Parental Investment in Animals and the Division of Care by Gender
Author: Elsby, D.
Introduction
Parental care is not required for all organisms. Innate, genetically programmed behaviours allow the offspring of many species to survive and reproduce without the need for any parental involvement whatsoever. Where present, however, parental care provides a multitude of additional and in many cases vital benefits to the offspring. Such care can be provided by the male, the female or both parents and the levels of investment vary dramatically by species.
Parental Investment
Parental investment is defined as the activities carried out for the benefit and increased survival of one offspring, but that also decreases the ability of the parent to invest in other offspring (Trivers 1972 cited in Goodenough et al. 2010, p.334). Parental Investment grants offspring greater protection from malnutrition, disease, parasitism, exposure and predation and may include the imparting of valuable learned skills such as how to hunt, how to avoid predators, where to forage etc. (Goodenough et al.2010).
A species is described as altrical when innate behaviours alone are not sufficient to ensure survival. In such cases, the removal of the parent(s) will usually result in the death off the offspring. However, even in cases where innate behaviours can be enough to allow offspring to survive, parental care can still be required to ensure appropriate levels of fitness e.g. to ensure appropriate mate selection (Dockery and Reiss 1999, p.97).
Benefits and Costs of Parental Investment
The overall benefit of parental care is an increase in the direct fitness of the parent; the offspring benefits by being more capable of surviving and reproducing while the parent succeeds in passing on their genes – and in the case of learned behaviours, their knowledge – to the subsequent generation.
The cost to the parents in providing care is often a reduced ability to reproduce again in the future; there is often a trade-off between the reproductive rate of adults and the parental care they invest in their offspring (Klug and Bonsall, 2007). This is an important factor in longer lived species; if a parent invests all its energy into one brood, it will be less likely to produce further offspring in the future.
Models of Parenting
Table 1 shows data from six taxa of animals and the patterns of parental care observed in each (Reynolds et al. 2002). Within the taxa of teleost fish, anurans and squamate reptiles, the majority of species provide no parental care to their offspring. Since these individuals spend no energy investing in their offspring, beyond the preparation of and combining of gametes, they are capable of reproducing more frequently and/or in greater numbers during their lifetime. The trade-off is that each of their offspring, individually, will be less likely to survive and reproduce than if the parent(s) had invested in care. This cost is often offset by increased brood numbers (Bekoff ed. 2004).
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Table2 : Parenting Models of Select Animal Taxa (data from Reynolds et al. 2002) |
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|
Taxa |
No Care |
Male-only Care |
Female-only Care |
Biparental Care |
| Teleost Fish |
79 % |
10 % |
1 % |
3 % |
| Anurans |
92 % |
9 % |
1 % |
9 % |
| Squamate Reptiles |
97 % |
0 % |
3% |
0 % |
| Crocodilia |
0 % |
0 % |
62 % |
38 % |
| Aves |
1 % |
1 % |
8 % |
81 % |
| Mammalia |
0 % |
0 % |
91 % |
9 % |
| Notes: 9% of Aves demonstrate co-operative breeding whereby three or more individuals care for young, hence the 91% total. Some anurans demonstrate more than one form of care, hence the >100% total. |
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Determination of Male or Female Care
While there are exceptions, there is a significant correlation between the provision of male parental or female parental care in a species, with internal or external fertilisation. In species that utilise internal fertilisation it is often the female who provides the bulk of the parental care; conversely, those species which utilise external fertilisation often exhibit male parental care (Dockery and Riess, 1999, p.96). In the case of internal fertilisation and gestation only the female is physiologically capable of supporting the young; the male is freed from parental duties. Certainty of Paternity also affects whether the male or female will provide care. With internal fertilisation, the female can be 100% certain that the offspring is her own – carries 50% of her genes – and thus will be more motivated to ensure its survival by providing care. Males on the other hand, especially in a competitive breeding environment, may be less certain. Rather than invest energy into offspring that may or may not be carrying his own genes, a male might better increase his fitness i.e. the chances of his own genes being passed on, by investing his energy into creating more offspring. As Dockery and Riess (1999, p.86) aptly put it, “males generally put their reproductive effort into mating, females into parenting”.
