According to all of my parent friends, raising girls is quite different from raising boys, from the toys to the tantrums, the clothes to the bathroom habits, even for the most liberal, gender-neutral of my friends. In most sexually dimorphic species, raising boys is actually physically more costly than raising girls because males are larger than females. This trend usually begins at birth and continues into adulthood. In sexually dimorphic species, males generally fight amongst themselves for access to females during mating season, so size is an important factor for male reproductive success. The bigger a male baby is when he’s born or weaned, the bigger he is likely to be as an adult. Male offspring are therefore more energetically expensive to raise than females. In fact, raising sons versus daughters early in life can influence how quickly the mother ages later in life!
Senescence, or aging, is the progressive deterioration of physiological function as individuals age, ultimately leading to decreased health, decreased reproduction, and increased incidences of disease and death (Douhard et al. 2020, Austad 2005). Evolutionary theories on aging (e.g., the disposable soma theory and antagonistic pleiotropy – see Kirkwood & Rose 1957 and Williams 1957 for more details) predict that investing more resources into raising offspring early in life causes faster reproductive or physiological declines later in life. In other words, the more your offspring cost you early on, the less you have to give later. Using 40 years of data on bighorn sheep from Ram Mountain, Alberta, Canada, Mathieu Douhard and colleagues (2020) investigated if having more male or female offspring in the first half of one’s life affected a female sheep’s survival or reproduction in the second half of her life.
The authors split a female’s reproductive output into ‘early life,’ or years 1-7, and ‘later life,’ or 8 or older and quantified the survival of each offspring and the sex ratio of each female’s offspring during each period. Male bighorn sheep are significantly larger than female ewes, with males weighing 58–143 kg (128–315 lb) and females weighing 34–91 kg (75–201 lb; Wikipedia). Male offspring suckle more frequently and for longer and are so costly to raise that a female ewe is 10-12% less likely to wean an offspring the year after weaning a son (Bérubé et al. 1996).
For bighorn females at least, the more offspring you succeed in raising early in life, the fewer offspring you can successfully support later (Figure 2A; Douhard et al. 2020), as evidenced by the decline in the pink line for females with 3 or more offspring early in life. Further, if you compare females that weaned the same number of offspring, females that weaned more male offspring early in life showed faster declines in their reproductive output than females that weaned mostly female offspring (Compare the green and orange lines; Figure 2B). However, the decline shows up mainly in the area of offspring winter survival rather than a female failing to give birth at all or failing to wean her offspring.
There is good news though – Although increased early-life reproductive effort negatively influenced later life reproduction, it positively influenced later life survival! In other words, females with better reproductive success early on were more likely to survive later in life across every age. So as long as you have your sons early, you can live long and prosper, undisturbed by later offspring.
S. N. Austad, Diverse aging rates in metazoans: targets for functional genomics. Mech Ageing Dev 126(1), 43-49 (2005)
M. Douhard, M. Festa-Bianchet, F. Pelletier, Sons accelerate maternal aging in a wild mammal. PNAS 117(9), 4850-4857 (2020).
C. H. Bérubé, M. Festa-Bianchet, J. T. Jorgenson, Reproductive costs of sons and daughters in Rocky Mountain bighorn sheep. Behav. Ecol. 7, 60–68 (1996).
T. B. L. Kirkwood, M. R. Rose, Evolution of senescence: Late survival sacrificed for reproduction. Philos. Trans. R. Soc. Lond. B Biol. Sci. 332, 15–24 (1991).
G. C. Williams, Pleiotropy, natural selection and the evolution of senescence. Evolution 11, 398–411 (1957).