Experimental protocol

We experimentally tested the assumptions of reproductive skew theory using injections of the contraceptive hormone Depo-provera (medroxyprogesterone acetate, at 7.5 mg kg−1) to prevent subordinate female reproduction in our long term study population of wild, habituated meerkats at the Kuruman River Reserve, South Africa (26°58'S, 21°49'E). Meerkats are cooperative mongooses, living in groups of 3–50, where reproduction is monopolized by a dominant pair15 and all subordinates contribute to offspring care by babysitting and provisioning pups. Dominant females breed two to four times per year, and pups remain in their natal burrow for 3 weeks before starting to forage with the group, when they are provisioned by adults until c. 3 months. Groups contain several (1–12) adult subordinate females, who also attempt to breed. Subordinate breeding attempts usually trigger intense aggression by dominant females, culminating in temporary eviction from the group, or infanticide of subordinate pups11,15,16.

In year 1, in six treated groups, all subordinate females over 180 days old (n=35 females) were injected with contraceptive, and in six control groups, all subordinate females (n=38) were injected with an equivalent volume of saline solution. Initial injections were administered during the first week of July 2009, the middle of the dry season, when the probability of pregnancy is at its lowest. Two further injections were administered at 90-day intervals (October 2009 and January 2010), giving a total treated duration of 9 months (July 2009–March 2011 inclusive). During this period, 14 of the control females (at least one in each of the 6 Control groups, range 1–5 per group) were detected to have conceived at least once (range of conceptions 1–3 per female), while no treated females were detected to have conceived. In year 2, the protocol repeated, starting in July 2010 and ending in March 2011, with control groups from year 1 now receiving Depo-provera (n=72 females), and treated groups from year 1 now receiving saline (n=38 females). During this period, 14 of the control females were detected to have conceived: two Control groups showed no subordinate conceptions, while all the remaining groups had at least three subordinates who conceived at least once (range 3–4 conceiving females, with conceptions ranging from 1–3 per females). In contrast, no treated females were detected to have conceived during this period. Overall, the experiment affected 59 dominant female breeding attempts (33 control and 26 treated).

Dominant female behaviour

Throughout the experiment, we visited the groups at least twice per week, to collect behavioural data, record group composition and life history events, and weigh animals (who have been trained to step onto portable electronic lab scales). Every week, we conducted at least two 30-min focal watches on each dominant female (to give a total of 1,067 h of observations on 12 females), recording every instance of aggression directed towards subordinate females. Dominant females attacked treated subordinates at lower rates (linear mixed model (LMM), F 1,1951 =8.74, P=0.003; Fig. 1a.; Supplementary Table 1). We also recorded the total amount of time when at least one subordinate female was within 2 m of the dominant, finding that dominants were more tolerant of the presence of treated subordinates (LMM F 1,1951 =8.03, P=0.005; Supplementary Table 2). Similar focal observations on the two largest subordinate females currently present in each group (960 h of focal observations on 99 females) recorded the outcome of each foraging attempt. These revealed that treated females were less likely to be interrupted by the dominant female during a foraging bout (generalized linear mixed model (GLMM), F 1,1812 =11.13, P<0.001; Supplementary Table 3).

Figure 1: The effect of experimental suppression of subordinate female reproduction on: (a) the rate at which dominant females attack subordinates (analysis conducted on 1,952 focal watches of 12 dominant females); (b) the probability that a subordinate female was evicted during a breeding attempt (analysis conducted on 128 subordinate females, present at 59 breeding attempts (33 control and 26 treated) by 12 dominant females; (c) the ratio of adult females to dependent pups during the period of peak pup provisioning (20 to 40 days after birth; analysis conducted on 59 breeding attempts (33 control and 26 treated) born to 12 dominant females); and (d) provisioning rates by subordinate females (mass of food per minute, square root transformed; analysis was conducted on 1,050 focal watches, of 72 subordinate females). Means ±s.e. Sample sizes may be less than complete experimental sample because it was not always possible to observe specific animals within target time windows. Full size image

To determine whether the reduction in aggression affected subordinate eviction, we investigated the probability of eviction during the dominant female’s gestation for 128 subordinate females, present at 59 breeding attempts (33 control and 26 treated). Treated females were less likely to be evicted by the dominant female during her gestation (GLMM F 1,127 =8.38, P=0.004; Fig. 1b; Supplementary Table 4). The analysis also revealed that larger (closer in size to the dominant) and older subordinates were more likely to be evicted (GLMM, size: F 1,127 =6.51, P=0.012; age: F 1,127 =4.82, P=0.029; Supplementary Table 4), confirming previous findings that dominants target those most likely to attempt to breed themselves15. It is unclear how dominants detected the suppressed reproductive state of subordinates: it is likely that olfactory cues played a role22, and there were some behavioural changes in subordinates (for instance subordinates were less submissive: LMM F 1,1951 =6.77, P=0.009; Supplementary Table 5);. However, there were no overall changes in activity patterns or affliative behaviour: no change in movement (proportion of time observed spent actively moving; LMM F 1,1812 =0.14, P=0.71); no change in vigilance behaviour (time observed standing on hind legs, scanning for predators; LMM F 1,1812 =0.03, P=0.87); no change in allogrooming behaviour with other subordinate females (time spent grooming with another subordinate female; LMM F 1,1812 =1.18, P=0.28).

