Behavioral Ecology Vol. 12 No. 5: 590-599
© 2001 International Society for Behavioral Ecology
Offspring food allocation by parents and helpers in a cooperative mammal
a Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK b Mammal Research Institute, University of Pretoria, Pretoria, 0002, Republic of South Africa
Address correspondence to P.N.M. Brotherton, who is now at English Nature, Northminster House, Peterborough PE1 1UA, UK. E-mail: peter.brotherton{at}english-nature.org.uk .
Received 20 September 1999; revised 20 May 2000; accepted 17 November 2000.
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ABSTRACT |
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In cooperatively breeding species, helpers and parents commonly face two decisions when they find a food item: first, whether to feed the item to a young group member or to eat it themselves; and second, which offspring to feed. Little is known about the factors that influence these decisions in cooperative mammals, though optimal foraging theory provides a basis for a range of predictions. In this article we describe pup feeding behavior by helpers and parents in a cooperative mongoose, the meerkat (Suricata suricatta). When meerkat pups begin accompanying the group, they beg food from older group members, who dig up dispersed prey items. As predicted, the probability of a prey item being fed to a pup shows a positive relationship with prey size and a negative relationship with pup distance. Meerkats apparently follow a "feed the nearest pup rule" and are more likely to feed the nearest pup if it is hungry. Hungrier pups beg more and follow older group members more closely. Across all age categories, females feed pups more frequently than males, both in terms of the relative frequency of feeds, and the proportion of prey biomass found by each individual that is fed to pups. Females also feed female pups significantly more than male pups, while males feed pups of both sexes equally. These sex biases in feeding contributions may result from female group members benefiting more than males from higher pup survival, and in particular higher female pup survival, because females are the philopatric sex.
Key words: cooperative breeding, group augmentation, kin selection, meerkat, offspring feeding rules, sex-biased parental care.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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In cooperatively breeding mammals and birds, individuals exhibit parentlike behavior toward offspring that are not genetically their own. This helping may include constructing nests or burrows, feeding, incubating, grooming, and guarding (see Solomon and French, 1997
; Stacey and Koenig,
1990). Perhaps the most widespread helping behavior is providing
food to young. Through increasing the provisioning rate of young, helpers may
increase the number and improve the condition of offspring raised by the
group, and this is likely to explain both why there is a positive correlation
between helper number and recruitment in many cooperative species (e.g.,
Brown et al., 1982;
Emlen and Wrege, 1989;
Moehlman, 1979) and why
cooperative mammals often have larger litters than noncooperative relatives
(Moehlman and Hoffer, 1997).
Although offspring typically benefit from helping behavior, it is often not
clear whether helpers derive direct or indirect benefits from their actions
(see Emlen, 1991). Answers to
two questions may help us distinguish between these possibilities. First,
which helpers provide the most help? Second, do particular helpers favor
particular offspring and, if so, which? These questions are most relevant when
individuals experience different costs and benefits to helping, perhaps due to
variation in their relatedness to the offspring, their capacity to help, or
the likelihood that the offspring will provide some future benefit. Almost no
field studies of cooperative mammals have quantified the contributions made by
different helpers (but see Owens and
Owens, 1984), and even studies of cooperative birds have typically
measured only relative contributions to helping, running the risk that these
may not accurately reflect different levels of effort if foraging abilities
differ between helpers. Similarly, little is known about the factors
influencing food allocation between offspring in cooperative mammals.
Here we describe the provisioning behavior of helpers and parents in a
cooperative mammal, the meerkat (Suricata suricatta). Meerkats live
in groups of 2-30 individuals in the drier regions of southern Africa
(Skinner and Smithers, 1990).
Multiple adult males and females are normally present in each group, with one
member of each sex socially dominant, and these dominant individuals are the
parents of more than 75% of pups born in the group
(Clutton-Brock et al., 2000;
Griffin, 1999). Meerkat groups
typically contain males and females of varying ages who were born in the group
and are related to the dominant female, and one or more adult immigrant males
(who are normally brothers). The dominant male in each group is normally an
immigrant, and incest has never been recorded
(Griffin, 1999). Meerkat pups
are born in a burrow and are initially entirely dependent on their lactating
mother for nutrition (although occasionally other females allolactate;
Clutton-Brock et al., 1999b).
When the pups begin moving with the group, they are initially incapable of
finding prey themselves and instead noisily beg food, which consists mainly of
burrowing invertebrates, from older group members
(Doolan and Macdonald, 1996).
