APPENDIX A1
HOW SHOULD COSTS OF REPRODUCTION BE MEASURED?
Working with parental investment requires longitudinal data covering at least two successive
reproductive events. By simply comparing individuals at one reproductive occasion, one could
only conclude that reproducing individuals have a greater reproductive effort than nonreproducing individuals, but no inference can be drawn about parental investment. Demonstrating
the existence of costs of reproduction must be done by comparing fitness components (e.g.
survival and reproduction) at time t+1 or later (t+2, …) between individuals that reproduced and
individuals that did not at time t. Several categories of reproductive states at time t may be
necessary to adequately assess costs of reproduction, since investment may occur only at one or a
few specific reproductive events (e.g. before mating, at mating, during gestation or lactation).
Survival costs of reproduction must be quantified from long-term, individual based data,
comparing survival rates between t and t+1 of individuals that were and were not in a
reproductive state at t. Similarly, reproductive costs of reproduction should be properly estimated
by comparing the breeding frequency or success of individuals at t+1 that did and did not
reproduce successfully at t (Fig. A1). If such a comparison is rather straightforward for pulsebirth populations (i.e. populations showing marked seasonality and synchrony in birth dates),
different measures of costs of reproduction for birth-flow populations must be considered (i.e.
populations breeding all year-round, such as ungulates living under the tropics and most
primates; Bronson 1989).
The birth-interval (i.e. the time elapsed between two consecutive births) can provide an
appropriate measure to assess reproductive costs in birth-flow populations (Fig. A1), although
misinterpretations of reproductive costs based on birth-intervals can occur (e.g. Cheney et al.
2004; Robbins et al. 2006). Indeed, birth-interval is a composite variable that encompasses the
time needed to give birth and to wean an offspring successfully (or not), and the time needed for
the female to recover from her previous reproductive attempt. Clearly, the former is compulsory
and will last as long as the young survives until it reaches the age of weaning, preventing a
female from starting a new oestrus. Therefore, only the latter component should be considered to
measure reproductive costs of reproduction in birth-flow populations with the prediction that the
longer the recovery time, the heavier the investment must have been. Note, however, that the time
used by a successful female to wean an offspring can result in costs of reproduction since taking a
longer time to wean offspring will reduce the number of future reproductive opportunities. For
birth-flow populations, we propose that a comparison between the time elapsed between a
successful weaning and the next ovulation/birth with the time elapsed between the loss of an
offspring before weaning and the next ovulation/birth (Fig. A1) should provide a correct measure
of reproductive costs.
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Figure A1. Comparison of the reproductive sequences in birth-pulse and birth-flow populations
and the proposed estimators of costs of reproduction (CoR). In birth-pulse populations, costs of
reproduction should be estimated by comparing the proportion of females in oestrus at time t+1
that did and did not successfully reproduce at time t ( F 1 and F 0 ). In these populations with a
highly seasonal and synchronous reproduction, these proportions will be N11 out of N 1 females
and N10 out of N 0 females respectively for mono-oestrous species (A). In poly-oestrous species
(B), the estimation of the costs of reproduction does not differ from the mono-oestrous case,
since the number of successful and unsuccessful breeding females at time t are directly
comparable ( N 1 and N 0 ). However, poly-oestrous females may ovulate several times in a row at
1
0
time t+1 if not fertilized. Hence, all oestrus from the first to the last ( N1+
and N1+
) should be
0
1
included in the comparison of frequencies ( F vs. F ), independently of the ovulation rank.
Assessing costs of reproduction in birth-flow populations (C) requires comparing birth-intervals
(d) of females that successfully weaned an offspring with that of females that skipped (0) or
failed ( ) reproducing at an early stage of breeding (i.e. comparing the time elapsed between a
successful weaning and the next ovulation/birth with the time elapsed between the loss of an
offspring before weaning and the next ovulation/birth). To make direct comparisons possible
with birth-pulse populations, birth-intervals may be transformed into breeding frequencies (F).
For instance, dividing the observed birth-interval between two successful reproductions in
months by a year (12 months defining the time unit T), the resulting yearly breeding frequency
can be compared directly with the proportion of breeding females observed in a birth-pulse
population.
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References
Bronson, F.H. (1989). Mammalian reproductive biology. University of Chicago Press, Chicago.
Cheney, D.L., Seyfarth, R.M., Fischer, J., Beechner, J., Bergman, T., Johnson, S.E., et al. (2004).
Factors affecting reproduction and mortality among baboons in the Okavango Delta,
Botswana. Int. J. Primatol., 25, 401-428.
Robbins, A.M., Robbins, M.M., Gerald-Steklis, N. & Steklis, H.D. (2006). Age-related patterns
of reproductive success among female mountain gorillas. Am. J. Phys. Anthro., 131, 511521.
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