Helper:pup ratio

Under natural conditions, evicted females frequently fail to return, either because they die, or because they establish new groups with unrelated males23. Eviction may therefore reduce the number of helpers present in a group, with negative effects on pup development24. Reducing the eviction rates may therefore increase the number of helpers present during pup provisioning, which we assessed by calculating the average number of adult females (>360 days old) present between 20 and 40 days after birth (the period during which pups are primarily dependent on provisioning by adults). We restricted the analysis to females because (i) they contribute more than males to pup provisioning25; and (ii) the number of males fluctuates due to temporary absences while prospecting for mating opportunities in other groups26. The ratio of female helpers to pups was greater in treated groups (LMM F 1,58 =5.89, P=0.019; Fig. 1c; Supplementary Table 6), which is likely to have substantial positive effects on subsequent pup development.

Dominant female weight gain and pup emergence weight

To test whether dominants benefited from reduced aggression toward subordinates, we analysed dominant foraging efficiency, since foraging may have been less interrupted. Our focal watches recorded time spent foraging and prey biomass captured, and revealed that dominant females in treated groups captured more food per minute (interaction between dominance and treatment, F 1,3764 =4.63, P=0.032; Supplementary Table 7). This increase in foraging efficiency, coupled with the reduced effort invested in evicting subordinates, meant that dominant females in treated groups gained more weight during pregnancy (LMM F 1,53 =5.62, P=0.022; Fig. 2a; Supplementary Table 8), and pups born to dominant females in Treated groups were heavier when they first emerged from their burrows (LMM F 1,214 =6.46, P=0.021; Fig. 2b; Supplementary Table 9; n=128 Treated, 87 control pups). The strength of the effect is emphasized by the fact that pup weight was also affected by the number of subordinate females allolactating (F 1,214 =24.59, P<0.001), and there were fewer allolactating females in treated groups (LMM F 1,58 =7.59, P=0.008; Supplementary Table 10). The positive effect of additional allolactators implies that successful reproduction by subordinates carries additional costs to dominants, since surviving subordinate pups would detract from the milk available to dominant pups.

Figure 2: The effect of experimental suppression of subordinate female reproduction on: (a) weight gain by dominant females during pregnancy (analysis conducted on 12 females over 54 pregnancies (23 treated and 31 control)).; and (b) pup weight at emergence (analysis conducted on 215 pups (128 treated, 87 control) from 51 litters (22 treated and 29 control) born to 12 dominant females. Means ±s.e. Sample sizes may be less than complete experimental sample because it was not always possible to weigh animals within target time windows. Full size image

Subordinate female helping effort

Helpers in cooperative breeders are thought to face a trade-off between investment in cooperative offspring care and investment in their own reproduction27,28. Therefore, we expected subordinate females in treated groups to increase their helping effort, both because they were unable to invest in their own reproduction, and because they were less subjected to the metabolic cost of eviction. We confirmed this by analysing pup provisioning rates, finding that subordinate females in treated groups provided more food (LMM F 1,1049 =5.80, P=0.016; Fig. 1d; Supplementary Table 11), but with no change in dominant provisioning rate (LMM F 1,1165 =0.48, P=0.49; Supplementary Table 12). In contrast, provisioning rates declined in Control groups when females (both dominant and subordinate) were themselves pregnant (LMM F 1,1471 =6.02, P=0.014; Supplementary Table 13).

Pup growth rate

Pups in treated groups started life heavier, but were also in groups with more helpers, many of whom were feeding at higher rates, so we expected them to show elevated growth rates after emergence. Analysis of pup morning weight revealed that pups in treated groups grew faster between emergence and 95 days (LMM interaction between treatment and age F 1,7619 =4.35, P=0.03; interaction between treatment and age2 F 1,7619 =30.66, P<0.001; Fig. 3; Supplementary Table 14). Pup weight at emergence and independence is likely to have profound long term effects on pup fitness: size at emergence determines competitive ability in early life18; experimental feeding to increase pup weight increases survival24; and size at adulthood affects probability of attaining dominance, dominance tenure and reproductive success29.