In this study, we considered the factors that affect pup feeding rates and
compared the contributions of different sex and age categories of helper.
Because we were able to record every food item captured by foraging meerkats,
we were able to make comparisons based on both the proportion of captured prey
biomass that individuals fed to pups and the relative frequency of pup feeding
events. We also investigated how pup hunger levels and spacing behavior
influence feeding rate and considered what factors determine which pups are
fed by older group members.
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METHODS |
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Study area
Data were collected at two study sites in the South African Kalahari, one in the Kalahari Gemsbok National Park (25°17' S, 20°32' E; the "Park") and one on ranchland (26°59' S, 21°50' E) close to Van Zyl's Rus about 120 km to the southeast (the "Ranch"). Both areas comprise a mixture of dry river-beds, river terraces, and sparsely vegetated sand dunes (Leistner and Werger, 1973
;
Rooyen et al., 1991). Annual
rainfall at both sites averages around 250 mm
(Clutton-Brock et al., 1999a),
most of which falls in the hot, wet season between October and April.
Temperatures can exceed 40°C during the hottest months, while minimum
temperatures may drop below freezing in the cold, dry season
(May—September).
Data collection
We collected data during more than 7000 h of observation of seven Park and
nine Ranch meerkat groups between 1995 and 1999. All meerkats in the study
groups could be recognized individually and were habituated to observation
within a few meters. We sexed individuals soon after they emerged from their
natal burrow and classified them as pups from 0 to 3 months, juveniles from 3
to 6 months, subadults from 6 to 12 months, and adults at 12 months and
older.
During group foraging sessions, we attempted to record all pup feeds, locating pups by the begging calls, which they emit almost continuously. The proportion of feeding events that we detected is likely to have been affected by various factors including the number of observers, size of the group, number of pups, group dispersion, and habitat type (e.g., height of grass). We therefore used these ad lib data to make comparisons within breeding events to determine relative feeding contributions by different group members and to establish which pups were fed most and what types of prey items they were fed. We were careful to ensure that all individuals were observed equally to avoid any systematic bias in our data. For each pup feeding event, we noted the identities of the donor and recipient, and the type (e.g., scorpion) and size of prey item.
We divided prey items into three size categories: small (items fitting entirely within the mouth of an adult), medium (items too large to fit inside the mouth, but where less than half the item was protruding), and large (items which mainly hung outside the mouth). Analysis of dropped or intercepted prey items revealed that small items had an average (wet) mass of 0.172 g, medium items 0.721 g, and large items 3.990 g. We used these average weights to estimate the biomass of prey items captured and fed to pups.
In addition to the ad lib records of pup feeds, we conducted continuous focal watches of foraging individuals during which we recorded the size and identity of every prey item captured and whether or not the prey was fed to a pup. If the prey was fed, we recorded the distance and identity of the recipient and whether or not it was the closest pup to the foraging individual. If the foraging individual consumed the prey instead of feeding it to a pup, we recorded its distance to the nearest pup. Each focal lasted 20 min, and we usually conducted at least 10 focals on each helper and parent during the first 6 weeks of pup feeding. When focal data are used in analyses, they are referred to as "proportion of items captured," distinguishing them from ad lib data, which are termed "relative contributions."
We estimated time budgets of pups and older group members by conducting
instantaneous scans (Altmann,
1978) at 10-min intervals during foraging periods. When a begging
pup was recorded within 2 m of a foraging adult, we recorded the identity of
the adult (hereafter this behavior is termed "social foraging").
We calculated time budgets by taking the proportion of scans an individual
spent in each activity each day and then taking the average across days.
Because our analysis of focal data revealed that most pup feeds occurred during the first 40 days after the pups began accompanying the group, our other analyses were restricted to this period unless otherwise stated. The end of this 40-day period is, on average, approximately 70 days after the pups were born, which is equal to the minimum interbirth interval for females. Restricting the analyses to this period therefore also removed any confounding effects of another litter being born into the group.
Pup feeding experiment
To investigate the effect of pup hunger levels on helper feeding and pup
behavior, we provisioned pups in 12 litters. We randomly assigned half the
pups in each litter to an experimental group and fed each of these pups with
about 12 g of hard-boiled egg at the start of morning and evening activity
periods. During each day of the experiment, we recorded the number of feeds
given to each pup and compared the average number of prey items given to
experimental and control pups. We also used scan data to calculate the
proportion of time that each pup spent social foraging, and, in nine of the
experiments, the proportion of time pups spent begging. Each experiment lasted
between 2 and 5 days; this variation was due to pups in some litters
disappearing through natural events.
