth & Morgan

2009).

Studies on birds and mammals provide mixed evi-

dence concerning who controls the transition to

nutritional independence. For example, cross-foster-

ing experiments on black kites (Milvus migrans) sug-

gest that the transition to independence is under

offspring control. In this species, parents prolong the

food provisioning period when provided with foster

chicks that are younger than their own, while older

chicks placed into nests that are cared for by foster

parents whose own chicks are younger rejected care

beyond their normal age of independence (Busta-

mante 1994). However, similar experiments on gui-

nea pigs (Cavia aperea) suggests that the transition to

independence is under parental control, as foster pups

that were younger than the female’s own pups were

weaned at a much younger age than control pups

and foster pups that were older than the female’s

own pups benefited from continuing lactation of fos-

ter mothers (Rehling & Trillmich 2007). The lack of a

consistent pattern between studies suggests that there

is considerable variation between species in which

party controls the transition to independence, which

begs the question of which conditions will tend to

favour parental or offspring control?

Currently, we have a poor understanding of the

conditions under which offspring or parents may be

expected to have an increased degree of control over

the transition to independence. We propose three

alternative explanations for the apparent offspring

control over this transition in N. vespilloides. First,

there may be little or no divergence in parent and

offspring optima with respect to the timing of the

transition to nutritional independence. This situation

might be satisfied when the costs of food provision-

ing to the parents are very low, as might be the case

for N. vespilloides where food is provided from the

carcass within which the larvae and parents reside.

In this situation, there may only be very weak selec-

tion on parental traits that would advance the tran-

sition to independence. In contrast, parental birds

may pay a high energetic cost of food provisioning

as they obtain food from the surrounding environ-

ment (Drent & Daan 1980). Thus, there may be rela-

tively strong selection on parental traits that advance

the transition to independence in birds, which may

have favoured the evolution of parental traits that

advance this transition, such as increased latency of

feeding (Davis 1976) and aggression towards the off-

spring (Leonard et al. 1991).

Second, offspring may have a greater degree of

control when parents have poor or incomplete infor-

mation about the offspring’s developmental stage

because of variation in environmental conditions.

For example, in N. vespilloides and other insects, par-

ents may have incomplete information about the

progression of the offspring’s development because

of variation in temperature, brood size and level of

hatching asynchrony (Smiseth et al. 2006). When

this is the case, parents may pay a price if they

employ a fixed strategy for when to terminate care

as they risk abandoning their offspring prematurely,

thereby jeopardising their offspring’s future survival.

Thus, a greater degree of offspring control may be

inevitable when parents benefit from adjusting the

termination of care in response to environmental

conditions that affect their offspring’s development.

Finally, offspring control may be associated with

the absence of parental traits that can advance the

timing of the transition to nutritional independence.

For example, there is no evidence that N. vespilloides

parents alter the cuticular chemical cues that trigger

larval begging, abandon their larvae, increase their

feeding latency when responding to larval begging,

or show some form aggression towards larvae at the

time of independence. In the absence of such paren-

tal traits, parents may have no means for advancing

the transition to nutritional independence even if it

were in their best interest to do so.

In conclusion, we show that begging larvae do not

behave differently when presented with parental

chemical cues from different stages of offspring

development and that larvae of different ages still

behave differently when presented with parental

chemical cues from the same stage of development.

Based on our results, we conclude that there is no

evidence for the hypothesis that N. vespilloides par-

ents can advance the transition to independence by

altering their cuticular chemical profile.

Acknowledgements

We thank Edinburgh Countryside Rangers for

permission to collect beetles at Corstorphine Hill,

Parent–Offspring Conflict over the Transition to Independence D. Leigh & P. T. Smiseth

464 Ethology 118 (2012) 460–465 ª 2012 Blackwell Verlag GmbH

Allen Moore for beetles from Kennel Vale and

Danny Rozen for beetles from Sunbank Wood. We

thank Ronnie Mooney for help with animal hus-

bandry and two anonymous reviewers for valuable

comments. PTS is funded by a grant from NERC

(NE ⁄ G004293 ⁄ 1).

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