DOI: 10.5061/dryad.79cnp5hsb
title: Higher sociability leads to lower reproductive success in female kangaroos

description: In social mammals, social integration is generally assumed to improve
 females’ reproductive success. Most species demonstrating this
 relationship exhibit complex forms of social bonds and interactions.
 However, female eastern grey kangaroos (Macropus giganteus) exhibit social
 preferences, yet do not appear to cooperate directly. It is unclear what
 the fitness consequences of sociability could be in species that do not
 exhibit obvious forms of cooperation. Using four years of life history,
 spatial, and social data from a wild population of approximately 200
 individually recognizable female eastern grey kangaroos, we tested whether
 higher levels of sociability are associated with greater reproductive
 success. Contrary to expectations, we found that the size of a female’s
 social network, her numbers of preferential associations with other
 females, and her group sizes all negatively influenced her reproductive
 success. These factors influenced the survival of dependent young that had
 left the pouch rather than those that were still in the pouch. We also
 show that primiparous females were less likely to have surviving young.
 Our findings suggest that social bonds are not always beneficial for
 reproductive success in group-living species, and that female kangaroos
 may experience trade-offs between successfully rearing young and
 maintaining affiliative relationships.
description: <strong>Social network data</strong>:
 <em>Collection</em>: Field observations were
 conducted between January 2010 and December 2013 at Sundown National Park
 in Queensland, Australia (28°55’03’’S, 151°34’46’’E). Each month between
 10 and 16 surveys were conducted within the study site in the two hours
 following sunrise or the two hours prior to sunset. From January 2010
 until September 2012, surveys were conducted once a day, alternating
 between mornings and evenings, when the kangaroos were feeding. Between
 October 2012 and December 2013, surveys were conducted either once or
 twice a day (once in the morning and once in the evening). The 15 m chain
 rule was used to assign group membership; any individual who was within 15
 m of at least one other group member was included in the group. Female
 kangaroos at the study site had been previously observed to maintain
 social and spatial cohesion at this distance. All members of a group were
 considered to be associating. Estimates of group size included all adult
 and sub-adult females within a group but did not include males of any age.
 This was done because we were primarily interested in females’ association
 patterns with each other. We recorded the location of the center of the
 group (or, where possible, the approximate location of each group member)
 using a Garmin eTrex H GPS (Garmin International Inc., Olathe, KS, USA).
 During field sessions in which surveys were not being conducted, GPS
 locations were taken for females ad libitum from 2010 until
 mid-2012.  <em>Processing</em>: Only
 females of reproductive age that were recorded on association surveys at
 least 10 times in a year were included in the analyses. We restricted our
 analysis only to females whose reproductive state had been recorded at
 least once every two months because of the possibility that we may have
 missed reproductive milestones for less frequently seen animals. For
 Approach 2, association survey data were grouped into three overlapping
 two-year periods: Jan 2010–Dec 2011 = time period A, Jan 2011–Dec 2012 =
 time period B, and Jan 2012–Dec 2013 = time period C.
 <strong>Reproductive state data:</strong> 
 <em>Collection</em>: The reproductive state of female
 kangaroos was visually assessed based on the presence and developmental
 stage of their young. Kangaroos give birth after 36 days of gestation to a
 small (approximately 800 mg), blind and hairless young that then continues
 its development inside the mother’s pouch. Young are permanently excluded
 from the pouch (PEP) at approximately 46 weeks after birth, although
 they begin to leave the pouch on temporary excursions approximately 1-2
 months prior to PEP. After PEP they continue to nurse for approximately
 another seven months before they are weaned. The reproductive state of
 each female we encountered during association surveys was recorded in one
 of six categories: females with no visible pouch-young (NPY), females with
 a small pouch-young (SPY), females with a medium pouch-young (MPY),
 females with a large pouch-young (LPY), females with a young-at-foot (YAF)
 (young that had reach PEP but continued to nurse and remain close to its
 mother), and females who were caring simultaneously for both a small
 pouch-young and a young-at-foot (SPY, YAF).
 <em>Processing</em>: For approach 1 that investigated
 annual reproductive success, for each female for a given year, we analyzed
 (1) whether she produced a young that survived to the LPY stage, (2) to
 PEP, and (3) to weaning. For approach 2, a longer-term dataset was used to
 assess the relationship between individuals’ sociability and offspring
 survival, incorporating a measure of females’ association preferences. We
 analyzed (1) whether she produced a young that survived to the LPY stage,
 (2) to PEP, and (3) to weaning in the second year of the two-year time
 period. This ensured that the same young were not counted in two time
 periods.
description: <strong>Social network data</strong>: There are
 four social network files, one for each year of the study. These data list
 the individuals and the groups in which they were seen.
 <strong>Reproductive state data</strong>: These
 tables indicate whether each female produced a young that survived to the
 LPY stage (Yng.LPY), to PEP (Yng.PEP), and to weaning (Yng.Wean) for the
 given year (binary 0 = no, 1 = yes). It also includes data on: 
 <em>Numrec.Yr</em>: Number of recorded sightings in a
 given year <em>Year / Period</em>: The year
 or period (A, B, or C) of the study
 <em>MedianGS</em>: The median group size in which the
 female was found <em>Prim.mum</em>: A
 binary variable indicating whether the female was primiparous (1) or not
 (0)