Statistical analysis
Our analyses involve data collected from between 1 and 8 breeding events in
each of 16 study groups. This represents a total of 97 breeding events (each
of which was assigned a unique litter code), though some analyses used fewer
litters (specified in the text) because all types of data were not available
for every litter. We present analyses that are based on both within-litter and
between-litter comparisons. The former typically involve the use of
proportional data which were normalized using an arcsine-square root
transformation. Where this transformation failed to normalize the data, we
either fitted the data to a binomial distribution using generalized models or
used nonparametric tests. The comparisons across breeding events usually
involved data that were unbalanced and not orthogonal and so we used residual
maximum likelihood (REML) models or, if the data could not be normalized,
generalized linear mixed models (GLMM;
Schall, 1991). REML and GLMM
models are equivalent to general linear regression (with normal errors) and
generalized linear models (with non-normal errors), respectively, except they
allow both random and fixed effects to be defined. In our analyses it was
necessary to define litter code as a random term because of repeated sampling
(multiple helpers and multiple pups) within breeding events. In all analyses,
we found no significant effect of group identity, and hence breeding events,
rather than groups, were used as our independent points. We included all
likely explanatory variables in the maximal regression models and dropped
terms sequentially until the models only included terms whose elimination
would significantly decrease the explanatory power of the model. The
significance of terms was tested using the Wald statistic, which is
distributed as chi-square. Statistical analyses were performed using Genstat
5.4 and Minitab 12. Multiple comparisons tests follow Zar
(1999).
Pup age analysis
To investigate the effect of pup age on the probability of an item being
fed, we pooled all focal data collected on each day from each litter. These
data were analyzed using a GLMM fitting the total number of prey items fed on
each day to a binomial distribution with the number of items found as the
denominator.
Analysis of biomass of captured prey fed to pups
For each individual, we pooled all data from focal watches conducted during
the first 40 days of pup feeding and calculated the proportion of the total
biomass of captured prey that was fed to pups. These data were then
arcsine-square root transformed and analyzed using a REML model.
Analysis of the effect of pup sex
To investigate whether individuals preferentially fed pups of one sex, we
excluded single-sex litters and used the ad lib data to calculate, for each
group member, the relative proportion of prey items fed to each pup each day
and then took the average across days. Because these relative proportions are
affected by the number of pups present, we obtained a standardized measure for
each feeder by taking the average contribution to pups of each sex and then
substracting the value for males from the value for females and dividing the
result by the mean of these two values. This gives a standardized index of the
degree of any sex bias, with negative values indicating that males are fed
more, and positive values indicating that females are fed more. These data
were then compared across litters using a REML model. We used a similar method
of analysis to investigate whether pups preferentially associated with helpers
of a particular sex.
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RESULTS |
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The onset and duration of pup feeding behavior
Pups began accompanying the group an average (± SE) of 29.3 ± 0.43 days after they were born (range 17-42 days, n = 97 litters). For these 97 litters, there was no difference in the age at which pups began moving with the group at the Ranch or Park (Ranch: 29.0 ± 0.53 days, Park: 29.7 ± 0.72 days, t test: t = 0.57, n = 70, 27, p =.57). Pups in larger litters began accompanying the group later: for 76 breeding events in which only one female gave birth, there was a positive correlation between the size of the litter and the age at which the pups first began accompanying the group (r =.384, n = 76, p <.001). Pup feeding behavior effectively began on the first day that the pups moved with the group. Lactating mothers were not fed by other group members, and little food was taken to pups down their natal burrow: fewer than 30 out of more than 32,000 observed feeding events were to preemergent pups.
Most pup feeds occurred within the first 40 days after the pups began accompanying the group. A GLMM of focal data showed that feeders gave pups approximately 10% of all medium and 25% of all large prey items captured on the first day that the pups accompanied the group. This increased to approximately 20% and 45%, respectively, by the 20th day and decreased thereafter, until pup feeding had effectively ceased by the 60th day. On average, less than 5% of small items were fed, even at the peak feeding age (Figure 1, Tables 1 and 2).