As shown in Table 2 most patterns of parenting involve only one parent. Solitary parenting occurs in cases where only one parent is required to nurture and protect the young; without even a single parent, the offspring would die. This is usually the female (Dockery and Reiss 1999, p87; Reynolds et al. 2002). By the same token, biparental parenting only occurs in species where the investment by both parents is required to successfully raise the offspring. Over time, and through selection pressures, the parental model of a species may change (Reynolds et al. 2002).
Female Care
Physiologically, there are some forms of parental investment that only the female of the species is capable of providing i.e. gestation and nursing. With internal fertilisation and uterine gestation i.e. mammals, a female is providing her offspring a safe, compatible environment during the most vulnerable life stage. This has an incredible energy cost as the female must continue to meet her own basic needs while also meeting the needs of the growing offspring.
Depending on the length of gestation, the benefit of this initial care in the womb can allow the offspring to become almost completely functional in terms of being able to survive the dangers of the external environment, either immediately or very soon after parturition i.e. precocial species (as opposed to altrical species). Well-developed offspring is an especially invaluable benefit to prey species that exist in open environments. For the blue wildebeest (Connochaetes taurinus) of the Serengeti and Masai Mara plains in Africa, the ability to be on the move with the herd, literally within minutes of birth (Kostyal 2010; Nowack, 1999), is crucial if they are to avoid becoming an easy meal for the spotted hyena (Crocuta crocuta), their primary predator. Despite this initial development, however, such offspring are still completely dependent upon their mother for nutrition in the form of milk.
Following parturition – or fertilisation, in the case of egg laying species – a female may continue to invest in other ways. The key demands of offspring are food and protection. In African Lions (Panthera leo), a uniquely social species among wild felids, the females are solely responsible for providing food for the entire pride – themselves, offspring and males – by hunting (MacDonald and Loveridge 2010, p.19). Males, in contrast, play no part in the hunt whatsoever.
Female Nile crocodiles (Crocodylus niloticus) keep a close watch over their nest of buried eggs to protect them from predators (O’Shea and Halliday, 2002). Once hatched the female will then carry the young from the nest to a suitable body of water where the young will continue their development.
Male Care
Male only care is most common in Teleost fishes; the female chooses a suitable male and deposits eggs then leaves the male to fertilise and tend them (Goodenough et al., 2010; T.J. Pitcher ed., 1986). While nourishment is provided by the yolk of the egg, the male is responsible for everything from suitable nest selection and preparation (vital to attract a mate in the first instance), protection from predation, maintaining suitable environmental conditions e.g. water/oxygen flow over the developing eggs, and cleaning the nest to keep it free from parasites (Pitcher ed., 1986).
In the biparental care model, males may provide parental investment initially by supporting and protecting the female. The role of protection can continue, after parturition, as the offspring grows. In crested auklets (Aethia cristatella) the male plays a greater role in defence of the nest than the female does (Fraser et al. 2002).
Among African lions the males act together to prevent takeover of the pride by nomadic coalitions of other males. In doing this they prevent the infanticide of their own offspring that would occur in a pride takeover (MacDonald and Loveridge 2010, p.147). The presence of a male lion also dissuades the theft of any kill made by the lionesses by other predators e.g. spotted hyenas.
Non Gender Specific Care
While physiologically better suited to certain care behaviours, there are many that both male and female parents take part in. Both male and female parents are capable of providing models of behaviour from which offspring can learn foraging skills, appropriate social structures and interactions (including mate selection criteria)(Goodenough et al. 2010, p.86-87). Despite being unable to nurse or gestate its offspring, a male may invest by foraging for the pregnant/brooding female – this is common in aves where biparental parenting is the dominant model and in some cases the male shares the task of incubating the egg with the female (Bekoff ed. 2004; Fraser et al. 2002).
Conclusion
Fecundity ensures that, while offspring are individually less likely to survive, the inclusive fitness of the parents is upheld. Internal fertilisation grants a greater chance of survival to the offspring, but the female (usually) is ‘left holding the egg’ and must therefore invest energy in it i.e. provide care; by doing so she increases her inclusive fitness. Males, while untethered physically from their offspring, can further increase their own inclusive fitness by aiding the female and/or offspring directly. In many species, particularly birds, offspring benefit from, and actually require, the care of both parents.
The gender divide of parenthood amongst animals (excluding humans) is ultimately the result of both physiology and evolutionary pressures on the fitness of the individual – male or female.
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