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Factors affecting pup feeding decisions
Prey type and size
Of 21,578 identified prey items fed to pups, the majority were reptiles
(mostly geckos and lizards), beetles (larvae, pupae, and adults) and scorpions
(Table 3). Focal data showed
that the proportion of captured prey items fed to pups increased with prey
size (percentage of captured large items fed = 41.1 ± 3.3%, medium =
20.6 ± 1.7%, small = 4.2 ± 0.6%; two-way ANOVA [blocking within
litters]: F2,96 = 147.0, p <.001; multiple
comparisons tests revealed all sizes differ from each other). Consequently,
although most prey items captured by foraging individuals were small (an
average of 75.0 ± 1.5% of captured items were small, 20.1 ± 1.1%
were medium, and 4.9 ± 0.6% were large), ad lib data revealed that only
31.5 ± 2.3% of prey items actually fed to pups were small, while 46.4
± 1.6% were medium, and 22.1 ± 2.2% were large (n = 44
litters). Although pups' capacity to handle larger prey items presumably
improves with age, there was no greater tendency to feed larger prey items to
older pups (Figure 2, two-way
ANOVA: F4,116 = 1.56, p =.19). This analysis used
all ad lib records of pup feeding events to each litter, pooled within five
age categories of pup (Figure
2).
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Pup distance
Captured prey items of all sizes were more likely to be fed if there was a
pup nearby: the average distance that prey was carried to a pup was
significantly shorter than the distance to the nearest pup when items were not
fed (Figure 3; paired
t tests: large items, t = 3.48, n = 24 litters,
p <.001; medium items, t = 4.80, n = 26,
p <.001; small items, t = 8.44, n = 25,
p <.001). Medium and large prey items fed to pups were carried
significantly farther than small items
(Figure 3; two-way ANOVA:
F2,46 = 4.64, p =.015; multiple comparison tests
reveal no difference between large and medium items, but both were carried
farther than small items).
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Age and sex of helper
Across all age categories, females contributed a significantly greater
proportion of the total number of prey items fed to pups than did males (ad
lib data, Figure 4; Wilcoxon
tests: juveniles z = 2.67, n = 15, p <.01;
subadults z = 2.54, n = 24, p <.01, adults
z = 3.26, n = 44, p <.001). Within each sex
there was a significant effect of age category
(Figure 4; Freidman tests:
females, n = 11 litters, 
2 = 14.36, df = 2,
p <.001; males, n = 11 litters, 2 =
13.64, df = 2, p =.001). Multiple comparison tests revealed that in
both sexes this result was due to juveniles feeding significantly less than
other age categories, but there was no difference in the feeding rate of
adults and subadults. Across the feeding period as a whole, there was no
difference between the relative contributions of helpers and parents of the
same sex (ad lib data, Wilcoxon tests: females z = 0.823, n
= 31, p =.41; males z = 1.581, n = 23, p
=.11). However, the contribution of mothers, relative to that of other group
members, was significantly less to young pups than to older ones
(Figure 5, Freidman test,
n = 27 litters, 2 = 13.64, df = 2, p
<.001; multiple comparisons tests revealed that the contribution to pups at
1-10 days was significantly less than at all other ages).
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The higher overall feeding rate by females may in part be attributed to
differences in time budgets because adult females spent a slightly higher
proportion of time foraging during the pup feeding period. This was
particularly true among the parents, among which males spent substantially
less time foraging than females (Tables
4 and
5). Other terms significant in
this REML analysis were group size (individuals spent more time foraging in
larger groups) and study site (individuals spent more time foraging on the
Ranch than in the Park). Both of these results are likely to be due to a
trade-off between foraging and vigilance because individuals are more vigilant
both in smaller groups and in the Park, where the risk of predation is higher
than on the Ranch (Clutton-Brock et al.,
1999a).
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Overall, foraging individuals gave pups an average of 18.4 ± 1.4% of
the total biomass they caught during the first 40 days of pup foraging.
Females of all age categories fed pups a significantly higher proportion of
the biomass of prey items that they captured than males (Tables
6 and
7). Parents fed pups a similar
proportion of prey items as other adults of the same sex (when parentage is
included as a term in the REML, it is insignificant: 2 = 0.0,
df = 1, p =.99). The proportion of prey biomass fed to pups was
higher as group size decreased and as litter size increased (Tables
6 and
7). Adults and subadults also
fed pups a higher proportion of prey biomass than juveniles of the same sex
(Tables 6 and
7).
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Factors affecting which pup is fed
Proximity to pup
Helpers almost invariably fed the closest begging pup: of 1072 feeding
events by 112 individuals from 9 different groups, 1033 (96.4%) were to the
nearest pup. When the pups began accompanying the groups during foraging
periods, they initially spent approximately 40% of their time begging within 2
m of a foraging group member ("social foraging"). This increased
to about 55% 17 days after they began accompanying the group and declined
thereafter (Figure 6).
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Social foraging data for each pup were pooled for the first 40 days after
the pups began following the group and fitted to a REML model. This analysis
showed that pups in larger groups spent a higher proportion of time socially
foraging than pups in small groups. In addition, they socially foraged more if
the sex ratio of helpers was female biased (Tables
8 and
9). Female pups spent
significantly more time socially foraging than male pups, although the
difference was small (< 2% of their time budgets; Tables
8 and
9). Pups socially foraged more
with older individuals than with younger individuals, and more with adult
females than with adult males (Figure
7; Friedman test, n = 8 litters, 2 =
14.5, df = 5, p =.01; multiple comparisons tests revealed a
significant difference between adult females and all other categories apart
from sub-adult males, and a significant difference between adult males and
subadults of both sexes and juveniles of either sex). There was no significant
difference in the proportion of time that pups spent social foraging with
parents and other adults of the same sex (Wilcoxon signed-ranks tests:
females, T = 147, n = 25, p =.69; males, T
= 110, n = 20 p =.87).
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Pup feeding experiment
Pups that were provided with egg at the start of foraging sessions spent
less time begging and less time socially foraging than control pups (median
percentage of time spent begging by fed pups = 51.8%, interquartile range
[IQR] = 47.4-59.0%; control pups = 64.3%, IQR = 51.0-69.1%; Wilcoxon
signed-ranks test: T = 0, n = 9, p <.01; median
percentage of time spent social foraging by fed pups = 45.8%, IQR =
36.3-56.6%; control pups = 55.4%, IQR = 38.9-66.8%; Wilcoxon signed-ranks
test: T = 10, n = 12, p <.05). Probably as a
result of this, they were fed fewer prey items by older group members than
were control pups (Figure 8;
Wilcoxon signed-ranks test: T = 1, n = 12, p
<.005).
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Pup sex
Female group members fed female pups significantly more prey items than
they fed to male pups, whereas male group members fed pups of both sexes
equally (Tables 10 and
11). REML analyses showed that
this was not due to female pups associating more strongly with females, as
male and female pups spent the same proportion of their social foraging time
with female group members. This was the case when considering all group
members, and when the analysis was restricted to adults (REML analyses with
litter code fitted as random term: all group members, Wald statistic for pup
sex, 2 = 0.11, df = 1, p =.74; adult group members,
Wald statistic for pup sex, 2 = 0.06, df = 1, p
=.81).
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DISCUSSION |
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In meerkats, helpers begin provisioning pups when they leave their natal burrow and start begging noisily for food. Unlike helpers in some other social carnivores (e.g., black-backed jackals: Moehlman, 1979
; wild dogs:
Malcolm and Marten, 1982),
meerkats do not feed lactating females, nor do they feed the young at the
burrow as do dwarf mongooses (Rood,
1978), black-backed jackals
(Moehlman, 1979), and
Ethiopian wolves (Sillero-Zubiri,
1994). This is probably because meerkats feed on small, dispersed
food items, and repeated trips back to the burrow would be both uneconomical
and dangerous (Clutton-Brock et al.,
1999a). As a result, pup feeding in meerkats is perhaps more
analogous to fledgling feeding in birds than it is to offspring feeding in
many other social carnivores.
Feeding rules in meerkats
When foraging individuals capture a prey item, they are faced with the
choice of feeding it to a pup or eating it themselves. The probability of
captured prey being fed to a pup increases as the size of the item increases,
and all sizes of prey items are more likely to be fed if there is a pup
nearby. In addition, large and medium items are carried farther than small
items. These observations are consistent with the marginal value theorem,
which predicts that it only pays individuals to work hard (in this case
deciding to feed or traveling farther) if the return (in terms of biomass of
food given to a pup) is high (Krebs and
Kacelnik, 1991).
When individuals do feed a pup, it is almost invariably the nearest, and a
similar tendency to feed the closest chick has been observed in a number of
studies of nestling birds (e.g., Kacelnik
et al., 1995; McRae et al.,
1993). From a parent's perspective, this strategy may lead to
optimal food allocation among the brood if the chicks are free to compete for
position in the nest, and they compete more strongly when they are hungry
and/or larger (Kilner and Johnstone,
1997; Mock and Parker,
1997). In contrast, because meerkat pups are dispersed, the
nearest will often not be the hungriest, so following a strategy of always
feeding the closest pup would appear at odds with status signaling
explanations of resource allocation among offspring
(Godfray, 1995). However,
despite their overwhelming tendency to feed the nearest pup, meerkat group
members are nonetheless responsive to pups' needs. Manser
(1998) showed that meerkats
increase their feeding rates in response to playbacks of pup begging calls and
preferentially approach louder playbacks (which simulated closer pups) with
food items. Thus, hungrier pups would be expected to increase their
probability of being fed by begging more and by following group members more
closely. This is precisely what control pups did relative to pups fed with egg
in the provisioning experiment described here, and control pups were indeed
fed more frequently.
Despite following a feed the nearest pup rule, helpers and parents still preferentially feed hungry pups because the decision that they make is not which pup to feed but whether to feed at all. This decision appears to be a graded response dependent on the strength or type of the begging signal from the nearest pup (it may also be influenced by the sex of the pup, as discussed below). Because meerkat groups are dispersed while foraging, this may be a more efficient strategy than attempting to compare hunger levels among pups. The extent to which this will prove a general feeding rule in other situations where offspring are dispersed remains to be seen.
What determines who helps?
The factors that determine which sex helps most in cooperative species have
been the subject of considerable debate (see
Cockburn, 1998). In many
cooperative bird species it is primarily or exclusively only one sex that
helps, typically males (e.g., long-tailed tits:
Hatchwell and Russell, 1996;
superb fairy wrens: Mulder et al.,
1994; pied kingfishers: Reyer,
1990), although there are some species where only females help
(e.g., Seychelles warblers: Komdeur,
1992). In his comprehensive review, Cockburn
(1998) suggested that most
cases of helping by one sex may ultimately be explained as an epiphenomenon of
sex-biased dispersal to avoid inbreeding. However, some cooperative bird
species (e.g., splendid fairy wrens:
Russell and Rowley, 1993;
Florida scrub jays: Woolfenden and
Fitzpatrick, 1990) and most cooperative mammals (e.g., banded
mongooses: Rood, 1975; dwarf
mongooses: Rood, 1978; brown
hyenas: Owens and Owens, 1984;
hunting dogs: Malcolm and Marten,
1982) have helpers of both sexes. Quantifying the relative
contributions by the sexes in these species may help us understand the
function of helping behavior, yet few studies have done so (but see
Owens and Owens, 1984).
Female meerkats fed at higher rates than males in all age categories, and
this was true both in terms of the total number of items delivered and the
proportion of captured prey biomass that they gave away. In doing so, females
also spent a higher proportion of their time foraging, and this may have
exposed them to higher predation risk
(Clutton-Brock et al., 1999c).
Within groups, juveniles fed less than older individuals, and this may be
because giving away prey items is more costly to juveniles, who face the extra
energetic demands of growth while also being less successful foragers than
older individuals (Barnard JA, unpublished data). Pups also spent more time
socially foraging with older individuals, especially adult females, who fed
pups more frequently than adult males. However, these association patterns may
result from these individuals taking more food to the pups, rather than any
tendency for pups to associate with the best feeders per se.
To what extent do these results help us determine why helpers actually help
(Emlen, 1991)? A number of
hypotheses have been proposed to account for the evolution of helping
behavior, including (1) indirect fitness benefits via kin selection derived
from feeding the offspring of close relatives
(Hamilton, 1964); (2)
unselected behavior arising as a by-product of older individuals' innate
response to begging stimuli (Jamieson,
1989); (3) a means of gaining parenting experience
(Skutch, 1961); (4)
"payment of rent" in order to remain in the group
(Gaston, 1978); and (5) future
benefits of delayed reciprocity or group augmentation
(Brown, 1987;
Ligon and Ligon, 1978;
Woolfenden and Fitzpatrick,
1978). In meerkats, some forms of help are performed exclusively
by nonbreeders, helping is often costly, and helpers contribute at least as
much as parents to feeding young and other cooperative behaviors; therefore,
helping is unlikely to be an unselected byproduct of parental behavior or a
means of gaining experience (Clutton-Brock
et al., 1998). A recent formulation of the payment of rent
hypothesis shows that it is only likely to explain the evolution of helping
where there is low relatedness within groups and where the benefits of group
living are low, at least for dominant individuals (Kokko H and Johnstone RA,
personal communication). Neither of these situations apply in meerkats
(Clutton-Brock et al., 1999b,
2000;
Griffin, 1999). This leaves
two remaining hypotheses that may account for helping in meerkats: kin
selection and group augmentation.
Meerkat groups typically contain highly related individuals; the average
relatedness between helpers and litters born into the group approximates that
of half-sibs (Clutton-Brock et al.,
2000). On average, helping behavior will therefore confer
inclusive fitness benefits to the helper. However, unrelated individuals help
as much as relatives, and close relatives help no more than less-related
individuals, suggesting that differences in relatedness do not account for the
variation in helping behavior observed within groups
(Clutton-Brock et al., 2000).
Nor does it explain why brothers help less than their sisters, as observed in
this study.
Group augmentation, or delayed reciprocity, would predict that individuals
that are most likely to associate with the pups in the future should help
most. This is consistent with the observed sex differences in helping behavior
because males are the dispersing sex (males disperse at 18-24 months), and
females are philopatric. By helping pups to survive, female helpers therefore
potentially have more to gain, both in terms of the antipredation benefits of
large group size (Clutton-Brock et al.,
1999a) and by increasing the number of potential future helpers
should they attain a breeding position. Owens and Owens
(1984) also found that female
helpers fed pups more than males in a clan of brown hyenas, where, again,
females are the philopatric sex. Similarly, young, female green woodhoopoes
feed at a higher rate than males, and this may be because females are more
likely to breed in their natal territories and subsequently benefit from
delayed reciprocity if the nestlings become helpers (Ligon and Ligon,
1990a,
b).
The tendency of female helpers to preferentially feed female pups, and for males to show no preference, may also be best explained in terms of future benefits. With the exception of older females who die before the male pups in the current litter disperse, females are likely to spend more of their lives associating with females from the current litter than with males. In contrast, male helpers will typically spend an equal proportion of their remaining time in the group with all pups in the current litter, and so would be expected to help both sexes equally, as observed. This study thus provides further evidence that fitness benefits accruing from group augmentation may play an important role in determining the levels of helping behavior in cooperative species.
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ACKNOWLEDGEMENTS |
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We are grateful to the National Parks Board of the Republic of South Africa for permission to work in the Kalahari Gemsbok National Park, and to Mr. and Mrs. H. Kotze for permission to work on their land at Van Zyl's Rus. In particular, we thank E. le Riche and D. Engelbrecht, Wardens of the Park, P. Novellie and A. Hall-Martin and the National Park staff based at Nossob, including D. Ras, J Herrholdt, G. de Kock, and S. de Waal. The study would not have been possible without the support of members of the Mammal Research Institute, University of Pretoria, including J. Skinner, J. du Toit, P. Richardson, A. MacKenzie, M. Haupt, and G. van Dyk, and of A. Griffin, T. Marshall, and J. Pemberton at the Institute of Cell, Animal and Population Biology (Edinburgh). We are grateful to the following for their assistance in data collection: D. Allsop, G. Avey, Z. Balmforth, C. Britten, J. and P. Chadwick, S. Clarke, A. Crichton, S. Davies, P. Dixon, P. Elsmere, J. Garner, N. Green, S. Hodge, J. Kewido, J. Kinns, C. MacCallum, A. MacColl, C. Macleod, T. Maddox, M. Manser, A. and G. Marais, K. McKay, S. Mercenaro, H. Nicholls, I. Olyn, M. Peterson, L. Postgate, M. Shaw, S. Slater-Jones, R. Smith, R. Tait, G. Telford, B. Themen, A. Toole, A. Turner, S. White, and R. Yarnell. For advice and assistance, we thank S. Balshine-Earn, J. Barnard, T. Coulson, R. Johnstone, R. Kilner, L. Kruuk, D. Macdonald, J. Nel, P. Roth, A. Russell, I. Stevenson, and R. Woodroffe. We are also grateful to J.D. Ligon and an anonymous referee for their comments on the manuscript. Our research is funded by grants from Natural Environment Research Council (NERC) and Biotechnology and Biological Sciences Research Council (BBSRC).
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